1. Introduction
Sandy soils are characterized by low nutrient content and water holding capacity, which leads to the frequent application of nutrients and water to meet crop requirements and improve soil quality [
1,
2,
3,
4,
5]. Virgin sandy soils, which were not used for cultivation before [
6], are found in nearly all regions of Egypt, from a few hundred square kilometers to more than a hundred thousand kilometers, covering most of Egypt’s total area [
7,
8,
9,
10,
11].
Soil can generally be enriched with nutrients by adding them in solid form or dissolved in water, and the plant can also be nourished directly with nutrients through foliar application. Historically, soil application is the most common fertilization practice, but it depends on several factors including soil and plant characteristics and the physiological state, as well as weather conditions [
12]. Generally, the availability of most nutrients is limited in soils with higher calcium carbonate content [
13], which are common throughout Egypt. However, foliar application improves the uptake and the efficiency of these nutrients [
14,
15]. Hence, to alleviate micro- and macronutrient deficiencies in sandy soils, foliar fertilization is increasingly adopted [
16]. Foliar fertilization results in rapid nutrient absorption and avoidance of several environmental factors such as antagonism, leaching, and deposition of elements [
17]. Hence, it can avoid some of the problems of challenges associated with soil application of nutrients in sandy soils. [
17,
18]. The foliar application does not substitute for soil application, but supplements it [
19,
20]. However, fertilization of major nutrients, especially NPK, is more effective via soil application, whereas secondary nutrients, e.g., calcium, magnesium, and sulfur, as well as micronutrients, e.g., zinc, iron, manganese, copper, molybdenum, and boron, proved to be more effective via foliar application [
19].
Chemical fertilizers are essential for plant nutrition, but they can cause environmental pollution [
21], particularly nitrogen, which leads to an increase of NO
3− ions in the soil [
20]; phosphorous may also cause soil contamination with Cd
2+, which is readily absorbed and translocated into different parts of the plant [
22]. As one of the basic organic fertilizers, leguminous compost can be utilized to reduce the quantities of chemical fertilizers applied [
23,
24]. The use of leguminous compost has multiple benefits including sanitation, reduced mass and bulk, and a lower C/N ratio [
21]. Besides, it has the potential to maintain the fertility of soils in agricultural systems [
25,
26]. Legume plants can contribute as much as 50–250 kg/ha of nitrogen [
27,
28], and their litter can be utilized as a highly valuable compost (organic fertilizer). In contrast, traditional organic materials, e.g., crop residues, animal manure, etc., cannot alone improve soil fertility, as they are usually not available in sufficient quantities and require intensive labor [
29].
Continuous use of inorganic fertilizers negatively affects soil fertility as it eventually leads to reduced crop yields. Moreover, its cost is much higher compared with organic fertilizers, which impacts the overall profits of agricultural production. Hence, it is of prime importance to appropriately combine inorganic with organic fertilizers [
23] to overcome the environmental impacts and maximize the overall income. The objective of the present study is to evaluate the effectiveness of chemical and organic fertilizers (leguminous biocompost) in improving the fertility of nutrient-deficient sandy soil and its reflection on the growth and yield of
Phaseolus vulgaris using a foliar and/or soil addition. We hypothesized that the combined application of soil and foliar chemical fertilizers with a bio-organic fertilizer would improve the fertility of the investigated sandy soil along with the growth and production of
Phaseolus vulgaris plants.
4. Discussion
Climate change, as one of the major challenges facing the world at present, has negatively affected agricultural lands. Furthermore, future changes in climatic events are expected to worsen [
58], particularly in arid and semi-arid regions [
59,
60,
61,
62,
63]. As a consequence, soils have lost much of their fertility, threatening the food security of the world [
58,
64,
65,
66,
67,
68,
69,
70]. Accordingly, to maintain sustainable agricultural development, virgin (unused) lands that can be cultivated with more organic and less chemical fertilization, are being, or should be, used to minimize pollutants to produce clean agricultural products free of pollution [
71].
Sandy soils suffer from high permeability coupled with poor water retention, which results in the loss of many important nutrients due to poor ability to retain many important nutrients, besides, the competition of cations, and consequently the rapid leaching [
1,
72,
73,
74]. Besides, if such soil was virgin, its suffering would have been greater. Therefore, it is necessary to improve the weaknesses of this soil by nourishing it with organic fertilizers, especially biocomposts, because they enhance microbial decomposition leading to humification of organic matter [
75] in favor of the soil and plants growing on it.
The virgin sandy soil used in this study had very low water and nutrient holding capacity and a poor cation exchange capacity (CEC), making it impossible to rely on adding nitrogen, phosphorus, and potassium (NPK) fertilizers only, otherwise, these fertilizers are rapidly lost. Therefore, to minimize fertilizer nutrient loss, it is appropriate to add a portion of the recommended NPK fertilizer (about 25%) to the plant as a foliar spray in combination with adding another portion (about 75%) to the soil with the addition of an appropriate amount of organic (S
CF75 + F
CF25 + C
L) or bio-organic fertilizer (S
CF75 + F
CF25 + C
LB) as a required strategy for the sustainability and productivity of plants growing on the tested soil. From this point of view, the S
CF75 + F
CF25 treatment went beyond the control treatment and significantly increased soil physicochemical properties (
Table 2 and
Table 3;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5 and
Figure 6) and
Phaseolus vulgaris plant growth, productivity, and physiobiochemical attributes (
Table 4,
Table 5 and
Table 6;
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12). Enrichment of the S
CF75 + F
CF25 treatment with leguminous compost (C
L) at 1.2 kg per m
2 (S
CF75 + F
CF25 + C
L) increased the treatment efficiency, as it exceeded the S
CF75 + F
CF25 treatment, resulting in a significant increase in the tested soil and plant attributes (
Table 3,
Table 4,
Table 5 and
Table 6;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5,
Figure 6,
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12). Moreover, enrichment of the C
L in the S
CF75 + F
CF25 + C
L treatment with bacteria (
Bacillus subtilis) further increased the treatment efficiency, as the S
CF75 + F
CF25 + C
LB treatment greatly exceeded the S
CF75 + F
CF25 + C
L treatment, resulting in a further significant improvement in soil and plant attributes (
Table 2,
Table 3,
Table 4,
Table 5 and
Table 6;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5,
Figure 6,
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12).
Therefore, from our findings of the tested virgin sandy soil and
Phaseolus vulgaris plant, the S
CF75 + F
CF25 + C
LB treatment was the best, with the best improvement results for the soil texture, pH, EC
e, and CaCO
3 content in both the 2019 and 2020 seasons (
Table 2,
Table 3,
Table 4,
Table 5 and
Table 6;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5,
Figure 6,
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12). These improvements were attributed to the leguminous biocompost (C
LB), which provided soil characteristics, in addition to the NPK nutrients that the plant acquired through two pathways: roots and leaves. In addition, compared to all treatments, including the control, the S
CF75 + F
CF25 + C
LB treatment significantly increased CEC, soil organic carbon (C) content, soil nutrient (N, P, K, Fe, Mn, and Zn) contents, and soil enzymes (e.g., cellulase, invertase, urease, and catalase) activities (
Table 3 and
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5 and
Figure 6).
Soil pH was decreased with soil supplementation using C
LB, which could be attributed to the increase in organic C content and soil enzyme activities (
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5 and
Figure 6), which led to the decomposition of the added organic matter (in C
LB). Besides, organic acids and phytohormones (e.g., indole acetic acid and cytokinins) resulting from the activity of bacteria added to the compost, leading to an increase in biological activity [
33,
76,
77,
78]. As obtained in this study (
Table 2), a decrease in pH is reported with the combined use of biocompost and inorganic fertilizer [
79]. This positive result is attributed to the production of organic acids due to biocompost decomposition followed by an increase in the salt content of the soil due to mineralization, which increases the EC of the soil. Sinha et al. [
79] also reported that soil pH decreased while EC increased, due to biocompost application, which also had a significant influence on soil organic C content and available nutrients (e.g., N, P, K, and S).
Furthermore, supplementing the soil with C
LB enhanced the microbial decomposition that leads to an increase in soil organic matter and thus CEC (
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5 and
Figure 6). Buragohain et al. [
75] found that the biocompost generates a large number of different bacteria that cause an increase in soil organic matter, which leads to improved crop growth. This subsequently results in increased exudations of plant roots and return of post-harvest residues, thus contributing to the organic content of the soil. They also documented that the observed high contribution of biocompost to soil organic C indicates more rebellious forms of soil organic C, thus resulting in enhanced soil C stabilization. Soil organic C elevation is also observed following the application of biofertilizers (
Bacillus megatherium and
Bacillus mucilaginous), as per Wu et al. [
80]. Increase of CEC after application of compost [
21], and in particular biocompost, is an indication of a high accumulation of soil organic C pool along with low inorganic P and N. The decreased inorganic P and N following the application of biocompost contribute to building a soil environment where a desirable microbial composition is created to stabilize the soil organic carbon pool [
75]. They also reported that the quality of organic matter and fertility of the soil is improved due to the increased humic/fulvic acid ratio due to the increased rate of humification, which helps to raise the stability of organic C as a result of biocompost supplementation to the soil [
81].
The buildup of available nutrients in nutrient-deficient soils, such as the soil tested in this study, can be attributed to the increased microbial proliferation due to the addition of organic manures, especially biocompost, which helps in mineralization as well as solubilization of native nutrients by complexation of nutrients by humic and fulvic acid contained in biocompost [
79]. The results of this study indicated that the application of NPK fertilizers only was not effective in maintaining soil fertility (
Table 2 and
Table 3;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5 and
Figure 6). The nutrients available in the soil and soil organic C were preserved in the treatment containing biocompost. It has been reported by Sinha et al. [
79] that soil fertility-contributing bulk density and pore spaces are improved effectively due to the accumulation of organic C content of the biocompost-treated soil. The beneficial influences of biocompost in improving the soil’s physical and chemical properties may be attributed to the improvement in the organic matter status of the soil treated with biocompost, which leads to the buildup of soil fertility for sustainable production of
Phaseolus vulgaris. Sinha et al. [
79] also explained that the use of biocompost greatly increases the soil microbial population, which utilizes the accumulated organic C as a source of energy, nutrients, and nourishment, which leads to the spread of microorganisms in the soil. This improved microbial community and activity due to the accumulation of organic matter by the application of biocompost help maintain soil fertility and productivity due to the faster decomposition rate and smooth mineralization of organic materials [
79].
As shown in the data of this study, the combined use of biocompost and inorganic fertilizers (applied to both soil and plant leaves) effectively increased the uptake of nutrients, which reflected positively on the higher yield (
Table 4 and
Table 5). Managing nutrients through organic and inorganic sources leads to more nutrient uptake. As of the effectively increased parameters due to the combined use of C
LB and inorganic fertilizers, the soil organic C content and availability of nutrients (e.g., N, P, K, Fe, Mn, and Zn) in the tested virgin sandy soil may be attributed to the increased content of organic C and nutrients in the C
LB and the beneficial advantages of inorganic nutrients. These nutrients are released from the compost into the soil through bacterial decomposition [
82] present in C
LB, which is activated by the added compost. Manirakiza and Şeker [
77] reported increased contents of soil nutrients and organic C which could be attributed to the compost’s richness in organic C and various nutrients. The release of nutrients from compost into the soil via the mineralization process can elucidate this result [
77,
83].
Compared to all other treatments, including the control, the S
CF75 + F
CF25 + C
LB treatment significantly increased soil enzymes (e.g., cellulase, invertase, urease, and catalase) activities, especially after adding C
LB, indicating a potentially greater source of beneficial microbes. The increased activity of soil enzymes contributed to the release of more nutrients for microbial use [
75]. Besides, the bacteria present in the C
LB have desirable influences on increasing and stabilizing bacterial populations [
84], which contributed along with the increased soil enzyme activities to repair limitations of the soil tested.
Macci et al. [
85] and Buragohain et al. [
75] demonstrated that compost and biocompost fertilizers increase soil quality due to enhancements of soil properties related to physicochemistry and biochemistry. This positive finding is confirmed by the results obtained for the tested virgin sandy soil, which reflected positively in the growth, productivity, and physiobiochemical indices of
Phaseolus vulgaris plants (
Table 4,
Table 5 and
Table 6;
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12). The increased availability of nutrients to the plants as a result of the improved physicochemical properties of the tested soil under supplementation of biocompost contributed to the increase in growth and physiobiochemical indices, and thus the productivity of
P. vulgaris plants. As reported in [
86], compost enriched with many bacterial strains (e.g.,
Bacillus,
Rhizobium,
Azotobacter,
Azospirillum,
Bradyrhizobium,
Acetobacter, and
Pseudomonas) has been reported to improve yields of different crops in some defective soils. Furthermore, it has been explained by Sinha et al. [
79] that the immediate and rapid supply of nutrients through an inorganic source for plant growth and a steady supply of nutrients by organics, especially biocompost, throughout the growth period leads to an increase in plant yield due to integrated use of organic and inorganic nutrients. The biocompost releases nutrients after decomposition and mineralization that will increase the availability of nutrients at a later stage and improve physical, chemical, and biological properties of soil, resulting in improved soil fertility and absorption of nutrients by the plant. The integrated use of organic nutrient sources with inorganic fertilizers has been shown to increase the potential of organic fertilizers [
79].
The growth and yield trend after two consecutive trial years of
P. vulgaris plants showed that reducing 25% of the recommended NPK fertilizer dose of soil addition for use in foliar spraying along with the application of biocompost resulted in a progressive increase in the growth and yield of
P. vulgaris plants (
Table 4). Thus, the S
CF75 + F
CF25 + C
LB treatment represented the highest sustainability for a period of two seasons (2019 and 2020). Allocating 25% of the NPK fertilizers to foliar spray conferred the opportunity to reduce the inorganic minerals added to the soil along with the use of C
LB to conserve the soil while increasing its fertility (
Table 2 and
Table 3;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5 and
Figure 6). Besides, it increased the efficiency of the growing
P. vulgaris plants in sandy soil to obtain all their nutritional needs, especially in the presence of C
LB that facilitates nutrient uptake. Foliar spraying with NPK nutrients has become a concern of scientists for its dynamic application to increase plant growth and yield [
17,
87] (
Table 2,
Table 3,
Table 4,
Table 5 and
Table 6;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5,
Figure 6,
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12).
As explained in [
88], the increase in plant growth and productivity can be attributed to the positive influences of biocompost and its content of microorganisms in raising root surfaces, root distribution, water-use efficiency, and photosynthetic activity. These positive results directly affect the physiological processes and carbohydrate metabolism due to the high levels of nutrients and organic matter in the applied biocompost. Similarly, our data attributed the increases in growth and productivity of
P. vulgaris plants to the application of C
LB, which contributed to improved phytonutrient contents (
Table 3), photosynthetic efficiency (chlorophyll content, chlorophyll fluorescence, and PSII performance index), and relative cellular water content (
Table 4), as well as the plant’s antioxidant defense system; osmoprotectants (proline and sugars) and both enzymatic and nonenzymatic antioxidants (
Table 6 and
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12).
The physicochemical properties and fertility of the virgin sandy soil, including the soil’s organic C content and organic acids, which were greatly improved by applying C
LB (
Table 2 and
Table 3;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5 and
Figure 6), led to an increase in water retention in the soil [
89], allowing
P. vulgaris plants to absorb more water and nutrients, leading to increased relative cellular water content and different nutrient contents (
Table 4,
Table 5 and
Table 6). Relative cellular water content indicates the water status in plants, reflecting the activity of plant metabolic processes, and is utilized as an indicator to distinguish between legumes with contrasting variations in drought tolerance [
90]. The increased plant water and nutrient contents through the application of S
CF75 + F
CF25 + C
LB played crucial roles in improving the efficiency of photosynthesis and the antioxidant defense system (
Table 4 and
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12), which need water as a medium for their reactions to take place [
22,
91]. The S
CF75 + F
CF25 + C
LB, including C
LB, notably improved
P. vulgaris plant water content due to the improved water uptake by roots and as a result of the accumulation of elemental K
+, free proline, and soluble sugars as osmoprotectant compounds [
47,
73]. Furthermore, the accumulation of ascorbate and glutathione with the S
CF75 + F
CF25 + C
LB treatment contributed to the improved tissue water status and membrane integrity by reducing the activity of the reactive oxygen species (ROS) [
47,
92,
93]. It has been explained that the accumulation of K
+, free proline, and soluble sugars as osmolytes contributes to osmotic adjustment, which helps to maintain the cell turgor and stabilize the membranes by activation of antioxidant plant defense system [
73,
94,
95,
96,
97,
98,
99]. This is also evident from improvement in antioxidant enzymes activities in this study (
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12), such as superoxide dismutase (SOD), catalase (CAT), and guaiacol peroxidase (GPOD), including leaf content of protein for the S
CF75 + F
CF25 + C
LB treatment in
P. vulgaris plants grown under the tested conditions. Enhanced activities of assayed antioxidant enzymes with the S
CF75 + F
CF25 + C
LB treatment have been also associated with improved photosynthetic activity and carbohydrates supply to growing sink [
73], which contributed towards increased growth and yield of
P. vulgaris plants. This increase in
P. vulgaris plant performance with the S
CF75 + F
CF25 + C
LB treatment is also associated with its influences on different physiological mechanisms and enzymes such as starch metabolism and glucose transport during photosynthesis and accumulation period [
100]. All these findings were reflected, positively, in
P. vulgaris plant growth and productivity (
Table 4).
Soil application with CLB (compost + bacteria) can integrate conventional NPK fertilizers that should be applied to both soil and plant (foliar spray) in the cultivation of P. vulgaris and other crops using the defective sandy soils to improve plants’ efficiency for good production and minimize contamination of farmland and agricultural products achieving agricultural sustainability.
All results of the 2020 season notably exceeded those of the 2019 season in terms of all the attributes examined for soil physicochemical properties and
P. vulgaris plant growth, yield, and physiobiochemistry (
Table 2,
Table 3,
Table 4,
Table 5 and
Table 6;
Figure 1,
Figure 2,
Figure 3,
Figure 4,
Figure 5,
Figure 6,
Figure 7,
Figure 8,
Figure 9,
Figure 10,
Figure 11 and
Figure 12). This may be attributed to the residual effects of the first year’s treatments, which provided the opportunity to liberate nutrients in excess amounts from the soil due to the elevated bacterial decomposition of the biocompost applied in the preceding season (2019), as well as the increased solubility of nutrients that contained in the biocompost.