Soil structure as a significant indirect factor affecting crop yields

© Slovak University of Agriculture in Nitra Faculty of Agrobiology and Food Resources


Introduction
Soil physical health is the ability of a given soil to meet plant and ecosystem requirements for water, aeration, and strength over time and to resist and recover from processes that might diminish that ability (McKenzie et al., 2011). Soil structure is often defined as the most important basic physical property of soil and is therefore considered an important indicator of soil quality (Kay et al., 2006). Soil structure refers to the shape, size, and strength of soil aggregates (the basic unit of soil structure) and pores (voids between particles and soil aggregates). These characteristics are differentiated through the capacity of soil pores, and their ability to retain water, organic and inorganic substances in them, allowing them to move, solubility and ability to support the growth and development of soil microorganisms and plant roots (Lal & Shukla, 2004). A change in the condition of the soil structure will usually also induce a change in other soil properties (Kay et al., 2006).
From the above context, soil structure does not directly affect plant growth and development. This is significant as it affects virtually all growth factors. Soil and plant water supply, aeration, availability of plant nutrients, microbial activity, root growth and many physical properties of soil are significantly affected by the soil structure. Therefore, an unfavourable soil structure is a factor that indirectly limits the growth of plants. Conversely, a good waterresistant soil structure allows growth factors to act in optimal performance (Fulajtár, 2006;Nayak & Mishra, 2019). The soil structure is influenced by the whole complex of external and internal factors, and there are numerous interactions between them (Idowu, 2003;Bronick & Lal, 2005;Šimanský et al., 2013). Organic matter is very important for maintaining structural stability in soils as well as improving the physical, chemical, and biological properties of soils (Iwai et al., 2019). The quantity and quality of organic matter that affects the physical condition of soils, including the soil structure as is known, can be modified through soil management practices (Šimanský et al., 2013;Iwai et al., 2019;Nayak & Mishra, 2019). The application of organic materials for soil amendment, especially the composed manures, plays important roles in reclaiming and improving the physical health, including the soil structure of degraded soils .
The following hypotheses were verified: added organic amendments to the soil would improve the soil physical properties, including the soil structure, and that the intensity of the soil physical properties improvement would primarily depend on the quality of the applied organic amendments, though the quantity may have a lesser bearing. Taken together better soil structure should result in a higher crop yield. Therefore, the aim of this study was to: 1. evaluate the effect and extent of applied organic amendments on the soil physical properties with an emphasis on the soil structure, 2. determine whether there is a linear relationship between the soil structure and crop yield.

Material and methods
In 2018, the Department of Vegetable Production of SUA-Nitra established a field experiment in the Botanical Garden at the Slovak University of Agriculture in Nitra. The local climate was warm and dry, with annual rainfall of 539 mm approx. and a long-term an average temperature of 10.2 °C (Špánik et al., 2002). The Botanical Garden is located on the left bank of the river Nitra. Hence, the original soil-forming substrate was Quaternary loamy-clayey alluvial sediments above gravel facies, on which were formed a Hortic Calcaric Fluvisol (Polláková & Šimanský, 2015). The average soil content before the experiment was 9.7% sand, 55.8% silt and 34.5% clay on average. Soil organic carbon content was 2.01% and the average soil pH H 2 O was 7.75. More chemical characteristics of soil (such as: sorption parameters, humic substances etc.) in the experimental field are available in Polláková and Šimanský (2015a).
A small-plot experiment at the Botanical Garden was established in spring 2018. The preceding crop was winter squash. Cabbage (Brassica oleracea var. capitata f. alba) was planted in 2018. The area of one plot was 42 m 2 and the plots were divided by protective belts.
The experiment was established by using the method of random arrangement with the triple repetition and it consisted of the following treatments: 1. control (non-fertilization), 2. compost at a rate of 44 t ha -1 , 3. farmyard manure at a rate of 44 t ha -1 , 4. farmyard manure together with compost at rates of 44 and 44 t ha -1 , respectively.
Organic materials (farmyard manure and compost) were applied on the soil surface in autumn 2017 and incorporated at a depth of 15-25 cm. As farmyard manure was used poultry manure which contains in dry matter 55% of organic substances with pH 6-8. Compost contains in dry matter 47% of organic substances and its pH on average was 8.2. Conventional practices were applied at the site. The seedlings of the model variety (Kamienna Glowa) were grown according to the usual methodology in the Botanical Garden greenhouses at the Slovak University of Agriculture in Nitra. Sowing was carried out on April 17 th , 2018. The seedlings were planted on the site on May 31 st , 2018 in plantation spacing of 60 × 60 cm. Additional irrigation was carried out by spraying in the morning, according to the current water deficit in the soil (saturation to 65% of field water capacity). The soil crust was disturbed by a hand hoe. As production was simulated in an ecological farming system, no pesticides or supplementary fertilizers were applied throughout the vegetation period. Cabbage was harvested on November 7, 2018.
Based on the contents of DSA, the mean weighted aggregate diameter obtained by dry sieving (MWD d ) was calculated: (1) x i -the mean diameter of each size fraction (mm); w ithe total sample weight within the corresponding size fraction; n -the number of size fractions Mean weighted aggregate diameter obtained by wet sieving (MWD w ) -based on the contents of WSA: ( 2) where: xi -the mean diameter of each size fraction (mm); WSAthe total sample weight within the corresponding size fraction; n -the number of size fractions Soil structure vulnerability coefficient (K v ) (Valla et al., 2000): The structure coefficient (K) was calculated according to Equation (4): where: A -the weight of air-dried aggregates in size fractions from 0.25 to 7 mm; B -the sum of the weight of air-dried aggregates in size fraction more than 7 mm and less than 0.25 mm We also determined the soil organic carbon content (C org )measured using the wet combustion method -oxidation of soil organic matter by a mixture of 0.07 mol dm -3 H 2 SO 4 and K 2 Cr 2 O 7 with titration using Mohr's salt (Hrivňáková et al., 2011). The labile carbon content (C L ) was determined using 0.005 mol dm -3 KMnO 4 (Loginow et al., 1987) and the fraction composition of humic substances according to Belchikova and Kononova (Hrivňáková et al., 2011).
The statistical analyses were performed with the statistical package Statgraphics Centurion XVI. programme (Statpoint Technologies, Inc., USA). The effects of organic materials on physical properties were tested using oneway ANOVA and then the least significant difference (LSD) method was used to compare treatment means at the significant level of α = 0.05.

Results and discussion
The addition of organic amendments would normally improve soil properties including soil physics (Lal and Shukla, 2004;Belmonte et al., 2018) however, in our experiment organic amendments (compost, farmyard manure and their combination) did not have statistically significant effects on bulk density, total porosity (TP), soil moisture or aeration (Table 1).
There were expected that a finding to be consistent with Iwai et al. (2019) and show the addition of compost in soil resulted in physical properties such as bulk density, TP, moisture in soil were improved. However, applied compost decreased bulk density and increased TP, moisture, and aeration but the changes were not significant compared to the control, the exception being the quantity of added organic matter to the soil, a key factor in the quality of organic amendments. Manure improves cation exchange capacity (CEC), a nutrient regime in the soils and physical properties through added humic substances (Vachalová et al., 2016). The quality of humic substances depends on their maturity. Besides, younger humic substances are responsible for CEC and nutrient regime in the soils, and mature (old) humic substances are responsible for improvements of physical properties in the soils. In this context, it is evident that organic amendments applied to the soils occur due to the production process (because of the conditions employed in the production process) and maturity of organic matter used. If the results of TP were evaluated there should not be any significant differences between organic amendments applied to the soil. However, when TP was evaluated to ascertain the volumes of an individual energetic category of soil pores (capillary, non-capillary and semi-capillary pores), the one-way ANOVA analysis showed significant differences between treatments, for a volume of capillary pores (Figure 1). The highest average volume of capillary pores was found in the compost treatment and then in control > farmyard manure + compost > farmyard manure, however statistically significant differences between compost vs. farmyard manure were also observed. This indicates that the quality of applied farmyard manure was poorer compared to applied compost. Volumes of non-capillary and semi-capillary pores were not changed significantly due to organic amendments application (Figure 1).
In this study, the results of soil structure parameters are summarized in Table 2. The ratio between dry-sieved macro-(DSA ma ) and micro-aggregates (DSA mi ) is a sensitive indicator of soil compaction and it indicates how much of the macro-pores are being altered concerning the volume of micro-pores. This ratio can be called the structure coefficient (K). A higher value of K results in optimal soil structure and better overall soil physical properties. No significant effects of organic amendments on changes in K values were observed. Based on the one-way ANOVA results, no significant differences in MWD d were between the following treatments: compost > farmyard manure + compost > control > farmyard manure. It is estimated that for proper soil aeration and plant development, the relationship between DSA ma and DSA mi within TP should be 67% DSA mi and 33% DSA ma (Oliveira et al., 2015). In the present study, the content of DSA mi and DSA ma was lower than 67% and higher than 33%, respectively for all treatments (Table 2). Higher content of DSA mi and reversely a lower content of DSA ma can induce aeration deficiency in soil and reduce gas exchange processes (Colombi et al., 2017). In this study, the contents of DSA ma and DSA mi ranged from 85.9 to 96.5% and from 3.52 to 14.13%, respectively. The application of organic amendments had a statistically significant influence on the total contents of DSA ma and DSA mi ( Table 2). The highest content of DSA ma was in compost > farmyard manure + compost > farmyard manure > control. The effects of organic amendments were dependent on size fractions of DSA ma . No significant effect of organic amendments in higher size fractions of DSA ma (>1 mm) on one and statistically significant effect in smaller size fractions of DSA ma (<1 mm) were observed on the other (only between fertilized treatments; no between control vs. fertilized treatments).
From a structural stability point of view, the content of WSA is crucial. The total contents of WSA, contents of agronomically favourable macro-aggregates (WSA ma ) in size fractions 0.5-3 mm (Šimanský & Bajčan, 2014), MWD w and coefficient vulnerability (Valla et al., 2000) are summarized in Table 2. Content of water-stable microaggregates (WSA mi ) and water-stable macro-aggregates (WSA ma ) ranged from 1.47 to 23.47% and from 76.56 to 98.53%, respectively. According to the Dolginov classification of WSA (Šimanský et al., 2017), the content of WSA ma >70% represents excellent water-resistant soil structure. The addition of organic amendments had a statistically significant influence on the increase of total contents of WSA ma and the decrease of WSA mi . The average value of WSA ma was higher by 10 and 11% in compost and farmyard manure + compost treatments, respectively compared to control. In WSA ma size fractions 2-1 and 1-0.5 mm, statistically significant differences between fertilizer treatments were observed. Fertilization of organic manures did not have any significant effects on changes in higher size fractions of WSA ma as well as in agronomical favourable size fraction of WSA ma 0.5-3 mm  Table 2). The stability of aggregates was higher in compost and farmyard manure + compost than in farmyard manure treatment compared to control. Also, better soil structure stability evaluated by vulnerability coefficient (K v ) was in farmyard manure + compost (2.38 ±0.38) and compost (2.53 ±0.68) than in farmyard manure treatment (3.13 ±0.30).
The addition of organic amendments to the soils has been suggested for improving structural stability by many researchers (Šimanský et al., 2018;Nayak & Mishra, 2019). Šimanský et al. (2013) reported that the addition of manure improved the stability of soil aggregates due to the increased organic matter. Organic matter is the main agent for binding mineral and organic particles together to soil aggregates (Bronick & Lal, 2005) and the application of organic amendments also improves the water-resistance of the soil structure (Gosling et al., 2013) through soil organic matter (Dörner et al., 2010). Soil organic matter affects WSA by decreasing their wettability and increasing their mechanical strength (Idowu, 2003;Onweremadu et al., 2007). This positive effect of applied organic amendments was also confirmed ( Table 3). The highest content of soil organic carbon and humic substances were found in the farmyard manure + compost treatment and then in compost > farmyard manure > control. In this study, added farmyard manure C -control, Com -compost, FYM -farmyard manure, DSA ma -the content of dry-sieved macro-aggregates, WSA ma -the content of water-stable macro-aggregates, WSA ma 3-0.5 -the content of water-stable macro-aggregates in size fractions 3-0.5 mm, MWD d -mean weight diameter for dry-sieved aggregates, MWD w -mean weight diameter for water-stable aggregates, K v -vulnerability coefficient, K -structure coefficient Different letters (a, b) between lines indicate that treatment means are significantly different at p <0.05 according to the LSD test C -control, Com -compost, FYM -farmyard manure, C org -soil organic carbon, C L -labile carbon, HS -humic substances, HA -humic acids, FA -fulvic acids did not have any significant effect on changes in the quality of humic substances compared to compost and farmyard manure + compost treatments (Table 3). Riffaldi et al. (1998) stated that farmyard manure is a source of unstable organic matter and its application to the soil leads to more intense changes in organic matter content and a robust release of carbon from the soil supply, making the organic matter more unstable (Shein et al., 2001). Labile components of organic matter can affect the stability of the soil structure. After the addition of organic materials, the content of labile carbon (C L ) increased the most in compost and the least in farmyard manure treatments. However, as stated by Harris et al. (1966) supplying organic amendments with higher instability will result in a temporary reduction in the stability of the soil structure, but in the long-term it is beneficial (Zaujec & Šimanský, 2006). As mentioned above, except for quantity, the quality of added organic amendments to the soil is crucial.
The cabbage yields after the addition of organic amendments are shown in Figure 2. In this study, in the control treatment, the average yield of cabbage was 50.9 t ha -1 and the addition of organic amendments to the soil increased the yield statistically significantly. In compost, farmyard manure and farmyard manure + compost treatments cabbage yield increased by 52, 22 and 72%, respectively compared to the control treatment. Significant differences were observed between organic amendments treatments too. The highest cabbage yield was in farmyard manure + compost probably because of the highest content of organic materials and nutrition. In compost treatment, the cabbage yield was higher than in farmyard manure. In these treatments, the same rates have been added, however, there were probably differences in the contents of other components and this could be the reason for differences in yields. The addition of compost to the soil has shown to have a more beneficial effect on soil physical properties (Tables 1-3 and Figure 1) compared to farmyard manure. The results of Iwai et al. (2019) also showed the beneficial effects of compost in increasing maize growth and the yield of Cassava. Also, the study of Nayak and Mishra (2019) confirmed that farmyard manure, composts, poultry manure and bio-fertilizers in various combinations along with a recommended dose of mineral fertilizers were applied to impacted soils to evaluate their effect on plant biomass. In this study, the final yields of cabbage in 2019 were also compared to the average cabbage yields in Slovakia (Figure 2). The average yield of cabbage in Slovakia was reported 70 t ha -1 (Vaněk et al., 2013). 70 t of cabbage yield per hectare was reached in compost and farmyard manure + compost treatments and the yield of cabbage in control and farmyard manure treatments was observed to be below the country's average.
Since organic amendments are a significant source of carbon, we assumed their application to the soil would increase its content in the soil, which increase in crop yields. The linear relationships between total, labile carbon and crop yields are shown in Figure 3. It is evident from the data that the higher content of soil organic carbon, labile carbon but as humic substances in the soil resulted in higher crop yields, with a statistically significant effect. Conversely, the soil structure parameter had no direct effect on increasing the cabbage yield ( Figure 4). We found no statistically significant linear relationships between mean weigh diameters, structure coefficient, vulnerability coefficient and cabbage yields.

Conclusions
All in all, the highest cabbage yields and the most significant improvement in soil physical properties including soil structure were produced using the combination of farmyard manure and compost. Adding compost significantly increased cabbage yield and improved physical properties than using farmyard manure.
It is evident that soil structure is an important soil quality parameter but results in this study show; it does not have a direct effect on crop yield even after its improvement with organic amendments. Yield linear increased after organic amendments were introduced due to an increase in soil organic carbon and humic substances and indirectly through an improvement of the physical properties. The increase in yield and improvement of soil properties depends not only on the quantity but also on the quality of the organic amendments incorporated into the soil.