1. Introduction
Collard greens (
Brassica oleracea var. acephala) are among the vegetables that make a favorite dish at restaurants and attractive isles in food sections of major stores across the southern USA, South America, and China. People prefer it as a part of healthy diets, usually comprising fibrous leafy vegetables rich in vitamins and low in calories [
1,
2]. There are intense demands for fresh and processed collard greens.
Production of collard greens requires nutrient-rich soil. Efficient nutrient management is essential for optimum crop yield; it accounts for nutrient dynamics below crops’ rootzone. A lack of awareness of the dangers in-built in the mere use of fertilizer alone for enhancing crop productivity deteriorates soil health and the environment [
3]. For healthy soils and a clean environment, the use of synthetic fertilizers can be reduced by using organic sources of plant nutrients without compromising crop production [
4].
Applying fertilizers to soils enhances their health and provides crops with nutrients needed for their development and growth [
5]. Organic agriculture uses organic manures as the only source of plant nutrients. Poultry manure is a rich source of major macronutrients [
6]. It also contains high concentrations of micronutrients, including Cu and Zn [
7], apparently derived from growth promoters in poultry feed [
8]. Dairy manure, poultry manure, and Milorganite (a biosolid) are sources of necessary crop macro- and micronutrients [
9,
10,
11,
12]. Cattle manure is a good source of nutrients, such as N, P, K, S, and Mg, coming from livestock diets and other trace elements. Reported values of nutrients found in cattle manure include 7.89 g kg
−1 for P, 38.45 g kg
−1 for K [
13], and 2–8.1 g kg
−1 for N [
14].
Milorganite™ (MILwaukee ORGAnic NITtrogEn), introduced by the Milwaukee Metropolitan Sewerage District (Milwaukee, WI), is a Class A biosolid [
15] that meets the US EPA standards for arsenic, cadmium, Cu, lead, mercury, molybdenum, nickel, selenium, and Zn limits [
16]. Having an N–P–K ratio of 6–2–0 and low solubility compared to commercial fertilizers, Milorganite
TM is distributed without restriction and labeling their origin or pollutant content [
17]. It has been reported to be significantly more phytoavailable than triple superphosphate (TSP) in Candler Florida soil [
18] and for its property of least cumulative P release than biological P removal (BPR) and other BPR-like biosolids [
19].
The mobility of nutrients in the vadose zone is a function of the rate and time of application of manure and/or biosolids applications. Excessive application of chicken manure (CM) can result in nitrate (NO
3-N) abundance in the crop rootzone. Once leached below the rootzone, NO
3-N is beyond plant uptake and eventually ends up in groundwater aquifers [
20]. Phosphorous is readily available for plant uptake in most soils. Most of the added P tends to be attracted to soil particle charges resulting in its minimum leaching down the soil profile.
Furthermore, P is cycled from roots to aboveground parts of the plant and is redeposited in crop residues on the soil surface. Therefore, most agricultural fields need lower quantities of P than N that may be supplied with slow-releasing soil amendments. Applying manure or biosolids to meet plant N requirements often results in the excessive application of P [
16]. Excessive P accumulation is not harmful to crops but could lead to several environmental problems, especially in poor P-sorbing soils [
19]. Once applied to agricultural fields, P has a tendency of attachment with soil particles [
21]. Potassium, as a macronutrient, is a fundamental element for plant growth, which markedly affects biomass accumulation and biomass partitioning [
22]. Showing an elusive behavior, K exchange in heterogeneous systems has been reported to be in equilibrium rather than a dynamic nature [
23]. Other macro- and micronutrients have been reported to have miscellaneous trends in their dynamics depending upon their source of origin and the type of soils they were considered in [
16,
24,
25] resulting in good crop growth but at some times soil and environmental degradations.
Organic agriculture supports environmental stewardship. Commercial-scale recycling of agricultural and municipal wastes into organic soil amendments facilitates safe disposal of waste, improves soil health, and reduces environmental degradation [
26]. Tanha [
27] conducted a field experiment to examine the changes in pedostructure-based soil characterization under different soil management methods involving three organic treatments (chicken, dairy manure, and Milorganite) applied at three different rates and found that the dairy manure had improved soil aggregate structure better than chicken manure and Milorganite. Organic amendment types and rates influence the rhizosphere and phyllosphere microbiota profiles of collard greens [
28]. Kebrom et al. [
29] evaluated phytotoxicity of chicken manure, Milorganite, and dairy manure to collard greens and identified phytotoxic effects of chicken manure and Milorganite, but not dairy manure, to collard greens. Particularly, nickel in chicken manure and Milorganite aqueous extracts was 28-fold and 21-fold, respectively, higher than previously reported toxic levels to wheat seedlings. Kebrom et al. [
26] identified phytotoxic levels of copper and nickel in commercial organic soil amendments recycled from poultry farms and municipal wastes and found that commercial chicken manure and Milorganite (recycled from municipal waste) were more phytotoxic than poultry manure. Application of exogenous N fertilizers (poultry or chicken manure and/or biosolids such as Milorganite) provides agronomic benefits but carries environmental liabilities [
30]. For example, higher cumulative CO
2 emissions for the soils amended with chicken manure and Milorganite but the lowest for the soils amended with dairy manure were reported by Ray et al. [
31]. Organic agriculture faces challenges of balancing the use of organic manures either in the form of dry manure or in the shape of manure slurry to fulfill crop nutrient requirements on efficient basis.
Application of exogenous organic matter, sourced from natural fertilizers including dry organic manures and/or slurry, to soil enriches it with micro- and macro-elements necessary for the growth and yield of crops [
32]. However, managing benefits and liabilities of N-based fertilizers in conventional and organic cropping systems (managed by the application of only organic manures or organic manures mixed with synthetic fertilizers) might be improved with better knowledge of nutrient dynamics, the generation of intrinsic N, and maintenance of soil organic matter [
31]. The knowledge about nutrient dynamics in the soil is pivotal for sustainable agriculture and a safe environment.
Effective nutrient management can address issues of sustainable agriculture and environmental stewardship as water, soil, and nutrient management are highly interconnected [
33]. Thus, the best agricultural management practices help developing solutions to the broader challenges that restrict farm profitability [
34]. Effective nutrient management facilitates farmer learning about nutrient dynamics within and below rootzone as well as augments the learning from the soil and nutrient monitoring [
35]. Innovative agricultural management practices have been discussed in detail elsewhere [
36,
37]. Good agricultural management practices include practices to manage soil, water, and nutrients for ideal soil health and productivity.
Organic soil amendments provide crops with their nutrient needs and maintain soil health by modifying their physical, chemical, and biological properties [
29,
38]. Small-scale vegetable and fruit crop growers are transitioning from conventional farming to environmentally friendly and more profitable semi-organic or organic farming [
39]; therefore, they are shifting from using conventional inorganic fertilizers to organic fertilizers and soil amendments prepared from municipal, industrial, and farm wastes [
40]. Agronomic management through better use of inputs benefits farmers both by enhancing productivity and profitability [
41].
An extensive literature review did not reveal any detailed study on the effect of organic amendment types and rates on dynamics of macro- and micronutrients within and below the rootzone of collard greens under southeast Texas edaphic conditions. It was hypothesized the dynamics of macro- and micronutrients within and below the crop rootzone would vary with organic amendment types and rates. There is a need to quantify the effects of different organic amendment types and application rates on nutrient availability and fate within and below the rootzone during such conditions. Therefore, this study’s objective was to quantify the effect of organic amendment types and rates on dynamics of macro- and micronutrients within and below the rootzone of collard greens under southeast Texas edaphic conditions.
4. Conclusions
This study was conducted to understand the complex dynamics of nutrients sourced from different types and rates of soil amendment applications in the collard greens grown on sandy loam soil in Prairie View, TX, USA. There were significant and highly significant effects of amendment types and their application rates on the dynamics of the studied macro- and micronutrients within and below the rootzone. Most of the macronutrients, except Na, remained within the rootzone with some movement below the rootzone. However, the releases of these nutrients varied with amendment types; this might be due to differences in manures’ mineralization rates. Chicken manure had significantly higher B, Cu, and Fe releases than the other amendments as evident from the higher concentrations of its micronutrients within the rootzone of this treatment. These concentrations were significantly higher than those in the rootzone of dairy manure and Milorganite TM treatments. Chicken manure released more TN (presumably the combination of organic N and mineralized contents of N), P, K, Na, Ca, Mg, B, Cu, and Fe than the other two amendments. However, dairy manure treatments had the lowest concentrations of TN (supposedly comprising the organized and mineralized contents of N), Ca, and Mg; meanwhile, Milorganite TM had the lowest presence of P, K, Na, B, and Cu in their leachates.
Higher organic amendment application rates can substantially increase nutrient leaching, especially under excess rainfall and irrigation events, which further increase nutrient leaching from agricultural fields, especially sandy and sandy loam soils of the experimental field used for this study. The dynamics of nutrients depend on their nature, source, and the amendments application rates necessitating the need to monitor each nutrient’s level in the plant material and the environment. An applied outcome of studying nutrient dynamics in the rhizosphere and below the rootzone of a crop is the ability to incorporate concepts of the best nutrient management for progressive plant growth and environmental stewardship. Selection of the best nutrient source among organic manures and the biosolids that ensure a nutrient-rich rhizosphere through the controlled release of nutrient contents from the applied soil amendments is key to profitable farming and environment-friendly sustainable agriculture.