Factors affecting soil quality among smallholder macadamia farms in Malawi
Agriculture & Food Security volume 12, Article number: 17 (2023)
Declining soil fertility limits smallholder macadamia productivity in Malawi. To reverse this trend, it is essential to apply organic and inorganic fertilisers in an efficient and effective manner. Yet, fertilizer recommendations for smallholder macadamia (Macadamia integrifolia) production in Malawi are not site-specific. Nutrient imbalances can occur if fertilisers are applied without a clear understanding of whether they are required or not. This may lead to yield losses, unnecessary costs, and other environmental issues associated with excess fertiliser application. To address this research need/ knowledge gap, our study examined the current soil fertility status among smallholder macadamia farms in Malawi. Specifically, the objective was to establish an evidence base for promoting soil fertility restoration interventions for smallholder macadamia production. One hundred and eighty nine soil samples at a depth of 0–15 cm were collected from sixty three smallholder macadamia farms belonging to the Highlands Macadamia Cooperative Union Limited members in central and southern Malawi. We found that the majority of the soils were sandy loams (52%), strongly acidic (mean pH ≤ 5.1), and deficient in essential nutrients required for the healthy growth of macadamia. The soils had an average low cation exchange capacity of 1.67 cmol ( +) kg−1, which is inadequate for macadamia cultivation. More than half of the sampled soils had very low organic matter content (≤ 1%). The low soil organic matter content, coupled with the sandy texture and high acidity, contributed to the observed low concentrations of essential nutrients and cation exchange capacity. Poor agronomic practices and inherent soil characteristics are responsible for this low soil fertility. Altogether, our findings underscore the urgent need to identify and implement more sustainable and effective soil nutrient management practices that help to improve the soil fertility of macadamia farms under smallholder systems.
Soil fertility status of smallholder macadamia farms in Malawi is very low.
The low soil fertility status among the smallholder macadamia farms is mainly attributed to poor agronomic practices and soil minerology.
Building soil organic matter among the smallholder farms is key to improving soil fertility.
Fertile and productive soils play a key role in agricultural production and crop yield [5, 50]. Globally, particularly in Africa, soil fertility is declining due to intense and mismanaged farming. This has resulted in significantly lower crop yields in the region relative to other continents. For example, the average yield productivity of maize in southern Africa increased from 1.6 metric tonnes per hectare (t ha−1) in 2016 to 2.0t ha−1 in 2020, whereas in South America and Asia, yields increased from 3 to 4.5t ha−1 during the same period (The World Bank). In Malawi, declining soil fertility has been identified as a major factor limiting crop production [26, 59]. Continuous cropping and lack of agricultural inputs are the common sources of this problem in the country [5, 48]. Studies conducted in Malawi show that long-term cultivation of a single crop, especially maize and tobacco, depletes soil fertility [9, 50, 62]. Further, soil fertility loss is linked to weathering, erosion, and blanket fertiliser applications [5, 18]. Understanding the soil fertility status of previously cultivated arable lands where high-value perennials such as macadamia are currently grown is essential for Malawi's long-term agricultural productivity.
Macadamia (Macadamia integrifolia, M. tetraphylla, and hybrids) is one of the world's most profitable export nut crops . The crop is native to the highly weathered acidic soils of north-eastern Australia but grows productively in subtropical climates (Moncur et al. 1985). More than forty countries are actively engaged in the cultivation of macadamia nuts, with a market value of over $1.14 trillion (INC, 2021). The crop is essential to the economies of producing countries as it contributes to income generation and revenue from foreign exports [7, 76]. The growing public knowledge of the health benefits of consuming macadamia nuts has led to a 45% increase in macadamia nut production over the past decade compared to the previous decades (INC, 2021). Because of this, the international retail market prices for first-grade macadamia nuts are higher than those for other nut crops (≥ $25 kg−1) (INC, 2021).
Macadamia nuts have high socio-economic value for smallholder producers in rainfed agricultural economies of the world, including Malawi. Producing two percent of total global production, Malawi is ranked the world's eighth-largest producer of macadamia nuts (INC, 2021). The nuts are a high-value export crop with an estimated value of more than $30 million. As a result, Malawi’s macadamia industry is rapidly expanding. Additionally, the Malawi government's Vision 2063 commercialisation programme on strategic crops like macadamia is expected to facilitate a further increase in the production and marketing of the crop (Malawi National Planning .
Macadamia production in Malawi is divided into two distinct subsectors: commercial estate (highly intensive with hectarages above 100 hectares) and smallholder (small-scale production with limited use of mechanization and usually no more than two hectares for production), and a growing intermediate scale of growers between these two. Production is dominated by the commercial estate subsector accounting for more than 90% of overall output [15, 75]. However, smallholder production has also rapidly increased, particularly during the past decade, starting from a low base. This expansion has provided many smallholders with a unique option to support their livelihoods. In addition, with an estimated net carbon sequestration potential of 3 t CO2e ha−1 year−1 , macadamia is attractive for contributing to both economic development and decarbonisation.
Despite the expansion of the smallholder macadamia subsector in Malawi, smallholder crop yields are substantially lower (≤ 8 kg tree−1) than those of commercial estate producers (≥ 20 kg tree−1) . The low input context of smallholder farmers on already nutrient-deficient soils has led to these massive yield reductions (due to inherent fertility issues and land mismanagement) . On top of the general scarcity and suboptimal management of organic fertilisers (i.e., farmyard manure, mulch, and crop residues), the lack of adequate replenishment of soil nutrients is one of the factors for the low macadamia yields among the smallholders .
The importance of soil fertility for macadamia production and productivity cannot be over-emphasized, as it impacts nut retention, quantity, and quality, all of which determine the yield and market value of the nuts produced [10, 16]. For optimal growth and quality yields, macadamia trees require a soil pH between 5.5 and 6.5, adequate amounts of soil organic matter (SOM), and essential nutrients, especially during the sensitive phenological stages of flowering, nut development, and oil accumulation [10, 12]. For example, a study indicated that an insufficient supply of essential nutrients results in nutrition deficiencies in macadamia trees . A related study showed that nutritional imbalance promotes floral abortion and contributes to macadamia yield losses . Zhao and Dong  found that excessive application of phosphorus (P) inhibits the development of cluster roots and rhizosphere processes, thereby decreasing the P-use efficiency and inducing iron deficiency. Research undertaken in Malawi has revealed that the majority of the soils lack soil organic matter (SOM) and many of the essential nutrients, including nitrogen (N), potassium (K+), boron (B), and zinc (Zn) [19, 28, 40]. Consequently, these nutritional deficiencies limit the production potential of macadamia in Malawian soils.
Soil micronutrients are essential to the global functioning of ecosystems and food production . Soil micronutrients, primarily B and Zn, are also crucial for macadamia nut set, yields, and quality . Boron is required for the development of new tissues and nut set . Zinc is essential for the fertility of the female parts of the macadamia flowers and for auxin metabolism, both of which contribute to fruit quality and disease resistance . As such, a thorough understanding of soil limiting factors among Malawian smallholder macadamia farms is essential to creating site-specific soil fertility management strategies and fertiliser recommendations for the crop. This is because applying fertilisers without knowing whether they are needed or not may lead to excessively low or high levels of some nutrients, which may negatively impact macadamia tree growth, yields, and quality. Moreover, available fertilisers high in N: P: K and without the correct mix of micronutrients are usually targeted for maize production and can be less appropriate for macadamia nutrition.
To date, soil fertility studies on smallholder macadamia farms in Malawi are still lacking. Because of this, smallholders still adhere to early recommendations provided in the 1990s for sustaining soil fertility on their macadamia farms. Assessing the soil fertility status of smallholder macadamia farms to identify underlying nutritional deficiencies is key to determining soil improvement recommendations. Furthermore, the lack of quantitative knowledge prevents smallholders from taking cost-effective corrective actions, thereby reducing the crop's potential yields. Realising the severity of these challenges, the present study was undertaken to better understand the soil fertility status among smallholder macadamia farms throughout Malawi and to compare the results to actual macadamia tree requirements. This should allow the first steps for effective nutrient management resulting in more efficient land use for sustainable smallholder macadamia production.
Materials and methods
The study was conducted in Malawi, a country located in southern Africa. The country has a subtropical climate with two distinct seasons, the rainy season from November to April and the dry season from May to October. Soil samples were collected from beneath age uniform trees (10-year-old macadamia orchards) at 63 locations among Highlands Macadamia Cooperative Union Limited (HIMACUL) members. These were Nachisaka (NSA) in Dowa district, Chikwatula (CTA), Malomo (MLM), Mphaza (MPA), and Tithandizane (TZE) in Ntchisi district, Mwanza (MA) in Mwanza district, and Neno (NN) in Neno district (Fig. 1). These cooperatives represent the country's primary smallholder macadamia production areas in terms of the number of growers and area under production .
Soil sampling procedure
Soil samples were collected from all study sites during the dry season in 2019 between August and September. Three undisturbed soil cores of 7 cm diameter were sampled from macadamia farms with an 8 m by 8 m tree-spacing (the middle and two other random locations) at 0 − 15 cm depth. A total of 27 soil samples were collected from nine locations in each cooperative, making an overall total of 189 soil samples. The study only focussed on the topsoil because macadamia has a shallow taproot and draws the majority of its nutrients through fibrous proteoid root systems near the soil's surface. Soil cores were trimmed at both ends immediately after collection, covered with plastic caps, and transferred to the Lilongwe University of Agriculture and Natural Resources (LUANAR) Plant and Soil laboratory situated in Lilongwe city. After air-drying, the soil samples from each core were sieved using 2 mm sieves to remove large particles, debris, and stones. A composite soil sample was generated by combining the three soil samples from each macadamia farm. Using soil standard preparation techniques outlined by Njoloma et al. , 10 g of the composite soil sample was weighed and used for each analytical method.
The soils were analyzed for pH in a 1:2.5 soil to water slurry (McLean, 1982) using a calibrated electrode pH meter at room temperature (OrionVersaStar®), particle size distribution (texture) using the Bouyoucos hydrometer method as described by Bouyoucos (1962) at the LUANAR Plant and Soils laboratory, and soil electro-conductivity (EC) using the method outlined by Wanda et al. (2013) at The Open University's, Ecosystems and Geobiology Laboratories (EGL). The cation exchange capacity (CEC) was determined by the ammonium acetate method (Metson, 1956), and available P was measured using the Olsen P method (Hodges & Sharpley, 2004) by the elemental analyser 146® also at EGL.
Soil organic carbon (SOC) was assessed by wet digestion and colorimetric scale using the Elemental Vario EL Cube® analyser. Total N (TN,) was extracted using the Kjeldahl method, available K+, and other micronutrients (B, Zn, sulphur/S, calcium/Ca2+, magnesium/Mg2+) were extracted using acid digestion and were analysed using the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES)®, Agilent 5110 at EGL. The soil’s physical and chemical properties were analysed over several analytical sessions, with three replication runs for each element, and mean values were used for the statistical analyses.
Analysis of variance (ANOVA) and mean comparisons were carried out using the general linear model (GLM) procedure in R® Statistical Computing Software version 4.2.0 (R Core Team, 2018). The assumptions of the ANOVA were tested by ensuring that the residues were random, homogeneous, and with normal distribution. Based on Bartlett’s test, all soil properties exhibited a homogeneous variance. Shapiro's test revealed normal distributions for the soil's physical and chemical properties. When the F-test showed statistical significance at p ≤ 0.01 or p ≤ 0.05, the Tukey Honest Significant Difference (HSD) post hoc test was used to evaluate the significance of differences between pairs of group means (Tukey, 1949). Pearson's correlation coefficient matrix was further used to describe relationships among the soil properties.
Using the USDA classification system, six distinct soil texture classifications were identified among the study sites (Table 1). The textural classes were principally sandy loams (52%) to sandy clay loams (15%), with some soil layers subtending clay loams (13%), loams (12%), and silty clay loams (5%). However, site-specific soil assessment revealed substantial variation within and between the farms among the cooperatives.
Soils in Nachisaka cooperative contained a high concentration of sand (57–81%) (Fig. 2a), followed by clay (13–29%) (Fig. 2b) and some low proportions of silt ranging from 2 to 14% (Fig. 2c). These soils have thus been categorised as sandy loams. Sandy clay loams (11.7%) were the most common type of soil in Tithandizane cooperative as they had considerable concentrations of clay and silt (Fig. 2b, c). Similar to Nachisaka, Malomo and Mphaza cooperatives had sandy loam soils (Fig. 2d).
Soil pH and cation exchange capacity
Soil pH is a measure of soil acidity or alkalinity. The pH of the soil sites did not differ significantly (p > 0.05) across the study sites. The soil pH ranged from 4.6 to 5.88, with a mean value of 5.10, indicating strongly acidic soil (Fig. 3a). Despite not being statistically different, we observed that Tithandizane soils had the highest mean pH values (5.26). Similar mean values were found for Mwanza (5.15), Mphaza (5.13), Neno (5.05), Malomo (5.02), Chikwatula (5.01), and Nachisaka (5.00) cooperatives.
The cation exchange capacity (CEC) indicates the ability of soil to hold onto and exchange cations, including plant nutrients such as Ca2+, Mg2+, and K+. In the current study, the mean CEC values did not differ significantly (p > 0.05). In general, the cation exchange capacity CEC in the smallholder macadamia farms evaluated was very low and ranged between 0.34 cmol ( +) kg−1 and 3.77 cmol ( +) kg−1, with an average of 1.67 cmol ( +) kg−1 (Fig. 3b). This showed that the CEC of the sampled soils were below (3–8 cmol ( +) kg−1) the global averages commonly reported for macadamia soils (Additional file 1: Table S1). The relationship between CEC as the dependent variable and soil pH as the explanatory variable was significant at p ≤ 0.001, and the adjusted R-square was 0.55 (Fig. 3c); that is, soil pH explained 55% of the variation in CEC. This showed that the soil pH among the sampled macadamia farms influenced the cation exchange capacity.
Soil organic matter
Soil organic matter primarily consists of organic carbon, which is commonly used to assess soil fertility. Our study has revealed that the SOM content among the sampled smallholder macadamia soils in Malawi is lower (Fig. 4) than the critical (2–5%) required level for healthy functioning of soil. Minimum and maximum values across the farms ranged from 0.26 to 2.96%, with an average of 1.13%. More than half of the study sites had soil organic matter content lower than one percent, 36.7% had less than 1.8% SOM content, and only 7.9% had a SOM content greater than 2%. Moreover, Chikwatula (1.31%) and Malomo (1.25%) cooperatives had the highest mean SOM content, whereas Mwanza (0.87%) cooperative had the lowest mean SOM content, suggesting that land management factors may be responsible for the SOM content on the farms.
The findings of our study indicate that the levels of total nitrogen (TN) and available potassium did not differ significantly across the sampled smallholder macadamia farms. In 70% of the study sites, the TN levels (≤ 0.08%) were below the values considered optimal (≥ 3%) for macadamia growth. We found that the TN content ranged between 0.065% and 0.102% (Fig. 5a). The total soil K+ concentrations were sufficient (Fig. 5b) in four of seven sites compared to the recommended range (200–300 mg kg−1) for macadamia production. Mwanza cooperative recorded the highest mean concentration of K+ (237 mg kg−1), whereas Chikwatula cooperative had the lowest mean concentration of K+ (176 mg kg−1). Significant differences (p ≤ 0.01) were observed regarding available P among the sampled smallholder macadamia farms. Median comparisons showed that Mphaza cooperative had the highest median phosphorus value of 49 mg kg−1, while Chikwatula cooperative had the lowest median P-value of 22 mg kg−1 (Fig. 5c). Overall, only 17% of the sampled soils met or exceeded the recommended threshold of 30 mg kg−1, indicating a general deficiency in soil available P.
Available sulphur concentrations significantly varied (p ≤ 0.02) between macadamia farms among the study sites. The available S concentrations ranged from 2.06 mg kg−1 to 27.03 mg kg−1, with an average of 10.9 mg kg−1 (Fig. 5d). Five of the seven cooperatives had farms with available S concentrations within the recommended range (10–300 mg kg−1) for macadamia production. No significant differences (p > 0.05) were observed in the concentration of available Ca2+ among the study sites (Fig. 5e). Nearly all of the farms under the study had lower Ca2+ levels than desirable for macadamia production. Further, the average calcium concentration of the soils examined (417.9 mg kg−1) was threefold lower than the minimum optimal level (≥ 1200 mg kg−1). Nachisaka cooperative had the highest average concentration of Ca2+ (677 mg kg−1), while Mphaza had the lowest concentration of the nutrient (267 mg kg−1). Only one farm in the Mwanza cooperative had optimal Ca2+ concentrations (1300 mg kg−1) Ca2 + , out of all the sampled farms.
Significant differences were observed in terms of the concentrations of magnesium among the sampled macadamia cooperative farms (p ≤ 0.024). Tithandizane cooperative had the highest average of Mg2+ (84.9 mg kg−1). Mwanza, in contrast, had the lowest average of Mg2+ (38.5 mg kg−1). Despite these differences, available Mg2+ levels at all the study sites were deficient, with an average of 60.4 mg kg−1 (Fig. 5f) below the optimal level of 170 mg kg−1 required for the healthy growth of macadamia trees.
Soil available boron and zinc
The total available boron concentrations among the smallholder macadamia farms ranged from 0.02 to 0.29 mg kg−1, with none exceeding the lower threshold concentration (≥ 1 mg kg−1) recommended for macadamia production (Fig. 6a). Compared to the recommended ranges, we observed that 95% of the soil samples in this study were deficient in B. Zinc, next to boron, is often the most limiting nutrient for macadamia production. Our study has shown that Zn exhibited similar patterns as those of boron, with very low concentrations in all the study areas (≤ 0.4 mg kg−1) than the optimal (≥ 3 mg kg−1) (Fig. 6b). Additionally, 98% of the sampled soils were below the threshold for Zn.
Relationships among soil physical and chemical parameters
This study has shown significant negative relationships between sand content with soil nutrients, cation exchange capacity (R2 = − 0.48), and organic matter content (R2 = − 0.33) (Fig. 7). This suggests that the higher sand concentrations impact the availability of soil nutrients in the soil. Correlations among soil nutrients were negatively significant for available S versus available P, and Ca2+, implying that these nutrients were affected by different factors. We also found significant positive relationships for soil pH with available B, Ca2+, K+, P, Zn, total N, and CEC. In contrast, a strong negative correlation (R2 = − 0.48) was found between available sulphur and soil pH. This suggests that the soil pH was affected by the concentration of sulphur. Furthermore, we found that cation exchange capacity was significantly inversely related to sand (R2 = − 0.48), clay (R2 = − 0.48), and silt (R2 = − 0.33) concentrations among the study sites.
Declining soil fertility and productivity is a major challenge among smallholder farmers in Malawi . Poor soil fertility is recognized 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.
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 [18, 24, 52]. 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 [17, 34].
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 to Lake Malawi (Malomo and Nachisaka cooperative) and Shire valley (Mwanza cooperative) explain why some of the farms in these areas have a higher sand content. However, some soils, as can be seen in (Fig. 2b, d), 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 (Figure 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 13% of the sampled macadamia farms have soil pH levels within the optimum range for the crop (Fig. 3a). This translates to 5% of macadamia farms belonging to Tithandizane cooperative and 2% 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% 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. , 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 [14, 63].
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 (Additionaf file 2: Table S2). According to Munthali et al. , 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 . We have observed that soils from all study sites barely exceed the lower threshold for CEC (Fig. 3b), which for sandy soils fall between 5 and 10 cmol ( +) kg−1 . 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.  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 (Fig. 7). 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.  that kaolinite clays do not contribute much to the CEC provides additional context for our results. Moreover, Bortoluzzi et al.  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 (Fig. 4), 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 (Fig. 5). 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+ (Fig. 5b). 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 (Fig. 5c). 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 (Fig. 5e, f).
Boron and zinc are essential micronutrients required in small but critical amounts for macadamia's normal growth and development . In general, we found that the B and Zn levels on smallholder macadamia farms were below the minimum reference levels for macadamia production (Fig. 6a, b). 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. , 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 .
The absent utilization 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  found that commercial estate producers in the country have increased their B and Zn levels through routine foliar applications.
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 . 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. 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.
Maintaining the proper soil pH is a crucial part of soil fertility management. This is because it optimises nutrient availability, minimises toxic element solubility, and boosts beneficial soil organism activity . As such raising the soil pH to near neutral is important among the smallholder macadamia farmers in Malawi. It is, therefore, recommended that smallholders effectively manage the soil acidity through the application of agricultural lime in conjunction with organic matter management.
Cover crops provide numerous benefits to agroecosystems. 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 [17, 23]. 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 utilize to improve the soil fertility of their farms.
The results of our study reflect that the soil fertility status of the study areas is very low for macadamia production. The study revealed that soils on smallholder macadamia farms in Malawi are predominantly sandy loams, highly acidic, and deficient in essential macro and micronutrients. In addition, the soil's cation exchange capacity and soil organic matter content are low. Poor agronomic practices among the smallholders’s macadamia farming communities have been identified as the primary drivers of the observed soil fertility decline. As there are no ‘‘one size fits all’’ solutions, a combination of management practices is recommended to assist in the restoration of soil fertility. We advise farmers to implement agricultural practices that encourage the build-up of organic matter (such as crop residue incorporation, application of manure, cover cropping, and intercropping) and protect the soil from erosion. This should be coupled with the application of blended foliar inorganic fertilisers containing important macro and micronutrients but should consider the nutrient limitations of each growing area.
Availability of data and materials
The data, compiled code, and files used in this study are freely available on GitHub: https://github.com/EJEYZiE01/Soil-analysis.
Abbas G, Zaman J, Khattak K, Hussain M. Effect of organic manures with recommended dose of NPK on the performance of wheat (Triticum aestivum L). The J Animal Plant Sci. 2012;22(3):683–7.
Aitken RL, Moody PW, Stephenson RA, Gallagher EC, Pignata AJ. Phosphorus fertilizer requirements for macadamia. Queensland: Australian Macadamia Society; 1990.
Amgain R, Khadka D, Joshi S, Malla R. Depth-wise variations of soil physicochemical properties in the apple growing area of Mustang district. Nepal J AgriNatural Resour. 2020;3(2):92–103. https://doi.org/10.3126/janr.v3i2.32486.
Arévalo-Gardini E, Canto M, Alegre J, Loli O, Julca A, Baligar V. Changes in soil physical and chemical properties in long term improved natural and traditional agroforestry management systems of cacao genotypes in Peruvian Amazon. PLoS ONE. 2015;10(7):1–29. https://doi.org/10.1371/journal.pone.0132147.
Asfaw, S, Orecchia, C, Pallante, G, and Palma, A. (2018). Soil and nutrients loss in Malawi: an economic assessment. www.fao.org/publications.
Austin M. Species distribution models and ecological theory: A critical assessment and some possible new approaches. Ecol Model. 2007;200:1–19. https://doi.org/10.1016/j.ecolmodel.2006.07.005.
Barrueto AK, Merz J, Hodel E, Eckert S. The suitability of Macadamia and Juglans for cultivation in Nepal: an assessment based on spatial probability modelling using climate scenarios and in situ data. Reg Environ Change. 2018;18(3):859–71. https://doi.org/10.1007/s10113-017-1225-2.
Bortoluzzi E, Tessier D, Rheinheimer D, Julien J. The cation exchange capacity of a sandy soil in southern Brazil : an estimation of permanent and pH-dependent charges. Eur J Soil Sci. 2006;57:356–64. https://doi.org/10.1111/j.1365-2389.2005.00746.x.
Bouwman TI, Andersson JA, Giller KE. Adapting yet not adopting ? Conservation agriculture in Central Malawi. Agri Ecosyst Environ. 2021;307:107224. https://doi.org/10.1016/j.agee.2020.107224.
Bright J. Macadamia plant protection guide 2015–16 NSW DPI Management Guide. Queensland: Australian Macadamia Society; 2018.
Chesters G, Corey RB. Contribution of organic matter and clay to soil cation-exchange capacity as affected by the pH of the saturating solution. Soil Soc Proceedings. 1964;12:24–34.
Cull BW, Mayer DG, Stephenson RA. Effects of site, climate, cultivar, flushing, and soil and leaf nutrient status on yields of macadamia in South East Queensland. Sci Hortic. 1986;30:227–35. https://doi.org/10.1016/0304-4238(86)90101-9.
Dhaliwal SS, Naresh RK, Mandal A, Singh R, Dhaliwal MK. Environmental and sustainability indicators dynamics and transformations of micronutrients in agricultural soils as in fluenced by organic matter build-up : a review. Environ and Sustain Indicators. 2019;1–2:100007. https://doi.org/10.1016/j.indic.2019.100007.
Dougill AJ, Twyman C, Thomas DSG, Sporton D. Nutrient balance studies for participatory degradation monitoring soil degradation assessment in mixed farming systems of southern Africa : use of nutrient balance studies for participatory degradation monitoring. Geogr J. 2002;168(3):195–210.
Evans N. Macadamia industry consultation: Summary of findings. Lilongwe: AgDevCo; 2020.
Evans N. A nationwide smallholder macadamia agronomic survey. Lilongwe: GIZ-MIERA; 2021.
Eze S, Dougill AJ, Banwart SA, Hermans TDG, Ligowe IS, Thierfelder C. Impacts of conservation agriculture on soil structure and hydraulic properties of Malawian agricultural systems. Soil Tillage Res. 2020;201:104639. https://doi.org/10.1016/j.still.2020.104639.
FAO. Soils for nutrition: State of the art. 2022. https://doi.org/10.4060/cc0900en.
Gashu D, Nalivata PC, Amede T, Ander EL, Bailey EH, Botoman L, Chagumaira C, Gameda S, Haefele SM, Hailu K, Joy EJM, Kalimbira AA, Kumssa DB, Lark RM, Ligowe IS, Mcgrath SP, Milne AE, Mossa AW, Munthali M, Broadley MR. The nutritional quality of cereals varies geospatially in Ethiopia and Malawi. Nature. 2021. https://doi.org/10.1038/s41586-021-03559-3.
Ghimire R, Lamichhane S, Acharya BS, Bista P, Sainju UM. Tillage, crop residue, and nutrient management effects on soil organic carbon in rice-based cropping systems : a review. J Integr Agric. 2017;16(1):1–15. https://doi.org/10.1016/S2095-3119(16)61337-0.
Habumugisha V, Mourad KA, Hashakimana L. The effects of trees on soil chemistry. Current Environ Eng. 2018;6(1):35–44. https://doi.org/10.2174/2212717806666181218141807.
Hazelton P, Murphy B. Interpreting soil test results. Clayton: CSIRO Publishing; 2016. https://doi.org/10.1071/9781486303977.
Hermans TDG, Dougill AJ, Whitfield S, Peacock CL, Eze S, Thierfelder C. Combining local knowledge and soil science for integrated soil health assessments in conservation agriculture systems. J Environ Manage. 2021. https://doi.org/10.1016/j.jenvman.2021.112192.
Huang J, Hartemink AE. Earth-science reviews soil and environmental issues in sandy soils. Earth-Sci Rev. 2020;208:103295. https://doi.org/10.1016/j.earscirev.2020.103295.
International Nut and Dried Fruit Council. Nuts and dried fruits statistical yearbook 2020/2021. Spain: Reus; 2021.
Ligowe IS, Patson CN, Joyce N, Wilkson M, Christian T. Medium-term effects of conservation agriculture on soil quality. Afr J Agric Res. 2017;12(29):2412–20. https://doi.org/10.5897/ajar2016.11092.
Mutegi, J, Vernon, K, and Zingore Shamie, Rebbie Harawa, and L. W. The status of fertilizer recommendation in Malawi gaps challenges, and opportunities. The Malawi Soil Health Consortium. 56. 2015.
Njoloma JP, Weldesemayat GS, Bruce GS, Patson CN, Betserai IN. Soil fertility status under smallholder farmers farms in Malawi. Afr J Agric Res. 2016;11(19):1679–87. https://doi.org/10.5897/ajar2015.10018.
Kumssa DB, Mossa AW, Amede T, Ander EL, Bailey EH, Botoman L. Cereal grain mineral micronutrient and soil chemistry data from geonutrition surveys in Ethiopia and Malawi. Sci Data. 2022. https://doi.org/10.1038/s41597-022-01500-5.
Lakudzala DD. Potassium response in some Malawi soils. Int Lett Chem Phys Astron. 2013;13:175–81. https://doi.org/10.1052/www.scipress.com/ILCPA.13.175.
Li BY, Zhou DM, Cang L, Zhang HL, Fan XH, Qin SW. Soil micronutrient availability to crops as affected by long-term inorganic and organic fertilizer applications. Soil Tillage Res. 2007;96:166–73. https://doi.org/10.1016/j.still.2007.05.005.
Jerzy L, Bogusław U. Quantifying cereal productivity on sandy soil in response to some soil-improving cropping systems. Land. 2021;10:1–16.
Jiménez EM, Maestre FT, Flagmeier M, Guirado E, Berdugo M, Bastida F, Dacal M, Martínez PD, Ochoa R, Rillig MC, Delgado TWCM. Soils in warmer and less developed countries have less micronutrients globally. Global Change Biol. 2022. https://doi.org/10.1111/gcb.16478.
Li G, Messina JP, Peter BG, Snapp SS. Mapping land suitability for agriculture in Malawi. Land Degrad Dev. 2017;28(7):2001–16. https://doi.org/10.1002/ldr.2723.
Lisuma J, Mbega E, Ndakidemi P. Influence of Tobacco plant on macronutrient levels in sandy soils. Agronomy. 2020;10:418.
Maida, J. H. A., & Chilima, Z. W. Changes in soil fertility under continuous cropping of tea. Technical Bulletin. 1976.
Maida JHA. The potassium status of Malawi soils. J Soil Sci. 1980;31(3):595–606.
Commission MNP. An inclusively wealthy and self-reliant nation: Malawi 2063. Lilongwe: Government press; 2022.
Malla R, Shrestha S, Khadka D, Bam CR. soil fertility mapping and assessment of the spatial distribution of Sarlahi District. Nepal Am J Agri Sci. 2020;7(1):8–16.
Matabwa CJ, Rowell DL. The factors limiting crop production on the Mubangwe Farm, Malawi. Soil Use Manag. 1997;13:107–10. https://doi.org/10.1111/j.1475-2743.1997.tb00567.x.
Matter, O. Soil pH and Organic Matter. Nutrient Management. 8. 2009.
Mloza-banda HR, Makwiza CN, Mloza-banda ML. Soil properties after conversion to conservation agriculture from ridge tillage in Southern Malawi. J Arid Environ. 2016;127:7–16. https://doi.org/10.1016/j.jaridenv.2015.11.001.
Mungai LM, Snapp S, Messina JP, Chikowo R, Smith A, Anders E, Richardson RB, Li G, Whitbread AM. Smallholder farms and the potential for sustainable intensification. Front Plant Sci. 2016;7:1–17. https://doi.org/10.3389/fpls.2016.01720.
Munthali C, Kinoshita R, Aiuchi D, Palta J, Tani M. Soil fertility status for potato production in the central highlands of Malawi. Afr J Agric Res. 2021;17(11):1472–9. https://doi.org/10.5897/AJAR2021.15768.
Mupangwa W, Thierfelder C, Ngwira A. Fertilization strategies in conservation agriculture systems with maize-legume cover crop rotations in Southern Africa. Exp Agric. 2017;53(2):288–307. https://doi.org/10.1017/S0014479716000387.
Murphy T, Jones G, Vanclay J, Glencross K. Preliminary carbon sequestration modelling for the Australian macadamia industry related papers. Agrofor Syst. 2012;87:689–98. https://doi.org/10.1007/s10457-012-9589-2.
Nagao MA, Hirae HH. Macadamia: cultivation and physiology. Crit Rev Plant Sci. 1992;10(5):441–70. https://doi.org/10.1080/07352689209382321.
Nájera F, Tapia Y, Baginsky C, Figueroa V, Cabeza R, Salazar O. Evaluation of soil fertility and fertilisation practices for irrigated maize (Zea mays L) under mediterranean conditions in central Chile. J Soil Sci Plant Nutr. 2015;15(1):84–97. https://doi.org/10.4067/S0718-95162015005000008.
Nazif W, Perveen S. Status of micronutrients in soils of District Bhimber (Azad Jammu and Kashmir). J Agric Biol Sci. 2006;1(2):35–40.
Ngwira A, Thierfelder C, Eash N, Lambert DM. Risk and maize-based cropping systems for smallholder Malawi farmers using conservation agriculture technologies. Exp Agric. 2013;1:1–20. https://doi.org/10.1017/S0014479713000306.
Pang Z, Tayyab M, Kong C, Liu Q, Liu Y, Hu C, Huang J, Weng P, Islam W, Lin W. Continuous sugarcane planting negatively impacts soil microbial community structure, soil fertility, and sugarcane agronomic parameters. Microorganisms. 2021;9:1–18.
Nalivata P, Munthali C, Makumba EW, Mbewe WI, Hastings EM. Optimizing fertilizer use within the context of integrated soil fertility management in Zambia. Fertilizer Use Optimization Sub-Saharan Africa. 2017. https://doi.org/10.1079/9781786392046.0210.
Pichakum A, Supaibulwatana K, Chintakovid W, Chanseetis C. Role of temperature and altitude on flowering performances of macadamia nut. Acta Hort. 2014;1024:127–32. https://doi.org/10.1760/ActaHortic.2014.1024.13.
Quiroz, D., Kuepper, B., Wachira, J., and Emmott, A. Value Chain analysis of macadamia nuts in Kenya research commissioned by CBI. Amsterdam, the Netherlands. Profundo. 2019.
Roba TB. Review the effect of mixing organic and inorganic fertilizer on productivity and soil fertility. Open Access Library J. 2018;5:1–11. https://doi.org/10.4236/oalib.1104618.
Rutkowska B, Szulc W, Sosulski T, Stępień W. Soil micronutrient availability to crops affected by long-term inorganic and organic fertilizer applications. Plant Soil Environ. 2014;60(5):198–203.
Saidian M, Godinez LJ, Prasad M. Effect of clay and organic matter on nitrogen adsorption specific surface area and cation exchange capacity in shales (mudrocks ). Natural Gas Sci Eng. 2016;56:1–12. https://doi.org/10.1016/j.jngse.2016.05.064.
Six J, Conant RT, Paul EA, Paustian K. Stabilization mechanisms of SOM implications for C saturation of soils.pdf. Plant Soil. 2002;241:155–76.
Snapp SS. Soil nutrient status of smallholder farms in Malawi. Commun Soil Sci Plant Anal. 1998;29(17–18):2571–88. https://doi.org/10.1080/00103629809370135.
Stephenson RA, Cull BW, Mayer DG, Price G, Stock J. Seasonal patterns of macadamia leaf nutrient levels in southeast Queensland. Sci Hortic. 1986;30(1–2):63–71. https://doi.org/10.1016/0304-4238(86)90082-8.
Stephenson RA, Gallagher EC, Doogan VJ. Leaf nitrogen as a guide for fertilising macadamia. Aust J Exp Agric. 1997;37:599–604. https://doi.org/10.1071/EA96143.
Stevens T, Madani K. Future climate impacts on maize farming and food security in Malawi. Sci Rep. 2016;6:1–14. https://doi.org/10.1038/srep36241.
Steward PR, Dougill AJ, Thierfelder C, Pittelkow CM, Stringer LC, Kudzala M, Shackelford GE. The adaptive capacity of maize-based conservation agriculture systems to climate stress in tropical and subtropical environments : a meta-regression of yields. Agr Ecosyst Environ. 2018;251:194–202. https://doi.org/10.1016/j.agee.2017.09.019.
Suci H, Gong Y, Li P, Sakagami N, Nishiwaki J. A cover crop and no-tillage system for enhancing soil health by increasing soil organic matter in soybean cultivation. Soil Tillage Res. 2021;205:104749. https://doi.org/10.1016/j.still.2020.104749.
The World Bank. Countries and economies. https://data.worldbank.org/country. Accessed in 2022.
Toit, J. P., Nankhuni, F. J., & Kanyamuka, J. S. Can Malawi increase its share of the global macadamia market? Opportunities and threats to the expansion of Malawi's macadamia industry. Poster presentation. Michigan State University. 2017.
Trueman SJ. The reproductive biology of macadamia. Sci Hortic. 2013;150:354–9. https://doi.org/10.1016/j.scienta.2012.11.032.
Tudela J, Martínez M, Valdivia R, Romo J, Portillo M, Rangel R. Cation exchange capacity of kaolinite. Nature. 2010;388:539–47.
Tully KL, Wood SA, Almaraz M, Neill C, Palm C. The effect of mineral and organic nutrient input on yields and nitrogen balances in western Kenya. Agr Ecosyst Environ. 2015;214:10–20. https://doi.org/10.1016/j.agee.2015.08.006.
Van Ranst, Eric, Verloo, M, Demeyer, Pauwel, J. (1999). Manual for the soil chemistry and fertility laboratory-analytical methods for soils and plants equipment and management of consumables.
Wani IA, Mehraj S, Ali MT, Hassan A, Wani SA, Hussain S, Bisati IA. Effect of inorganic and organic fertilisers on yield and soil nutrient status of Walnut Orchard. Int J Plant Soil Sci. 2017;16(2):1–13. https://doi.org/10.9734/IJPSS/2017/32310.
Wiyo KA, Kasomekera ZM, Feyen J. Effect of tied-ridging on soil water status of a maize crop under Malawi conditions. Agric Water Manag. 2000;45:101–25.
Xu Y, Li Y, Li H, Wang L, Liao X, Wang J, Kong C. Science of the total environment effects of topography and soil properties on soil selenium distribution and bioavailability (phosphate extraction): a case study in Yongjia. Sci Total Environ. 2018;633:240–8. https://doi.org/10.1016/j.scitotenv.2018.03.190.
Zhao X, Dong Q. rhizosphere processes and nutrient management for improving nutrient-use efficiency in macadamia production. HortScience. 2019;54(4):603–8. https://doi.org/10.21273/HORTSCI13643-18.
Zuza EJ, Maseyk K, Bhagwat S, Emmott A, Rawes W, Araya YN. Review of macadamia production in Malawi: focusing on what, where, how much is produced and major constraints. Agriculture. 2021;11(2):152. https://doi.org/10.3390/agriculture11020152.
Zuza EJ, Maseyk K, Bhagwat SA, De Sousa K, Emmott A, Rawes W, Araya YN. Climate suitability predictions for the cultivation of macadamia (Macadamia integrifolia) in Malawi using climate change scenarios. PLoS ONE. 2021;16(9):e0257007. https://doi.org/10.1371/journal.pone.0257007.
We thank Prof. Clare Warren and Richard Holliman of The Open University, Prof. Andy Dougill of York University, Dr. Michael G. Chipeta of Oxford University, and Dr. Thirze D.G. Hermans of Wageningen University for their insightful assistance during the research and writing of this paper. We should also thank HIMACUL smallholder farmers for allowing us to collect soil samples from their macadamia farms and for welcoming us. In addition, we would like to thank Mr. Ken Mkangala for his assistance during the fieldwork. However, mistakes and omissions are our responsibility.
The research was supported by The Open University and the UK Research and Innovation via Global Challenges Research Fund (GCRF).
Ethics approval and consent to participate
Consent for publication
The authors declare that they have no known competing financial interests or personal relationships that could be perceived as having influenced the work presented in this paper.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Zuza, E.J., Maseyk, K., Bhagwat, S.A. et al. Factors affecting soil quality among smallholder macadamia farms in Malawi. Agric & Food Secur 12, 17 (2023). https://doi.org/10.1186/s40066-023-00421-9
- Soil fertility
- Smallholder farmers
- Soil organic matter
- Cation exchange capacity