Next Article in Journal
Optimization of N Fertilizer Type and Ridge–Furrow Ratio to Improve Resource Use Efficiency and Grain Yield of Rain-Fed Winter Wheat in Loess Plateau, China
Previous Article in Journal
Biological Characteristics and Thermal Requirements of Telenomus podisi and Trissolcus basalis (Hymenoptera: Scelionidae) in Fresh and Cryopreserved Eggs of Euschistus heros and Nezara viridula (Hemiptera: Pentatomidae)
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Agronomy
Volume 14
Issue 1
10.3390/agronomy14010171
agronomy-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Effects of Different Cover Crops and Amendments on Soil and Crop Properties in Organic Vegetable Production †

by
Robert P. Larkin
New England Plant, Soil, and Water Laboratory, United States Department of Agriculture, Agricultural Research Service, University of Maine, Orono, ME 04469, USA
†
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
Agronomy 2024, 14(1), 171; https://doi.org/10.3390/agronomy14010171
Submission received: 29 September 2023 / Revised: 6 January 2024 / Accepted: 10 January 2024 / Published: 12 January 2024
(This article belongs to the Section Farming Sustainability)
Browse Figures
Versions Notes

Abstract

:
The impacts of two different cover crop (CC) strategies, as well as compost, silicon (Si), and biocontrol (BC) soil amendments were evaluated on soil chemical and biological properties, crop development and yield, and disease and pest issues in organic vegetable production, as represented by legume (green snap bean), cucurbit (green zucchini squash), and solanaceous (sweet pepper) vegetable crops, in a three-year field trial in Maine, USA. A multi-species CC mixture (6 crops, including legumes, grasses, and brassicas) was compared with a standard winter rye CC for potential benefits on soil properties and biomass production. Soil amendments included a commercial organic fertilizer alone, composted dairy manure, compost plus BC (commercial formulations of Trichoderma and Streptomyces spp.), and compost plus Wollastonite, a natural source of Si. Poor stand establishment of some components of the multi-species CC mixture resulted in lower biomass and ground cover relative to winter rye, but had no effect on crop development or yield. Compost amendments increased soil pH, organic matter, and nutrient contents, as well as yields of bean, zucchini, and peppers relative to a fertilizer-only treatment. Additions of Si increased Si content in plant leaves and reduced powdery mildew on squash and leaf necrosis of beans. In the absence of substantial soilborne disease, BC provided only marginal reductions in powdery mildew and leaf necrosis and no effects on yield. These results help define specific management practices to improve organic vegetable production and provide useful information and options for growers.

1. Introduction

Sustainable organic vegetable farming faces many production challenges, including maintaining adequate soil health and fertility, management of pests and diseases, and balancing multiple different vegetable crops [1]. The effective use of basic soil and crop management strategies, such as the establishment and development of crop rotations, cover crops, and soil amendments are crucial for achieving sustainable vegetable production systems [1,2].
Cover crops are any crop that is grown between periods of normal crop production, usually grown primarily to protect the soil from erosion and nutrient losses [2,3]. But cover crops can have many additional benefits and uses and can serve multiple functions in a cropping system. Cover crops have been shown to be useful for the addition of organic matter, improving soil structure and tilth, addition and recycling of nitrogen (N) and other nutrients, improving soil and crop productivity, and weed, pest, and disease control [4,5,6]. The type or species of cover crop is instrumental in determining the kind of impact it may have, and different types of cover crops will provide different ranges of benefits. For example, leguminous species provide additional N, high-biomass-yielding grasses and forage add more organic matter, crops with large tap roots improve aeration and drainage, and crops with extensive root systems help improve soil structure and microbial diversity [7]. Because no single species can deliver all of these benefits, there has been much recent emphasis on the use of mixtures of diverse species of cover crops. These multi-species mixtures can potentially provide multiple different benefits to cropping systems [7,8]. However, research results on the benefits of multi-species mixtures have been mixed, with some studies not showing any significant advantages in biomass production or soil properties relative to single species cover crops [9,10,11]. And the potential for using multi-species cover crop mixtures in organic production systems is still largely unknown. Winter rye (Secale cereale) has been established as a standard cover crop in the Northeastern US because it is well-adapted for use as a late-season fall cover crop, has an extensive root system and high biomass production, and has the ability to overwinter and regrow in the spring. But could the use of multi-species mixtures improve upon the performance and benefits of winter rye as a cover crop in an organic vegetable system?
Building soil organic matter is an important component for improving soil health and sustainable organic production [2]. Organic matter impacts all aspects of soil properties, physical, chemical, and biological, supplying food for the soil biota, stabilizing soil structure and water relations, and increasing soil fertility [2,12]. Additions of compost can supply larger amounts of organic matter to soil than most other types of amendments, and compost amendments have been shown to increase microbial biomass, activity, and diversity, reduce soilborne diseases, and provide other benefits well beyond supplying basic plant nutrition [13,14,15].
Recent reviews have highlighted the role of Silicon or silica (Si) in plants and its potential beneficial effects in reducing biotic and abiotic stresses [16,17,18,19,20]. Although Si is one of the most abundant elements in the world and is not considered an essential mineral nutrient for plant development, increased plant levels of Si have been shown to reduce plant stresses, such as insect pest or pathogen attacks, drought, and low temperatures [16,21,22]. Because plant-available forms of Si may be somewhat limiting, additions of Si can improve Si uptake and provide benefits [17]. Although the potential for Si amendments in organic vegetable production has not been established, a previous preliminary trial indicated some positive effects [23].
Successful management of soilborne diseases is another area of concern in organic production. Although good soil management practices may reduce soilborne diseases, supplemental practices may be needed, and biological control approaches are an attractive option. There are several commercially available biocontrol products, such as those containing the biocontrol fungi Trichoderma spp. and actinomycete Streptomyces spp., that have shown activity against a range of different soilborne pathogens and diseases [24,25,26,27,28]. However, their efficacy and utility in organic vegetable production has not been fully established.
As multiple different crop management practices are implemented, the compatibility and potential complementarity of these various practices is also important, such that different practices must work well together to provide beneficial results. Thus, how different factors affect each other, or the interaction between different cover crops and soil amendments, also must be assessed and considered. The purpose of this research was to assess how different cover crops would affect a multiple crop organic vegetable production system, as well as determine the potential benefits and interactions that selected soil amendments (compost, silicon, and biological control organisms) may provide to the system. Thus, in this research, two different cover crops and four soil amendments were assessed for their effects on soil properties, crop growth and yield, and pest and disease issues in three different organic vegetable crops (snap bean, zucchini squash, and sweet peppers) in an organic vegetable production system over three full cropping seasons in central Maine. The ultimate goal of these studies is to develop and optimize improved production systems for organic vegetables that maximize sustainability and productivity.

2. Materials and Methods

2.1. Field Design and Management

Field trials were conducted at an organic research site in St. Albans, ME (44°53′ N, 69°25′ W) over three consecutive growing seasons (2019–2021). This study was established as a split-block design (with cover crop as the main plot and soil amendment treatments as sub-plots) with four replicate blocks for each of three different vegetable crops (Figure 1). These trials were implemented in the same field and with the same plot configuration as a previous trial that also involved compost amendments [23]. A full season of a winter rye cover crop separated the trials, and plots receiving compost or no compost were consistent between the trials, but some of the compost effects may be cumulative based on this previous history. Details of the field site, including soil type, soil texture, slope, and previous cropping history have been described previously [23]. All plots were tilled with a chisel plow followed by a disc harrow prior to planting.
Crop water needs were mostly dependent on natural rainfall, with some supplemental watering provided by hand watering of plots when conditions were not suitable for plant growth. An on-site weather station was used to monitor environmental conditions (air temperature, relative humidity, and rainfall) and data were used to determine daily, weekly, and monthly average conditions throughout the cropping season. Additional monitoring of air temperature and relative humidity within plots at canopy level, and soil temperature and soil moisture (using Watermark sensors) within plots in each block at 15 cm depth for (a) under the mixed CC treatment plots, (b) under the winter rye CC treatment plots, and (c) outside the plots under bare ground was monitored using Watchdog data loggers (model 450, Spectrum Technologies, Plainfield, IL, USA) throughout the season.

2.2. Cover Crop Treatments

Two different cover crops were used prior to planting the vegetable crops to evaluate potential effects on soil properties, crop productivity and soilborne diseases. The cover crops used were (1) a winter rye (Secale cereale) planted at a seeding rate of 134 kg/ha, and (2) a multi-species cover crop mixture consisting of the legumes field peas (Pisum sativum), hairy vetch (Vicia villosa), and crimson clover (Trifolium incarnatum), planted at 13, 11, and 13 kg/ha, grains winter rye and annual ryegrass (Lolium multiflorum), at 34 and 11 kg/ha, respectively, and the Brassica tillage radish (Raphanus sativus) planted at 6 kg/ha. All cover crops were planted in the fall of each year in mid-late September using a cone seeder drill (Gandy Co., Owatonna, MN, USA), allowed to overwinter and regrow in the spring before preparing the fields for planting to vegetables. Prior to spring incorporation, estimates of ground cover and cover crop biomass were made from all plants within 3 randomly placed 0.581 m2 quadrats within each CC for each block (24 total per year). Ground cover was determined as the percentage of ground within a quadrat covered by vegetation and assessed via digital analysis. Cover crop biomass was determined from harvested quadrats as the total above-ground biomass (both fresh and dry wt).

2.3. Soil Amendment Treatments

Soil amendments included different soil fertility and biological treatments, and consisted of (1) commercial organic fertilizer alone, (2) composted dairy manure, (3) compost plus biological control organisms, and (4) compost plus Wollastonite (a natural mineral source of Si). The fertilizer-only plots received 1000 kg/ha of an organic fertilizer (Fertrell Feed-n-Grow, Fedco Seeds, Clinton, ME, USA), which contains a 3-2-3 NPK content applied to soil prior to mulch covering and planting [23]. The rest of the treatments received an initial application of 500 kg/ha of the commercial fertilizer, plus the additional amendments. Composted dairy manure was added at a rate of 60 m3/ha (~18 Mg/ha dry weight) and tilled into the soil prior to planting. Average compost composition (dry weight basis) was 21.2% C, 1.6% N, 0.7% K, and 0.5% P, with a pH of 7.2 and a C:N ratio of 13. Wollastonite, a soluble-grade natural mineral (52% SiO2, 48% CaO) (Fedco Seeds) was applied at the rate of 3.5 Mg/ha and incorporated into the soil prior to planting. The biological control treatment consisted of a combination of two different commercial biocontrol organisms, a bacteria, Streptomyces lydicus WYEC108 (Actinovate AG, Novozymes BioAg Inc., Brookfield, WI, USA), and a fungus, Trichoderma virens GL-21 (SoilGard, Certis USA, Columbia, MD, USA). A liquid suspension was added to the soil around each plant (1 g and 2.5 g/plot in 0.5 L water for S. lydicus and T. virens, respectively, as previously described [29]. In all plots, a permeable and reusable woven polypropylene fabric ground cover (DeWitt Sunbelt, Sikeston, MO, USA), was used as mulch, as previously described [23].

2.4. Soil Chemical and Biological Properties

Soil samples were collected in the spring of each year from each plot, after soil amendments were applied and prior to vegetable planting, for analyses of effects of the treatments on soil properties. Eight soil cores (2.5 × 15 cm) were collected from random locations throughout each plot and combined into one composite sample per plot. Samples were then sieved through a 2 mm screen and air-dried, to be used for soil physical and chemical analyses. Measured soil properties included pH, organic matter content, cation exchange capacity, and concentrations of nutritionally important elements and compounds. Potentially available N, as nitrate (NO3−) and ammonium (NH4+) was determined using cold water bath KCl extractions [30]. Soil concentrations of P, K, Ca, Mg, Al, B, Fe, Mn, Na, S, Cu, and Zn were estimated using Modified Morgan extraction procedures [31], and analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES) by the University of Maine Analytical Lab (Orono, ME, USA).
Testing of soil samples for selected biological properties was also performed in both 2020 and 2021. Microbial biomass was estimated through CO2 respiration using the Solvita CO2− burst assay [32], and an additional soil labile amino nitrogen assay (Solvita SLAN test, Woods End Laboratory, Mt. Vernon, ME, USA) was conducted in the fall of 2021.

2.5. Vegetable Crops

The three organic vegetable crops grown were green snap beans (Phaseolus vulgaris L., variety ‘Provider’), green zucchini squash (Cucurbita pepo L., variety ‘Dunja’), and sweet peppers (Capsicum anuum L., variety ‘Glow’), as representative examples of legume, cucurbit, and solanaceous vegetable crops. The beans and zucchini were planted by seed as described previously [23], whereas the pepper were transplanted into the plots as seedlings after being grown in the greenhouse for 6 weeks as described previously [29]. All vegetable seeds were obtained from organically grown sources (Johnny’s Selected Seeds, Winslow, ME, USA, and High Mowing Organic Seeds, Wilcott, VA, USA). In 2019, planting of all three vegetable crops in the upper half of the field (blocks 1 and 2) was completed on 5 June, and the lower half of the field (blocks 3 and 4) on 14 June, due to early season wet field conditions in 2019, with some limited replants done in areas of poor germination on 19 and 28 June in the upper and lower field portions, respectively. In 2020, beans and zucchini were planted on 9 June and peppers were transplanted on 10 June. In 2021, beans and zucchini were planted on 26 and 27 May, and peppers were transplanted on 4 June. After planting, and prior to emergence, squash plots were covered by an insect netting (ProtekNet, Johnny’s Seeds, Winslow, ME, USA) laid over steel hoops to protect plants from squash bugs (Anasa tristis), cucumber beetles (Acalymma vittatum), and other insects during early growth stages, as described previously [29]. Emergence (as percentage of emerged seedlings relative to total seeds planted) was assessed for bean and squash crops periodically through the first 30 days after planting (DAP). All seed, products, equipment, inputs, and methodologies used throughout these trials were certified organic and/or approved for use in organic production.

2.6. Crop Growth, Yield, and Disease Evaluations

Signs and symptoms of foliar and soil-borne diseases and insect pests were monitored in the field for all crops. Vegetables were harvested by hand as they ripened to maturity, weighed, and data recorded by row. Squash was harvested 2–3 times each week (due to fast growth), and beans and peppers were harvested once per week. In 2019, squash harvest ran from 18 July to 13 September (9 weeks), bean harvests from 26 July to 12 September (7 weeks), and pepper from 6 August to 16 September (6 weeks). In 2020, squash, bean, and pepper harvests began on 17, 30 July and 3 August, and continued over a period of 9, 8, and 7 weeks, respectively. In 2021, squash, bean, and pepper harvests began on 8, 26 July, and 10 August, and continued over a period of 10, 8, and 6 weeks, respectively. Yield was determined as the total weight of harvested vegetables per each plot row (6.1 m) at each harvest date as well as the total for all harvest dates, and expressed as Mg/ha. Visual assessments of symptoms of powdery mildew, leaf spots, or other plant diseases present, as well as insect infestation and damage, were recorded (as the percentage of total plot leaf area affected) during the latter half of the growing season.
To assess the nutritional characteristics of the growing crops, as well as to determine whether the Si treatment affected plant Si content, plant leaf tissue analysis was conducted on leaf samples from all crops and treatments in 2021. In early August, plant leaf samples were obtained by collecting a total of 30 young, mature (fully developed) leaves from near the top of 5–6 plants within each crop and treatment combination for each block. The leaf samples were analyzed for all standard elemental nutrient contents (N, P, K, Mg, Ca, S, B, Zn, Mn, Fe Cu), as well as for Si, by an independent commercial laboratory (Waters Agricultural Laboratories, Warsaw, NC, USA) and expressed as either % of total or parts per million (ppm) based on sample dry weight.

2.7. Statistical Analysis

Standard analysis of variance (ANOVA) for a split-block design with factor interactions was conducted for all soil property, yield, and disease assessment data. Data were analyzed for each crop year separately. An example ANOVA table showing the factors, interactions, and associated error terms and degrees of freedom for the split-block analyses is depicted in Table S1. In addition, data from all years were combined and analyzed together (with year and interactions as additional factors) to indicate overall effects over the course of the study. Normality was confirmed for all data sets using the Shapiro–Wilk test. Significance was evaluated at p < 0.05 for all tests. Mean separation was accomplished with Fisher’s protected LSD test. All analyses were conducted using the Statistical Analysis Systems v. 9.4 (SAS Institute, Cary, NC, USA).

3. Results

3.1. Environmental Conditions and Overall Crop Growth

Environmental conditions varied during the vegetable growing seasons over the three years of the study. Daily temperatures averaged above normal throughout the summer for all three years, with notably higher temperatures in June and July 2020 and August 2021 compared with the long-term (30-year) averages for the area (Table 1). Rainfall was variable from month to month and year to year, with 2019 having a wet spring and fall (May, June, August, September), but a dry July, and overall wetter than normal year. In 2020, a relatively dry start to spring (May, June) was followed by a wet July, and drier than normal August and September, for a relatively dry summer. In 2021, a dry spring (May-June) and August was interspersed with a wetter than normal July and September. Overall, there was a rain surplus of 16 cm in 2021, a deficit of 14 cm in 2020, and a slight deficit of 4.5 cm in 2021 for the summer season compared to long-term average conditions (Table 1).
Soil temperature and moisture conditions were monitored throughout the growing season for the two different cover crop treatments at a 15 cm depth under the fabric mulch in plots and also under bare ground. Soil moisture generally fluctuated with rainfall (Figure 2). In 2019, abundant rainfall kept soils moist most of the summer (except for a dry period in July). In both 2020 and 2021, a dry spring resulted in water stress through most of June, and substantially wetter soils under the fabric mulch than bare soil. Adequate moisture was temporarily restored by July rains, but dry conditions due to lack of rain continued until September. In general, soil under the fabric mulch for both cover crops retained moisture better than outside the fabric mulch, averaging fewer days under water stress (>−60 kPa) in both years, but no consistent differences between the two cover crops (Figure 2).
Crop growth was generally good in all three years despite the fluctuating moisture conditions. In 2019, wet early conditions and late planting dates resulted in lower overall yields for all three crops than in 2020 and 2021, averaging 10, 35, and 26% lower than in 2020 for beans, zucchini, and peppers, respectively, whereas earlier planting dates in 2021 resulted in overall higher zucchini and pepper yields, averaging 26 and 11% higher than in 2020. Plant disease issues were relatively minor in all years, with only some late season powdery mildew on zucchini, and some general decline (leaf chlorosis and necrosis) emerging late in the season on beans and zucchini.
Cover crop establishment, growth, and spring regrowth varied substantially from year to year, depending on environmental conditions. However, in all years, the winter rye cover crop established better in the fall and maintained over winter for better regrowth the following spring, resulting in greater ground cover, biomass fresh weight, and biomass dry weight than the mixed species cover crop treatment (Table 2). Averaged over all three years, the winter rye cover crop resulted in 65% greater ground cover, 55% greater biomass fresh wt, and 40% greater dry wt than the mixed species cover crop. Only a few of the species in the mixed species CC were observed to survive over winter to grow back in the spring, notably the winter rye, ryegrass, and hairy vetch.

3.2. Treatment Effects on Soil Properties

Soil amendment treatments had significant effects on many soil properties, whereas cover crop had only minor effects on a few parameters, and interactions between the two factors were not significant (p = 0.11 to 0.99) for any parameter. Similar results were observed each year (no significant year by cover crop or year by amendment interaction, p = 0.08 to 0.99), so soil properties are presented as averaged over all three years (Table 3). The fertilizer-only treatment resulted in the lowest pH and organic matter content, and the lowest concentrations of K, Mg, Na, and CEC than all other treatments, as well as lower NO3 than silica, but higher NH4 content than compost and silica treatments (Table 3), whereas the silica amendment (Wollastonite) resulted in higher pH and higher Ca content, and higher CEC than all other treatments. For other minor elements, silica treatment resulted in the lowest concentrations of Zn, Al, Fe, and Mn than all other treatments. Fertilizer-only treatment had the highest Al and lowest B concentration than all other treatments. Cover crop treatment had much smaller effects on soil properties, but soil from the winter rye cover crop showed higher K and Mg concentrations than the mixed-species CC, as well as higher Zn and Mn concentrations.
Soil microbial properties, as determined by CO2 respiration (Solvita test), were not affected by cover crop in either the spring or fall samples, but there were significant amendment effects, with both the compost and biocontrol treatments resulting in higher microbial respiration than the fertilizer treatment in both the assessments (Table 4). However, there was no amendment effect on labile amino-N in samples from Fall 2021, as detected in the SLAN test (Table 4).

3.3. Treatment Effects on Crop Growth and Yield

For emergence, once again there was no significant interaction between cover crop and amendment factors (p = 0.07 to 0.99). Crop emergence of beans was somewhat affected by cover crop, with slight increases in emergence observed with the mixed species cover crop each year, and the average emergence for over all three years showing significantly higher emergence than the rye CC for both the early and late emergence assessments (Table 5). However, there was no effect of cover crop on emergence of zucchini, either in any single year or the overall average over all three years. Soil amendment influenced early emergence for both bean and zucchini, with the fertilizer-only treatment resulting in lower emergence than all other treatments. However, this effect was not observed in the later (21 DAP) assessment, which showed no effect of soil amendment on final emergence, and all treatments producing emergence of 85% or greater for both beans and zucchini (Table 5).
Cover crop did not significantly affect total yields of green bean or zucchini in any year, but in 2020, the mixed species CC did result in higher yield of pepper than the rye CC, but only in that year, and not overall (Table 6). There was no significant interaction between cover crop and soil amendment factors in any year or across all years (p = 0.06 to 0.78). Soil amendments significantly affected yield on all crops in all years, with the fertilizer-only treatment associated with the lowest observed yield totals for all crops (Table 6). Compost amendment treatment resulted in a higher yield of green beans than all other treatments in 2019, averaging 21% higher than silica treatment and 34% higher than the biocontrol treatment. However, all three amended treatments produced comparable yield in 2020 and 2021. Averaged over all years, the compost treatment produced higher overall green bean yields than the biocontrol treatment (by 11%), and all amended treatments yielded better than the fertilizer-only treatment, by 30–44% (Figure 3). Other than the lower yields for the fertilizer-only treatment, no other yield differences among treatments were observed for zucchini in any year (Table 6), or when averaged over all three years (Figure 3). The amended treatments resulted in overall zucchini yields of 37–39% higher than the fertilizer-only treatment over all three years. For peppers, the biocontrol treatment showed significantly higher total yield than the silica treatment in 2021 (by 9%), but no yield differences among amended treatments in the other two years (Table 6). Over all three years combined, the biocontrol treatment averaged overall higher pepper yields than the silica treatment (by 8%), and all amended treatments resulted in overall higher pepper yields than the fertilizer-only treatment, by 22–33% (Figure 3).

3.4. Treatment Effects on Plant Leaf Tissue Nutrient Contents

Cover crop treatments did not significantly affect plant leaf tissue nutrient concentrations for any measured element, but soil amendment did result in significant effects for some parameters (Table 7). Leaf N content was similar across all amendment treatments, as well as across all crops, ranging between 4.5 to 5.3%. Silica content was significantly higher in the silica treatment than all other treatments for all crops, averaging increases of 19–37% for green bean and 15–24% for zucchini than all other treatments. Pepper leaves had lower Si contents than either green bean or zucchini by more than an order magnitude, averaging 2.60% for beans, 3.49% for zucchini, and only 0.17% for pepper across all treatments (Table 7). But the silica treatment still resulted in a dramatic increase in the silica content, with increases of 92–107% relative to the other amendment treatments. In green bean leaves, P and K content was lowest in the fertilizer-only treatment, but Ca and Mg content was higher than all other treatments, and Mn was higher in the fertilizer treatment than in the silica treatment. In zucchini leaves, the fertilizer treatment resulted in higher Mg and S content than all other treatments, as well as higher Ca content than the biocontrol treatment and higher Mn than silica and biocontrol treatments, but also lower K than the biocontrol treatment (Table 7). In pepper leaves, the fertilizer treatment resulted in higher P, Ca, S, and Mn contents than all other treatments, but again, had lower K content than all other treatments. The silica treatment also had a lower Mn content than all other treatments.

3.5. Treatment Effects on Pests, Diseases, and Other Issues

Overall, problems with insect pests were minimal throughout the study, but there were some yearly occurrences of minor insect issues. As expected, cucumber beetles and squash bugs were present on zucchini plants each year, but the insect netting protected the plants during the early growth stages and they never became a problem on the more mature plants after removal of the netting. In 2019, several tomato hornworms (Manduca quinquemaculata) were found on the pepper plants in late August. These were removed by hand when observed, but were not sufficiently abundant to cause much damage, and were not associated with any specific treatment. In 2021, a small infestation of Mexican bean beetle (Epilachna varivestis) was observed in 2 plots in late August. These were removed by hand when observed, the infestation only affected a few plants, and did not substantially increase in size through the end of the season.
Crop disease issues were also minor throughout the study. The only consistently recurring foliar disease was powdery mildew (Podosphaera xanthii) on zucchini plants, which was observed late in the season in all three years at similar levels. Soil amendment treatments significantly affected development of the disease, with lower disease incidence of powdery mildew in the silica treatment than in all other treatments (by an average of 50–60%), as well as higher incidence in the fertilizer- only treatment than all other treatments (Figure 4A). Another foliar symptom that was observed in all vegetables each year was a late-season leaf chlorosis or necrosis which occurred in the final two weeks of the season, but was not associated with any specific disease and may indicate senescence due to a variety of factors. This late-season necrosis varied by soil amendment, with higher incidence of necrosis on zucchini plants observed in the fertilizer-only than all other treatments (by 48–57%) (Figure 4B). On both green beans and peppers, the fertilizer-only treatment resulted in higher incidence of late-season necrosis than all other treatments, with each of the other treatments reducing the average incidence relative to the fertilizer-only treatment by 48–65% in beans (Figure 4C) and 84–91% in peppers (Figure 4D). In addition, on green beans, the compost treatment resulted in the lowest overall incidence, lower than the biocontrol and fertilizer treatments. In 2019, one soilborne disease was observed on beans, with some occurrences of white mold (Sclerotinia sclerotium) on bean plant stalks observed late in the season (September). This only affected a few plants, was not observed in any other year, and was not associated with any specific treatment.

4. Discussion

In this research, two different cover crop approaches were compared, the well-established single species winter rye cover vs. a cover crop mixture of 6 species with different characteristics, for their effects on soil and crop properties. In addition, organic soil amendments of compost, compost plus Si, and compost plus biocontrol organisms were compared with a fertilizer-only treatment for their potential benefits to organic vegetable production of three different crops. Overall, despite more biomass production and ground cover provided by the rye cover crop than the mixture, only very minor cover crop effects were observed on soil and crop properties, and no effect on vegetable yield was observed. Cover crop and soil amendment factors showed no significant interaction for any measured parameter. Thus, there was no direct impact or influence of the cover crop type on the effects of the soil amendments in this study. Compost amendments provided improvements in soil properties and crop yield relative to the fertilizer-only treatment, and the additional amendment of silicon increased the Si content in crop leaves and showed some activity in reducing foliar disease (powdery mildew on zucchini). However, the biocontrol amendments, in the absence of substantial disease pressure, did not provide any significant benefit to crop production.
The appeal of multi-species cover crop mixtures is that they have the potential to provide more types of benefits than a single species can, based on the types of different crops included. In other studies, mixtures have shown improvements in soil properties, crop yield, quality of residues, and various other ecosystem services [7,33,34,35]. Whereas some other studies have not shown these types of improvements, or have resulted in lower biomass production or negative effects [9,10,11,36]. Of course, the species included in the mixture is critical, and they must be compatible with one another, as well as suitable for the environment and conditions they will be exposed to. One of the issues with the cover crop mixture used in this study was that, under our conditions in the Northeast, only some of the components established well in the fall or were able to over winter and grow back in the spring. This resulted in only sporadic ground cover with many gaps and holes where the winter-killed crops did not come back. Although this could potentially be improved upon with a selection of better-adapted species, it remains an issue for this region because some of the plant groups needed to be included in the mixture for their contributions to ecosystem services, such as Brassicas, are just not able to over winter in a cold climate. This may be a serious limitation for a diverse mixture of species in this region, and something that remains an issue. The species included in the mixture used here were chosen based on recommendations for this area, consisting of multiple legumes (crimson clover, hairy vetch, and field peas), two grass crops (winter rye, ryegrass), and a Brassica root crop (tillage radish), all of which have been used as cover crops in this region, but not necessarily as part of a mixture. Potentially a better, more refined selection of component species might do better than the mixture used here, but our data thus far, under our conditions, does not support the concept that multi-species cover crops will provide superior performance to that obtained with a good single species cover such as winter rye. After a series of trials in the North-western US and review of the literature, McGuire [37] concluded that considering the higher seed costs and potential additional management needed to get a diverse stand with cover crop mixtures, a monoculture cover crop is best. There was no evidence in our study supporting the use of multi-species cover mixtures for organic vegetable production in this region.
As was observed in a previous study comparing compost amendment with organic fertilizer-only under similar conditions [23], all compost-amended treatments were superior to the fertilizer-only treatment, with the fertilizer treatment resulting in the lowest pH and organic matter content, as well as lower soil fertility and soil biological properties throughout the study, which then also resulted in lower crop yields for all three vegetable crops. However, the soil chemical analyses, and especially the plant leaf assays, indicated that sufficient N and other nutrients were available and being taken up by the plants, as there were no deficiencies noted, or substantial differences among treatments in the leaf analyses (except for K, which was at low levels in the fertilizer-only treatment). In fact, plant leaves from the fertilizer treatment maintained the highest contents of Ca, Mg, S, and Mn of all the treatments. Thus, the compost effect on increasing yield was not based solely on improved mineral nutrition, but additional factors that were improved by compost amendments, most likely related to increased organic matter content and potential changes in soil microbial communities. Compost amendments are known to improve soil structural stability, as well as improvements in bulk density, aeration, porosity, water-holding capacity, water movement, and nutrient cycling [38,39,40]. Compost amendments are also related to increases in microbial biomass and activity, changes in community structure and composition, and enhancements of specific groups of microorganisms [41,42,43,44,45]. Such changes are often associated with increased yields. In previous research, compost amendments affected soil microbial communities and successfully improved yield in both conventional and organic production systems [46,47,48,49,50,51].
The Si and biocontrol amendments were included here to determine whether these additives to the compost amendment could provide any benefits in addition to those provided by the compost alone, particularly as they may relate to insect or disease management. Silicon has been shown to provide reductions in insect feeding and damage, reductions in plant diseases, and tolerance to various abiotic stresses, such as drought and low temperature [16,21,22]. Increased Si content in plant tissue enhances the plant’s physical structure, increasing the mechanical strength and protective layer of the plant, and has been associated with stimulation of plant defense responses and biochemical processes that reduce the impact of abiotic stresses [16,17,18,19,20]. For most parameters, the effects of the silica amendment in this study were not significantly different from compost alone, but silica treatment did increase pH and Ca content of the soil (as Ca was also added in the amendment, as CaO) and reduced powdery mildew on squash. In a previous trial with a similar set-up, silica amendment improved early emergence of beans as well as reduced powdery mildew on squash [23]. The plant leaf tissue analysis verified that the additional Si added to soil was being taken up by the vegetable crops, with increases in Si content ranging from 15–37% in green bean and zucchini leaves. In pepper leaves, which maintained much lower Si content, Si amendment increased Si content by over 100%. The much lower Si content in pepper leaves is consistent with known Si concentrations in different types of plants and is considered a characteristic plant trait [52]. Thus, the treatment was effective in increasing Si in the plants, but the overall impact was minor in this trial. Possibly under greater insect or disease pressure, or other stresses, greater effects might be observed. At this point, the usefulness of silica as an amendment in organic vegetable production has not been reliably demonstrated and remains inconclusive.
Although there were no substantial soilborne disease issues with any of the crops, and foliar diseases were limited to some powdery mildew on squash and some early senescence, there were still some indications of reductions in diseases with the amendments. All of the compost amendments reduced powdery mildew on squash relative to the fertilizer-only treatment, and was associated with less incidence of late season yellowing and browning in all three crops. However, there were no observed effects of the addition of the biocontrol organisms to the compost amendment on any measured parameter. In a previous trial, this same biocontrol mixture treatment resulted in a small increase in yield of organic yellow squash (~7%), as well as a 10% reduction in powdery mildew [29], but no such effects were observed in the present trial. Thus, under these conditions (low disease pressure), the addition of the biocontrol treatment did not provide any additional benefits.

5. Conclusions

This research assessed the potential benefits of different types of cover crops and selected soil amendments within a diversified organic vegetable production system. A multi-species cover crop mixture resulted in less biomass and ground cover than a single species winter rye cover crop, but overall, the type of cover crop had only minor effects on soil properties and no effect on yield for any of the three vegetable crops. Thus, there was no evidence of any benefits derived from a cover crop mixture vs. a single species cover crop in this system. Soil amendments containing compost provided improvements in soil properties including pH, organic matter, and soil fertility, as well as vegetable yield for all three crops relative to an organic fertilizer-only treatment. Addition of silicon, in the form of Wollastonite, increased soil pH and Ca content, increased Si content of plant leaves, and reduced powdery mildew on squash, but had no significant effect on other soil or crop parameters. Addition of the biocontrol mixture (Trichoderma and Streptomyces spp.) had no effects beyond compost alone on any soil or crop parameter. Thus, the usefulness of multi-species cover crop mixtures and additions of these biocontrol products was not supported for organic vegetable production under the conditions of this trial. Silicon amendments can be used to increase the silica content of crop plants, but how much impact that may have on insects, diseases, and abiotic stresses is yet to be seen. Compost was once again verified as a valuable component to organic production that provides many benefits that fertilizer alone does not. These results further emphasized the importance of compost and additions of organic matter for improving soil quality and supplying the soil properties, nutrition, and microbiology needed for enhanced crop performance and yield in organic vegetable production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy14010171/s1, Table S1. Example ANOVA table for split-block analyses, showing source of variation (Source) and degrees of freedom (df) for all analyses, as well as the mean square (MS), F-statistic value (F), and probability (p) value results for the total yield of green bean, zucchini, and pepper in the 2020 field season.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

Special thanks to Jim Hunt, Ethel Champaco, and Dave Torrey for their technical support and roles in establishing, maintaining, and conducting these trials. Thanks to our summer workers Alex Baron, Nick Baron, Mackenzie Connor, Elizabeth Dee, Sarah DellaRatta, Isabel Passerini, Celeste Ramirez, and David Rondeau for all their help with planting, maintaining, harvesting, and distributing the vegetable crops, and a very special thanks to Christina Gee for all her help and cooperation in continuing to make the field site available for this work.

Conflicts of Interest

The author declares no conflicts of interest.

References

  1. Dorais, M. Organic production of vegetables: State of the art and challenges. Can. J. Soil Sci. 2008, 87, 1055–1061. [Google Scholar] [CrossRef]
  2. Magdoff, F.; Van Es, H. Building Soils for Better Crops, 3rd ed.; Sustainable Agriculture Research and Education: Waldorf, MD, USA, 2009. [Google Scholar]
  3. Sarrantonio, M.; Gallandt, E. The role of cover crops in North American cropping systems. J. Crop Prod. 2003, 8, 53–74. [Google Scholar] [CrossRef]
  4. Lu, Y.C.; Watkins, K.B.; Teasdale, J.R.; Abdul-Baki, A.A. Cover crops in sustainable food production. Food Rev. Int. 2000, 120, 121–157. [Google Scholar] [CrossRef]
  5. Clark, A. Managing Cover Crops Profitably, 3rd ed.; Sustainable Agriculture Network: Beltsville, MD, USA, 2000. [Google Scholar]
  6. Robacer, M.; Canali, S.; Kristenses, H.K.; Bavec, F.; Mlakar, S.G.; Jakop, M.; Bavec, M. Cover crops in organic field vegetable production. Sci. Hortic. 2016, 208, 104–110. [Google Scholar] [CrossRef]
  7. Chu, M.; Jagadamma, S.; Walker, F.R.; Eash, N.S.; Buschermohle, M.J.; Duncan, L.A. Effect of multispecies cover crop mixture on soil properties and crop yield. Agric. Environ. Lett. 2017, 2, 170030. [Google Scholar] [CrossRef]
  8. Chapagain, T.; Lee, E.A.; Raizda, M.N. The potential of multi-species mixtures to diversify cover crop benefits. Sustainability 2020, 12, 2058. [Google Scholar] [CrossRef]
  9. Wortman, S.E.; Francis, C.A.; Bernard, M.L.; Drijber, R.A.; Lindquist, J.L. Optimizing cover crop benefits with diverse mixtures and an alternative termination method. Agron. J. 2012, 104, 1425–1435. [Google Scholar] [CrossRef]
  10. Finney, D.M.; White, C.M.; Kaye, J.P. Biomass production and carbon/nitrogen ratio influences ecosystem services from cover crop mixes. Agron. J. 2016, 108, 39–52. [Google Scholar] [CrossRef]
  11. Appelgate, S.R.; Lenssen, A.W.; Wiedenhoeft, M.H.; Kaspar, T.C. Cover crop options and mixes for upper Midwest corn-soybean systems. Agron. J. 2017, 109, 968–984. [Google Scholar] [CrossRef]
  12. Larkin, R.P. Soil health paradigms and implications for disease management. Annu. Rev. Phytopathol. 2015, 53, 199–221. [Google Scholar] [CrossRef]
  13. Perez-Piqueres, A.; Edel-Hermann, V.; Alabouvette, V.; Steinberg, C. Response of soil microbial communities to compost amendments. Soil Biol. Biochem. 2006, 38, 460–470. [Google Scholar] [CrossRef]
  14. Saison, C.; Degrange, V.; Oliver, R.; Millard, P.; Commeaux, C.; Montange, D.; Le Roux, X. Alteration and resilience of the soil microbial community following compost amendment: Effects of compost level and compost-borne microbial community. Environ. Microbiol. 2006, 8, 247–257. [Google Scholar] [CrossRef]
  15. Bonilla, N.; Cazorla, F.M.; Martinez-Alonso, M.; Hermoso, J.M.; Gonzalez-Fernandez, J.; Gaju, N.; Landa, B.B.; de Vicente, A. Organic amendments and land management affect bacterial community composition, diversity and biomass in avocado crop soils. Plant Soil 2012, 357, 215–226. [Google Scholar] [CrossRef]
  16. Tubana, B.S.; Babu, T.; Datnoff, L.E. A review of silicon in soils and plants and its role in US agriculture: History and future perspectives. Soil Sci. 2016, 181, 393–411. [Google Scholar] [CrossRef]
  17. Debona, D.; Rodrigues, F.A.; Datnoff, L.E. Silicon’s role in abiotic and biotic stresses. Annu. Rev. Phytopath. 2017, 55, 85–107. [Google Scholar] [CrossRef] [PubMed]
  18. Bakhat, H.F.; Bibi, N.; Zia, Z.; Abbas, S.; Hammad, H.M.; Fahad, S.; Ashraf, M.R.; Shah, G.M.; Rabbani, F.; Saeed, S. Silicon mitigates biotic stresses in plants: A review. Crop Prot. 2018, 104, 21–34. [Google Scholar] [CrossRef]
  19. Seal, P.; Das, P.; Biswas, A.K. Versatile potentiality of silicon in mitigation of biotic and abiotic stresses: A review. Am. J. Plant Sci. 2018, 9, 1433–1454. [Google Scholar] [CrossRef]
  20. Kaushik, K.; Saini, D.K. Silicon as a vegetable crops modulator—A review. Plants 2019, 8, 148. [Google Scholar] [CrossRef]
  21. Ma, J.F. Role of silicon in enhancing the resistance of plants to biotic and abiotic stress. Soil Sci. Plant Nutr. 2004, 50, 11–18. [Google Scholar] [CrossRef]
  22. Fauteux, F.; Remus-Borel, W.; Menzies, J.G.; Belanger, R.R. Silicon and plant disease resistance against plant pathogenic fungi. FEMS Microbiol. Lett. 2005, 249, 1–6. [Google Scholar] [CrossRef]
  23. Larkin, R.P. Effects of selected soil amendments and mulch type on soil properties and productivity in organic vegetable production. Agronomy 2020, 10, 795. [Google Scholar] [CrossRef]
  24. Sabaratnam, S.; Traquair, J. Formulation of a Streptomyces biocontrol agent for the suppression of Rhizoctoinia damping-off in tomato transplants. Biol. Control 2002, 23, 245–253. [Google Scholar] [CrossRef]
  25. Bonaldi, M.; Kunova, A.; Sardi, P.; Cortesi, P.; Saracchi, M. Streptomycetes as possible biocontrol agents against soilborne pathogens. J. Plant Pathol. 2011, 93, S4.26–S4.27. [Google Scholar]
  26. Sharon, E.; Chet, I.; Spiegel, Y. Trichoderma as a biological control agent. In Biological Control of Plant-Parasitic Nematodes: Progress in Biological Control, 11th ed.; Davies, K., Spiegel, Y., Eds.; Springer: Dordrecht, The Netherlands, 2011. [Google Scholar] [CrossRef]
  27. Sharma, P.; Prashant, K.; Gothalwal, R. Trichoderma: A potent fungus as biological control agent. In Agro-Environmental Sustainability: Volume 1: Managing Crop Health; Singh, J., Seneviratne, G., Eds.; Springer International Publishing: Cham, Switzerland, 2017; pp. 113–125. [Google Scholar] [CrossRef]
  28. Vurukonda, S.S.K.P.; Giovnardi, D.; Stefani, E. Plant growth promoting and biocontrol activity of Streptomyces spp. as endophytes. Int. J. Mol. Sci. 2018, 19, 952. [Google Scholar] [CrossRef]
  29. Larkin, R.P. Effects of cover crops, rotation, and biological control products on soil properties and productivity in organic vegetable production in the Northeastern US. Org. Agric. 2019, 10, 171–186. [Google Scholar] [CrossRef]
  30. Gianello, C.; Bremner, J.M. Comparison of chemical methods of assessing potentially available organic nitrogen in soil. Commun. Soil Sci. Plant Anal. 1986, 17, 215–236. [Google Scholar] [CrossRef]
  31. Helmke, P.A.; Sparks, D.L. Lithium, sodium, potassium, rubidium, and cesium. In Methods of Soil Analysis Part 3; Chemical Methods SSSA Book Series No. 5; Sparks, D.L., Ed.; SSSA and ASA: Madison, WI, USA, 1996; pp. 551–574. [Google Scholar]
  32. Brinton, W. Proficiency of Solvita CO2-burst and Solvita SLAN in soil testing laboratories. J. Woods End Lab. 2015, 1, 1–9. [Google Scholar]
  33. Snapp, S.S.; Swinton, S.M.; Labarta, R.; Mutch, D.; Black, J.R.; Leep, R.; Nyiraneza, J.; O’Neil, K. Evaluating cover crops for benefits, costs, and performance within cropping system niches. Agron. J. 2005, 97, 322–332. [Google Scholar] [CrossRef]
  34. Couëdel, A.; Alletto, L.; Tribouillois, H.; Justes, É. Cover crop crucifer-legume mixtures provide effective nitrate catch crop and nitrogen green manure ecosystem services. Agric. Ecosys. Environ. 2018, 254, 50–59. [Google Scholar] [CrossRef]
  35. Mallinger, R.E.; Franco, J.G.; Prischmann-Voldseth, D.A.; Prasifka, J.R. Annual cover crops for managed and wild bees: Optimal plant mixtures depend on pollinator enhancement goals. Agric. Ecosyst. Environ. 2018, 273, 107–116. [Google Scholar] [CrossRef]
  36. Smith, R.G.; Atwood, L.W.; Warren, N.D. Increased productivity of a cover crop mixture is not associated with enhanced agroecosystem services. PLoS ONE 2014, 9, e97351. [Google Scholar] [CrossRef] [PubMed]
  37. McGuire, A. Cover Crop Monoculture vs. Mixture Update. Available online: https://csanr.wsu.edu/2018-cover-crop-update/ (accessed on 19 September 2023).
  38. Abiven, S.; Menasseri, S.; Chenu, C. The effects of organic inputs over time on soil aggregate stability. Soil Biol. Biochem. 2008, 41, 1–12. [Google Scholar] [CrossRef]
  39. Albaich, R.; Canet, R.; Pomares, F.; Ingelmo, F. Organic matter components and aggregate stability after application of different amendments to a horticultural soil. Bioresour. Technol. 2001, 76, 125–129. [Google Scholar] [CrossRef] [PubMed]
  40. Diacono, M.; Montemurro, F. Long-term effects of organic amendments on soil fertility. A review. Agron. Sustain. Dev. 2010, 30, 401–422. [Google Scholar] [CrossRef]
  41. D’Hose, T.; Cougnon, M.; De Vliegher, A.; Vandecasteele, B.; Viaene, N. The positive relationship between soil quality and crop production: A case study on the effect of farm compost application. Appl. Soil Ecol. 2014, 75, 189–198. [Google Scholar] [CrossRef]
  42. Ros, M.; Klammer, S.; Knapp, B.; Aichberger, K.; Insam, H. Long-term effects of compost amendment of soil on functional and structural diversity and microbial activity. Soil Use Manag. 2006, 22, 209–218. [Google Scholar] [CrossRef]
  43. Ros, M.; Pascual, J.A.; Garcia, C.; Hernandez, M.T.; Insam, H. Hydrolase activities, microbial biomass and bacterial community in a soil after long-term amendment with different composts. Soil Biol. Biochem. 2006, 38, 3443–3452. [Google Scholar] [CrossRef]
  44. Stark, C.; Condron, L.M.; Stewart, A.; Di, H.J.; O’Callaghan, M. Influence of organic and mineral amendments on microbial soil properties and processes. Appl. Soil Ecol. 2007, 35, 79–93. [Google Scholar] [CrossRef]
  45. Termorshuizen, A.J.; Moolenaar, S.W.; Veeken, A.H.M.; Blok, W.J. The value of compost. Rev. Environ. Sci. Biotechnol. 2004, 3, 343–347. [Google Scholar] [CrossRef]
  46. Delate, K.; Freidrich, H.; Lawson, V. Organic pepper production systems using compost and cover crops. Biol. Agric. Hortic. 2003, 21, 131–150. [Google Scholar] [CrossRef]
  47. Delate, K.; Cambardella, C.; McKern, A. Effects of organic fertilization and cover crops on an organic pepper system. HortTechnology 2008, 18, 215–226. [Google Scholar] [CrossRef]
  48. Nair, A.; Ngouajio, M. Integrating row cover and soil amendments for organic cucumber production: Implications on crop growth, yield, and microclimate. Hortic. Sci. 2010, 45, 566–574. [Google Scholar]
  49. Nair, A.; Ngouajio, M. Soil microbial biomass, functional microbial diversity, and nematode community structure as affected by cover crops and compost in an organic vegetable production system. Appl. Soil Ecol. 2012, 58, 45–55. [Google Scholar] [CrossRef]
  50. Bernard, E.; Larkin, R.P.; Tavantzis, S.M.; Erich, M.S.; Alyokhin, A.; Sewell, G.; Lannan, A.; Gross, S. Compost, rapeseed rotation, and biocontrol agents significantly impact soil microbial communities in organic and conventional potato production systems. Appl. Soil Ecol. 2012, 52, 29–41. [Google Scholar] [CrossRef]
  51. Bernard, E.; Larkin, R.P.; Tavantzis, S.M.; Erich, M.S.; Alyokhin, A.; Gross, S.D. Rapeseed rotation, compost and biocontrol amendments reduce soilborne diseases and increase yield in organic and conventional potato production systems. Plant Soil 2014, 374, 611–627. [Google Scholar] [CrossRef]
  52. Katz, O. Silicon content is a functional plant trait: Implications in a changing world. Flora 2019, 254, 88–94. [Google Scholar] [CrossRef]
Agronomy 14 00171 g001
Figure 1. Diagram of the field set-up and plot arrangement showing two of the four replicate blocks, depicting the three different vegetable crops (two rows per plot), the cover crop (main plot) and soil amendment (subplot) treatments. Crops were rotated each year but treatments within plots remained the same. Rye = winter rye cover, Mix = multi-species cover crop mixture.
Figure 1. Diagram of the field set-up and plot arrangement showing two of the four replicate blocks, depicting the three different vegetable crops (two rows per plot), the cover crop (main plot) and soil amendment (subplot) treatments. Crops were rotated each year but treatments within plots remained the same. Rye = winter rye cover, Mix = multi-species cover crop mixture.
Agronomy 14 00171 g001
Agronomy 14 00171 g002
Figure 2. Daily rainfall totals and daily average soil moisture (soil matric potential) readings throughout the (A) 2020 and (B) 2021 growing season at organic vegetable field site in St. Albans, ME, USA, as measured following a winter rye cover crop either within treatment plots (IN-Rye) or outside treatment plots (OUT-Rye) and following a mixed species cover crop either within treatment plots (IN-Mix) or outside treatment plots (OUT-Mix). Within treatment plots represents under the fabric mulch and soil treatments whereas outside the treatment plots represents bare soil conditions.
Figure 2. Daily rainfall totals and daily average soil moisture (soil matric potential) readings throughout the (A) 2020 and (B) 2021 growing season at organic vegetable field site in St. Albans, ME, USA, as measured following a winter rye cover crop either within treatment plots (IN-Rye) or outside treatment plots (OUT-Rye) and following a mixed species cover crop either within treatment plots (IN-Mix) or outside treatment plots (OUT-Mix). Within treatment plots represents under the fabric mulch and soil treatments whereas outside the treatment plots represents bare soil conditions.
Agronomy 14 00171 g002
Agronomy 14 00171 g003
Figure 3. Average total yield of (A) green snap beans, (B) zucchini squash, and (C) sweet peppers over three consecutive growing seasons (combined data for 2019–2021) and as affected by different soil amendments. Bars topped by the same letter for each vegetable are not significantly different based on Fisher’s protected LSD test (p < 0.05). Compost = composted dairy manure; Silica = compost plus silicon (Si) as Wollastonite; Biocontrol = compost plus the commercial biocontrol organisms Trichoderma virens and Streptomyces lydicus; Fertilizer = commercial organic fertilizer-only treatment (no compost).
Figure 3. Average total yield of (A) green snap beans, (B) zucchini squash, and (C) sweet peppers over three consecutive growing seasons (combined data for 2019–2021) and as affected by different soil amendments. Bars topped by the same letter for each vegetable are not significantly different based on Fisher’s protected LSD test (p < 0.05). Compost = composted dairy manure; Silica = compost plus silicon (Si) as Wollastonite; Biocontrol = compost plus the commercial biocontrol organisms Trichoderma virens and Streptomyces lydicus; Fertilizer = commercial organic fertilizer-only treatment (no compost).
Agronomy 14 00171 g003
Agronomy 14 00171 g004
Figure 4. Effect of soil amendments on incidence of (A) powdery mildew and (B) late season yellowing and browning on zucchini squash plants, and late season browning on (C) green bean plants and (D) sweet pepper plants, as averaged over three cropping seasons (2019–2021). Compost = composted dairy manure; Silica = compost plus silicon (Si) as Wollastonite; Biocontrol = compost plus the commercial biocontrol organisms Trichoderma virens and Streptomyces lydicus; Fertilizer = commercial organic fertilizer-only treatment (no compost). Bars topped by the same letter for each parameter are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05).
Figure 4. Effect of soil amendments on incidence of (A) powdery mildew and (B) late season yellowing and browning on zucchini squash plants, and late season browning on (C) green bean plants and (D) sweet pepper plants, as averaged over three cropping seasons (2019–2021). Compost = composted dairy manure; Silica = compost plus silicon (Si) as Wollastonite; Biocontrol = compost plus the commercial biocontrol organisms Trichoderma virens and Streptomyces lydicus; Fertilizer = commercial organic fertilizer-only treatment (no compost). Bars topped by the same letter for each parameter are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05).
Agronomy 14 00171 g004
Table 1. Average daily temperature and total rainfall for the months of May through September at the St. Albans, ME research site for 2019, 2020, and 2021 compared with long-term (30-year) average (LTA) conditions.
Table 1. Average daily temperature and total rainfall for the months of May through September at the St. Albans, ME research site for 2019, 2020, and 2021 compared with long-term (30-year) average (LTA) conditions.
Average Daily Temperature (°C)Rainfall (cm)
Treatment201920202021LTA201920202021LTA
May14.711.812.912.011.56.66.59.5
June21.920.019.417.012.35.33.09.9
July18.820.918.419.86.610.414.98.7
August16.619.620.518.916.64.45.48.6
September14.215.715.814.215.75.612.29.6
Season avg17.017.617.416.412.56.58.49.3
Table 2. Ground cover (%) and plant biomass (fresh and dry weights) produced by winter rye (rye) and mixed-species (mix) cover crops (measured in spring prior to planting vegetables) for each year and averaged over all years of the study.
Table 2. Ground cover (%) and plant biomass (fresh and dry weights) produced by winter rye (rye) and mixed-species (mix) cover crops (measured in spring prior to planting vegetables) for each year and averaged over all years of the study.
Cover Crop Treatment Spring Growth
Ground Cover (%)Plant Biomass (Mg/ha)
Fresh wtDry wt
YearRyeMixRyeMixRyeMix
201968.2 a z36.2 b17.6 a11.80 b3.35 a2.25 b
202018.5 a9.6 b2.7 a1.52 b1.04 a0.82 b
202135.6 a28.4 b10.1 a6.65 b1.86 a1.17 b
Avg40.8 a24.7 b7.6 a4.94 b1.81 a1.29 b
z Values within rows followed by the same letter for each parameter are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05).
Table 3. Selected soil chemical properties and nutrient concentrations as affected by cover crop and soil amendments as measured each spring and averaged over three cropping years (2019–2021).
Table 3. Selected soil chemical properties and nutrient concentrations as affected by cover crop and soil amendments as measured each spring and averaged over three cropping years (2019–2021).
OM(mg/kg Soil)
pH(%)NO3NH4PKMgCaNaCEC
Cover crop
Rye6.17 a z5.47 a31.2 a2.96 a155.5 a236.9 a217.7 a4894 a23.0 a9.8 a
Mixed 6.18 a5.41 a29.1 a2.71 a178.0 a209.8 b193.1 b4923 a23.1 a10.0 a
LSD0.080.293.20.3043.423.814.93931.30.4
Amendment
Silica6.58 a5.45 a33.2 a2.4 c193.9 a248.4 a222.8 a5727 a23.1 ab10.4 a
Compost6.10 b5.47 a30.3 ab2.7 bc171.1 a245.6 a224.9 a4675 b23.5 a10.1 a
Bicontrol6.09 b5.56 a30.5 ab2.9 ab178.3 a252.0 a226.0 a4842 b24.1 a10.1 a
Fertilizer5.94 c5.09 b26.3 b3.3 a127.1 a143.4 b135.1 b4393 b21.4 b8.9 b
LSD0.120.404.50.461.333.621.55551.90.6
z Values within columns followed by the same letter for each parameter are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05). Interaction between cover crop and soil amendment factors was not significant for any property (p > 0.05).
Table 4. Soil microbiological properties as affected by soil amendment, and as measured by CO2 respiration and soil Amino-N (Solvita CO2 burst test and SLAN test).
Table 4. Soil microbiological properties as affected by soil amendment, and as measured by CO2 respiration and soil Amino-N (Solvita CO2 burst test and SLAN test).
AmendmemtSpringFall
CO2 Resp. CO2 Resp. Amino-N
(mg/kg Soil)(mg/kg Soil)(mg/kg)
Silica105.8 ab z106.8 ab134.4 a
Compost117.3 a114.4 a133.2 a
Biocontrol120.8 a109.7 a136.4 a
Fertilizer93.3 b93.9 b136.9 a
LSD (p = 0.05)16.613.614.2
z Values within columns followed by the same letter for each parameter are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05). Interaction between cover crop and soil amendment factors was not significant for any parameter (p > 0.05).
Table 5. Effect of cover crop and soil amendments on early (12 DAP) and late (21 DAP) seedling emergence for green bean and zucchini crops averaged over all three seasons of the study (2019–2022).
Table 5. Effect of cover crop and soil amendments on early (12 DAP) and late (21 DAP) seedling emergence for green bean and zucchini crops averaged over all three seasons of the study (2019–2022).
Emergence (%)
Green BeanZucchini
TreatmentEarlyLate EarlyLate
(12 DAP)(21 DAP)(12 DAP)(21 DAP)
Cover crop
Rye73.4 b z84.7 b82.3 a93.8 a
Mix78.7 a89.7 a84.6 a93.4 a
LSD (p = 0.05)4.54.23.82.2
Amendment
Silica76.8 a87.9 a85.2 a93.0 a
Compost77.6 a88.2 a83.1 ab94.2 a
Biocontrol79.5 a88.4 a87.1 a94.4 a
Fertilizer70.5 b85.0 a78.3 b92.9 a
LSD (p = 0.05)6.36.05.33.1
z Values within columns followed by the same letter for each factor are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05). Interaction between cover crop and soil amendment factors was not significant for any crop or year (p > 0.05).
Table 6. Total vegetable yield of green bean, zucchini, and peppers as affected by cover crop and soil amendments in the 2019, 2020, and 2021 cropping seasons.
Table 6. Total vegetable yield of green bean, zucchini, and peppers as affected by cover crop and soil amendments in the 2019, 2020, and 2021 cropping seasons.
Yield (Mg/ha)
Green BeanZucchiniPepper
201920202021201920202021201920202021
Cover crop
Rye16.9 a z18.7 a17.9 a36.6 a60.8 a74.0 a13.8 a17.1 b20.1 a
Mixed 16.5 a18.4 a18.9 a39.7 a57.7 a75.6 a13.3 a19.7 a20.8 a
LSD1.80.91.56.95.14.21.61.21.4
Amendment
Silica17.4 b19.6 a20.5 a44.1 a66.4 a77.0 a13.3 ab19.1 a20.7 b
Compost 21.2 a19.9 a19.4 a41.6 a63.7 a80.9 a14.3 a19.1 a21.8 ab
Bicontrol15.8 b19.2 a19.8 a43.4 a61.2 a80.7 a15.3 a19.8 a22.8 a
Fertilizer12.4 c15.4 b14.0 b29.4 b45.5 b60.5 b11.4 b15.6 b16.5 c
LSD2.51.22.14.87.25.92.31.72.0
z Values within columns followed by the same letter for each parameter and year are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05). Interaction between cover crop and soil amendment factors was not significant for any crop or year (p > 0.05).
Table 7. Plant leaf tissue nutrient concentrations as affected by soil amendments in green bean, zucchini, and pepper vegetable crops as measured during vegetable production in 2020 season.
Table 7. Plant leaf tissue nutrient concentrations as affected by soil amendments in green bean, zucchini, and pepper vegetable crops as measured during vegetable production in 2020 season.
CropNutrient Content (%)
AmendmentNPKCaMgSSiMn (ppm)
Green Bean
Silica5.1 a z0.65 ab2.50 a2.09 b0.40 b0.33 a3.17 a49.0 b
Compost4.7 a0.65 ab2.40 a2.00 b0.41 b0.35 a2.66 b55.0 ab
Biocontrol5.0 a0.67 a2.50 a2.15 b0.42 b0.34 a2.37 b56.8 ab
Fertilizer5.3 a0.58 b1.99 b2.54 a0.46 a0.33 a2.32 b60.8 a
LSD0.70.070.330.230.020.030.578.9
Zucchini
Silica4.7 a0.59 a2.17 ab5.13 ab0.56 b0.44 b3.96 a59.1 b
Compost4.7 a0.57 a2.07 ab4.85 ab 0.54 b 0.46 b 3.20 b72.9 ab
Biocontrol4.9 a0.62 a2.34 a4.70 b0.56 b0.48 b3.43 b71.4 b
Fertilizer4.7 a0.58 a1.93 b5.76 a0.75 a0.59 a3.37 b87.0 a
LSD0.50.090.300.950.130.050.4714.3
Pepper
Silica4.8 a0.34 b2.99 a5.37 b0.73 a0.59 b0.27 a65.4 c
Compost4.8 a0.33 b3.18 a5.27 b0.73 a0.60 b0.13 b90.3 b
Biocontrol4.5 a0.35 b2.83 a5.57 b0.71 a0.59 b0.14 b73.1 bc
Fertilizer4.7 a0.40 a1.61 b6.67 a0.81 a0.67 a0.14 b115.9 a
LSD0.50.040.360.580.130.030.0722.1
z Values within columns followed by the same letter for each vegetable are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05).
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Larkin, R.P. Effects of Different Cover Crops and Amendments on Soil and Crop Properties in Organic Vegetable Production. Agronomy 2024, 14, 171. https://doi.org/10.3390/agronomy14010171

AMA Style

Larkin RP. Effects of Different Cover Crops and Amendments on Soil and Crop Properties in Organic Vegetable Production. Agronomy. 2024; 14(1):171. https://doi.org/10.3390/agronomy14010171

Chicago/Turabian Style

Larkin, Robert P. 2024. "Effects of Different Cover Crops and Amendments on Soil and Crop Properties in Organic Vegetable Production" Agronomy 14, no. 1: 171. https://doi.org/10.3390/agronomy14010171

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop