Declining soil fertility and productivity is a major challenge among smallholder farmers in Malawi (Kumssa et al., 2022). Poor soil fertility is recognised as a key obstacle to macadamia production globally. In Malawi, studies and recommendations on soil fertility improvement on macadamia farms have been tailored for the commercial macadamia subsector as opposed to the smallholder subsector (World Bank, 1994). For this reason, it is challenging for smallholders to address nutrient deficiencies on their farms. Therefore, this study sought to address this knowledge gap and to determine soil fertility improvement recommendations for the smallholder macadamia producers.
4.1. Current soil fertility status and macadamia needs
Soil texture and structure are important soil properties that define the general inherent capacity of soil and have profound implications on the soil's water holding capacity, drainage, nutrient retention and supply, and nutrient leaching (Nalivata et al., 2017; Huang & Hartemink, 2020; FAO, 2022). Our study reveals that the majority (67%) of the soils among smallholder macadamia farms in the study sites are sandy, i.e., sandy loam (52%) or sandy clay loams (15%), while only 16% are classified as clays, i.e., clay loam (13%) and clay (3%). These findings concur with descriptions of Malawi soils as generally sandy in texture (Li et al., 2017; Eze et al., 2020).
However, we observed variations in soil textural classes at the individual farm levels. We noted that some macadamia farms, especially those in hilly areas of Nachisaka, Neno, and Tithandizane cooperatives, had a greater sand content (≥ 70%) than the other cooperatives. One possible reason is soil erosion which was evident during the field survey. This was possibly enhanced by the previous sifting, as ridges were made for annual crops. Contrarily, the proximity of some areas in Malomo, Nachisaka, and Mwanza cooperatives to Lake Malawi and Shire valley explain why some of the farms in these areas have a higher sand content. However, some soils, as can be seen in Fig. 2b and Fig. 2c, have higher clay content (≥ 40%). This is because the farms are located in flood alluvial plains (locally known as dambos).
The high sand content in the study sites negatively impacted the availability of essential soil nutrients and contributed to the lower levels of CEC and SOM content (Fig. 7). Because of these characteristics, sandy soils have poor soil fertility status, necessitating regular and increasing levels of fertiliser applications to ensure the healthy growth of crops annually. However, this is becoming increasingly difficult for Malawi's smallholders to achieve and afford. Additionally, this has been made worse by the rapid increase in fertiliser costs (more than 130–160% higher than in 2020) and limited availability attributed to Russia's invasion of Ukraine, both of which are major global suppliers of fertilisers.
Soil pH is a crucial indicator of soil fertility since it influences the availability of all nutrients in the soil. This study has shown that only 11.1% of the sampled macadamia farms have soil pH levels within the optimum range for the crop. This translates to about 4.76% of macadamia farms belonging to Tithandizane cooperative and 1.59% of macadamia farms belonging to each of the four cooperatives (Chikwatula, Mwanza, Mphaza, and Neno). We learned from HIMACUL staff that some of the wealthy macadamia smallholders, especially those whose soils had a near-neutral pH, use agricultural lime to manage the pH of their soils. In contrast, 87.3% of the soil samples were strongly acidic (≤ 5.5), rendering them unsuitable for growing macadamia production.
Principal contributors to the soil's strong acidity were agronomic practices, loss of major cations (leaching and soil erosion), and higher nutrient uptake accompanied by lower nutrient replenishment. Some examples of agronomic practices include low input of organic materials, previous continuous monoculture of annual crops, and use of higher rates of compound inorganic fertilisers, especially NPK, in an effort to achieve higher growth and productivity of crops. Our results complement and, more importantly, extend the findings of Mutegi et al. (2015), who found that continuous monoculture and blanket inorganic fertiliser applications are responsible for soil acidification in Malawi. Moreover, soil acidification may have been exacerbated through the inorganic fertiliser only nutrition strategy among the smallholders (Dougill et al., 2002; Steward et al., 2018).
The strong soil acidity in some of the upland areas (≥ 1400 m.a.s.l) of Chikwatula and Tithandizane cooperatives can be partially attributed to the heavy precipitation amounts received in these areas (Table S2). According to Munthali et al. (2021), the intense precipitation received in the higher altitude areas (1200–1700 m.a.s.l) of Dedza district makes the soil vulnerable to acidification and nutrient losses due to soil erosion, confirming our study results. Thus, for areas that receive intense precipitation, water management technologies that promote infiltration are recommended. These may include constructing box, contour, and tier ridges, mulching, intercropping, and using live plants such as vetiver grass.
CEC is an important soil property that influences soil structure stability, nutrient availability, pH, and the soil's response to fertilisers and other ameliorants (Hazelton & Murphy, 2016). We have observed that soils from all study sites barely exceed the lower threshold for CEC, which for sandy soils falls between 5 and 10 cmol (+) kg− 1 (Van Ranst et al., 1999). This reflects the soil's high sand content, strong acidity, low organic matter content, and possibly the clay type (kaolinite). In Malawi, Mloza-banda et al. (2016) found that acidity lowered the CEC of soil, thus verifying our results. Furthermore, the lower CEC may be attributed to the rapid mineralisation rate resulting from previous conventional tillage practices by smallholder farmers.
The present study has also revealed a negative correlation (R2 = − 0.48) between cation exchange capacity and clay content. This suggests that the SOM fractions, rather than clay particles, are the source of CEC across our study sites. As kaolinite clays are predominant in Malawi, including our study areas, the finding by Tudela et al. (2010) that kaolinite clays do not contribute much to the CEC provides additional context for our results. Moreover, Bortoluzzi et al. (2006) found that organic matter fractions contribute more than 50% of the negative charges in the soil compared to clay particles (31%). This demonstrates that organic matter fractions have a greater impact on the CEC of the soil than clay particles.
Soil organic matter, is crucial for crop productivity and maintaining soil health (Belachew & Abera, 2010; Omuto & Vargas, 2018). Majority of the sampled macadamia farms in this study have very low levels of SOM (≤ 1%), below the recommended threshold (≥ 2%) for macadamia. This is partially attributed to previous conventional tillage practices, continuous cultivation, the inherent nature of sandy soils and the smallholders' low incorporation of organic residues. However, while most of the sampled macadamia farms had very low SOM levels, 3.2% and 4.7% of the sampled farms in Chikwatula and Malomo had optimal soil organic matter levels. Field observations and farmer conversations revealed that the incorporation of farmyard manure and crop residues were responsible for the observed higher SOM content. These farmers reported having easy access to farmyard manure due to their ownership of considerable herds of cattle and goats (made possible by livestock pass on programmes in the areas) and crop residues because of the cultivation of legumes. Thus, encouraging the incorporation of livestock manure and crop residues is also a viable option for increasing the content of SOM among smallholder macadamia producers in Malawi.
The results of this study have shown variability in terms of essential nutrient concentrations in smallholder macadamia farms. We found that the total N concentrations among the study sites were below the average values recommended for macadamia soils. However, our results have also revealed that the average potassium concentrations of the examined soils were adequate for macadamia production. Nevertheless, at the individual farm level, only 44.4% of the sampled soils had adequate levels of available K+. These results suggest that soil potassium reserves on some macadamia farms within the cooperatives are becoming inadequate for macadamia's needs.
Soil available phosphorus among the study sites was generally deficient. About 83% of the soils were below the critical value of 30 mg kg− 1 recommended for macadamia. The average concentration of available P was only sufficient for macadamia production in Mphaza cooperative. This is because five of the sampled macadamia farms were markedly high in soil available P (≥ 50 mg kg− 1), which can be attributed to previous monoculture tobacco production and the ongoing intercropping of tobacco in the rows of macadamia trees. With regard to available calcium and magnesium, we have established that nearly all of the study sites were deficient in both elements. We recommend the application of lime in order to increase the levels of calcium and the application of dolomite to increase the levels of magnesium in smallholder macadamia producing areas.
Boron and zinc are essential micronutrients required in small but critical amounts for macadamia's normal growth and development (Stephenson et al., 1986). In general, we found that the B and Zn levels on smallholder macadamia farms were below the minimum reference levels for macadamia production. This is due to the course texture of sandy soils, and the lack of organic matter in the soils. Our findings are consistent with Jiménez et al. (2022), who reported that low soil clay content stimulate rapid SOM decomposition in tropical ecosystems and thus reduce soil micronutrient concentrations. In addition, boron and zinc are naturally deficient in Malawian soils (Evans, 2020).
The absent utilisation of boron and zinc fertilisers may also be the reason for the low levels of B and Zn in the study areas, as the nutrients are taken up and not replenished. Evans (2021) found that commercial estate producers in the country have increased their B and Zn levels through routine foliar applications. In light of this, boron and zinc fertilisers should be made available to smallholder farmers, which are currently scarce within Malawi. Furthermore, mulching, intercropping, and cover crops should be encouraged as these systems reduce the direct exposure of soil to sunlight, thus maintaining cool temperatures in the soil and, subsequently, biological activities.
4.2. Implications of the study and recommendations for management
Nutrient management is one of the most important aspects of a successful macadamia crop. Based on our study findings, it is possible to conclude that nutritional imbalances and deficiencies are one of the factors affecting the productivity of macadamia among smallholder farmers in Malawi. According to the "Law of Minimum," a limited supply of one of the essential nutrients can limit crop yield (Austin, 2007). As such, the identified deficiencies and imbalances in the study areas will need to be addressed simultaneously to improve their soil fertility status in a reasonable amount of time.
Contrasted with what was reported in the 1990s, our findings show that the current soil fertility status of smallholder macadamia growing areas in Malawi is very different and in a poor state. Early recommendations for macadamia production were that farmers maintain and replenish soil fertility with the addition of only manure, as opposed to inorganic fertilisers. A key message from our findings is that "no one size fits all" or "silver bullet" solutions can be applied to maintain and replenish soil fertility loss in macadamia farms. Thus, soil organic matter and inorganic fertiliser application management are essential for sustainable macadamia productivity.
However, for the precise application of inorganic fertilisers, providing smallholders with local-scale information about their soil fertility status is beneficial. This can be achieved through the annual low-cost testing of soil nutrients by trained agricultural officers or lead farmers (LUANAR is already trialling this technology in some parts of Malawi). Nevertheless, this necessitates additional research to develop recommended application rates of proposed blended (mixture of micro and micronutrients) inorganic fertilisers and to understand the response of trees to fertilisers as well as the long-term effects on soil health.
With respect to crop productivity and speed in supplying soil nutrients, there is evidence that inorganic fertilisers produce higher crop yields at a point in time and readily supply nutrients to the soil for plant use. However, organic fertilisers such as manure and crop residues have longer-term benefits than inorganic fertilisers. This means that an integrated soil fertility management system is viable for cost-effective soil fertility management. For sustainable productivity, mixed use of organic (crop residues, farmyard, and green manure) and inorganic fertilisers has proved to be highly beneficial in terms of balanced nutrient supply (Li et al., 2007; Rutkowska et al., 2014; Wani et al., 2017) and significantly increased yields of various crops (Phiri et al., 2010; Mungai et al., 2016; Ghimire et al., 2017; Abbas et al., 2012; Roba, 2018). To improve the soil fertility status of smallholder macadamia farms in Malawi, we recommended that farmers utilise a combination of organic and inorganic fertilisers and practices to increase soil pH.
Soil acidity amelioration is a prerequisite for sustainable soil fertility management. When soil pH is maintained at the proper level, plant nutrient availability is optimised, the solubility of toxic elements is minimised, and beneficial soil organisms are most active (Malla et al., 2020). Therefore, raising the soil pH to near neutral (5.5–6.5) among the smallholder macadamia production areas will enable the availability of essential nutrients, particularly boron and zinc. It is therefore recommended that smallholders effectively manage the soil acidity through the application of agricultural lime or gypsum in conjunction with organic matter management.
Cover crops provide numerous benefits to agroecosystems. Specifically, they protect the soil from erosion, improve water infiltration, help to control weeds, help to reduce soil temperature and build soil organic matter (Suci et al., 2021). We thus recommend growing annual crops, especially legumes (groundnuts, pigeon peas, and soybeans), between the rows of macadamia trees. This will assist in increasing the amount of high-quality organic residues and N resulting from biological nitrogen fixation, provided that residues are retained or spread under the tree canopy. Maize-legume associations (cowpeas and pigeon peas) have been reported to enhance the status of SOM and improve soil hydraulic properties, including soil water storage pores, water transmission, and retention within the root zone in Malawi (Eze et al., 2020; Hermans et al., 2021). In addition, interplanting annual crops will ensure that farmers harvest an additional crop annually for food security, income generation, and resilience in case of crop failure. Figure 8 provides a summary of recommendations that smallholders can utilise to improve the soil fertility of their farms