1. Introduction
The productivity of cropping systems in the North Eastern (NE) region of South Africa, which is characterised by semi-arid climate, is limited by the physical constraints of inadequate water, high temperatures, and poor soils, amongst other factors. The poor fertility of soils in the region is associated mainly with low and declining soil organic carbon (SOC), which is exacerbated by extreme climatic conditions as well as poor soil management such as continuous monoculture, excessive burning of veld, uncontrolled grazing, and low organic matter application [
1]. Most soils in this region have relatively low levels of organic carbon [
2,
3], which are below the threshold level for sustaining soil quality [
4,
5]. Poor soil management leads to destruction of ecological soil processes, depletion of soil quality, loss of biodiversity, and direct loss of soil [
6]. Therefore, carbon (C) input is usually necessary to either decrease or reverse C loss in agricultural soils.
Increasing carbon inputs through agricultural management is likely to improve the quality of the soil’s organic matter [
7], soil quality, and crop productivity in NE South Africa. Indeed, the importance of soil organic matter in a soil ecosystem, and more specifically in alleviating soil degradation, has been documented [
8]. Greater organic input into the soil has been associated with high soil nutrient concentration, high water retention, and high crop yield [
9,
10]. Therefore, soil organic input management is important for soil quality improvement and the development of sustainable low agriculture input systems [
11] as well as sequestration of atmospheric C that leads to climate change mitigation [
12].
Organic materials (such as biochar, compost, and green manure legumes) may be used to improve and/or sustain SOC and consequently soil fertility and productivity of cropping systems, especially in areas characterized by low SOC [
2,
13,
14,
15]. However, most smallholder farmers do not have the capacity to produce biochar nor ready access to biochar manufactured elsewhere. Earlier efforts to incorporate green manure legumes in smallholder farming systems were largely unsuccessful due to their lack of immediate benefits [
16]. The use of compost may offer a sustainable option of improving soil organic carbon and soil fertility, especially in smallholder farms. However, the effects of compost vary with soil type, compost type, compost quality, application rate, application method, and the crop being grown [
17].
A wide range of raw materials (including crop residues, livestock and poultry manure, and organic wastes) are used as feedstock for production of organic composts. However, the use of compost may be limited by the availability of feedstock material. The northeastern region of South Africa is a major producer of macadamia nuts. After harvesting, the farms generate enormous quantities of organic waste, in the form of macadamia husks (the outer coating of the nut-in-shell), which end up being dumped as waste. Therefore, the use of macadamia husk for making compost would not only contribute to a sustainable improvement of soil fertility and crop productivity, but also provide farmers with a sustainable and cost-effective way to dispose of the macadamia husk waste [
18].
Despite the availability of macadamia husks in macadamia-growing areas and the potential benefits of macadamia husk compost (MHC), there is limited information in the literature on the effect of soil-incorporated MHC on SOC and overall soil productivity. For example, in an earlier study [
18], macadamia husk was mixed with cattle manure. In another study, MHC was applied as surface mulch rather than being incorporated into the soil [
19]. More recently, macadamia husk powder did not affect plant–parasite interactions or beneficial nematodes in a laboratory experiment [
20]. Clearly, there is a need for more extensive field studies on the effect of soil-incorporated macadamia husk compost on soil fertility and other soil-related properties, especially in soils characterized by low SOC.
Therefore, the objective of this study was to quantify the effect of soil-incorporated macadamia husk compost on the physicochemical properties of a sandy loam soil under field conditions in NE South Africa. We hypothesized that MHC would improve selected chemical and physical properties of the soil by increasing SOC. To the best of our knowledge, this is the first field study to assess the response of physicochemical properties of a sandy loam soil characterized by low SOC to soil-incorporated MHC.
4. Discussion
Organic matter is a source of nutrients and microbial activity in the soil, and it affects water holding capacity, soil structure, infiltration rate, soil aeration, and soil porosity [
33,
34]. The increase in SOC with the addition of compost, including MHC, to the soil has been reported previously and this was attributed to the high organic carbon in the compost [
19,
35,
36] and an increase in the rate of carbon sequestration [
14,
37,
38]. In the current study, the increase in soil organic carbon with compost application was likely due to the high C of the macadamia husk compost (
Table 1), which is consistent with recent findings that carbon inputs from compost resulted in greater SOC at lower soil depths [
15]. In addition, an increase in SOC with biochar application was associated with the high carbon content of the biochar used [
39].
The effect of inorganic fertilizer on soil C is generally less pronounced, particularly in the short-term, since it increases C indirectly by improving crop growth [
40]. Consistent with our findings, Lusiba et al. [
2] did not observe a significant effect of inorganic fertilizer on SOC in clay (both seasons) and loamy sand (one season) soils in NE South Africa, and Jalal et al. [
39] reported no effect of inorganic nitrogen fertilizer on soil carbon in a clay loam characterised by low organic matter. Similarly, soil amended with compost had higher SOC compared to that of soil treated with inorganic fertilizer [
41,
42].
Macadamia husk compost increased the soil potential available water (PAW) in both seasons, while the effect of inorganic fertilizer was non-significant, as expected. The greater PAW with application of MHC was likely due to a similar increase in soil organic matter [
43,
44] as reflected by the significant positive correlation between SOM and PAW (
Table 6). Organic matter affects water holding capacity by improving soil structure, soil aggregate stability, particle size, and total porosity [
45]. However, we did not determine aggregate stability, particle size, and total porosity, which is one of the limitations of the current study. Similar results have been reported with soil surface-applied MHC [
19] and other organic wastes [
46,
47,
48,
49] where the increase in water holding capacity was attributed to increased SOM, but our study is the first to report on the positive effects of soil-incorporated MHC on PAW of sandy loam soil that is characterized by low SOC.
Addition of organic amendments to soil reduces soil bulk density by increasing total soil organic carbon, which causes an increase in stable soil aggregates and soil porosity [
2,
34,
50,
51,
52,
53]. In this study, application of macadamia husk compost reduced soil bulk density, probably due to an increase of soil organic carbon (
Table 6). Although we did not determine aggregate stability and soil porosity in the current study, the effect of MHC on soil bulk density was consistent with that on WHC.
Soil organic matter and SOC play a significant role in both soil fertility and soil quality management [
36,
54]. Application of MHC increased total N in the soil, soil available P, exchangeable cations, and micronutrient levels in the soil in the current study (
Table 4 and
Table 5). The observed increase in soil available phosphorus with MHC application was probably due to a similar increase in SOC with MHC application (
Table 4 and
Table 5) and high concentration of P of the MHC (
Table 1). MHC supplied 855–1710 kg P ha
−1 in each season, which may in part explain the greater effect of MHC on soil available P compared to that of IF [
55,
56]. Moreover, it is likely that MHC released various organic acids, which led to increased P solubilization [
57]. In support of the association of high soil available P with high SOC due to MHC application, we observed a highly significant positive correlation between SOC and available P (
Table 7). More recently, application of rice straw biochar and rice husk ash led to a significant increase in available P and K in both surface and subsurface soil layers due to the high P and K content of these soil amendments [
58].
The application of soil amendments may increase total soil N either through a reduction in soil N losses via leaching and denitrification [
59], direct addition of N contained in the soil amendments [
58], or an increase in SOC [
60]. In the current study, the increase in total soil N due to MHC could be attributed to a combination of the high N content of the MHC (
Table 1), increase in SOC as evidenced by the highly significant positive correlation between total N and SOC (
Table 7), and a decrease in leaching losses. Although we did not explicitly determine the leaching losses in our study, MHC increased PAW (
Table 3). Previous studies with MHC compost have not been conclusive. For example, Cox et al. [
19] observed an increase in total N content in the 2–10 cm soil layer due to surface-applied MHC. In contrast, application of 10 t ha
−1 macadamia husk/cattle manure compost decreased soil nitrate levels and leaf nitrogen content in macadamia orchards [
18]. These contrasting results, which are likely due to the huge variability in compost quality, suggests the need for more widespread studies with MHC.
The increase of exchangeable cations and micronutrients content in soil with compost application was likely due to the high content of these nutrients from the compost (
Table 1) [
56,
61], an increase in adsorption of the cations, and a reduction in leaching losses [
58,
62]. Consistent with our findings, Bittenbender et al. [
18] observed an increase in K, Ca, Mg, and Na with application of macadamia husk/cattle manure compost.
Exchangeable cations and micronutrients levels in the soil were greater in plots amended with MHC compared to those amended with IF, as expected. The IF used in our study supplied only N, P, and K while MHC contained high amounts of exchangeable cations and micronutrients (
Table 1). Moreover, organic soil amendments have high cation exchange capacity, which bind more exchangeable cations [
58,
63]. The increase in exchangeable cations and micronutrients due to inorganic fertilizer application that we observed in the current study was unexpected since the mineral fertilizer used did not contain any exchangeable cations (other than K) or micronutrients.
Application of MHC increased soil pH in the current study probably due to (i) the high pH of the compost used (
Table 1), which is consistent with previous findings [
18,
19], and (ii) a similar increase in exchangeable cations as shown by positive correlations between exchangeable K, Na, Ca, Mg, and soil pH (
Table 7). Consistent with our findings, Macil et al. [
64] attributed an increase in soil pH of both clay and loamy sand soils with the application of biochar to high ash content and pH of the biochar, and Ojobor et al. [
38] concluded that application of rice husk compost increased soil pH due to the exchangeable Ca, Na, and K contained in the compost.
5. Conclusions
Application of macadamia husk compost improved soil fertility (N, P, K, organic C, exchangeable cations, and micronutrients) and overall soil health (pH, PAW, and bulk density). The large increase in SOC was particularly important due to the low SOC at this site. The beneficial effect of MHC on soil fertility and soil health, which was associated with the high content of carbon and other nutrient elements in the compost, was more pronounced at the higher compared to the lower application rate. Macadamia husk compost outperformed inorganic fertilizer, suggesting the huge potential of using MHC as a soil amendment in this region. However, we recommend further studies, which should include the 15–30 cm soil layer, across several sites with contrasting climate, soils, and cropping systems and in-depth analysis of soil physicochemical properties (e.g., aggregate stability, particle size, porosity, sodicity, etc.) that were not assessed in the current study.