Maize (Zea Mays) is an important staple crop in sub-Saharan Africa (SSA). It provides a significant proportion of the human daily intake of calories and mineral nutrition (Goredema-matongera et al., 2021). The production of maize in SSA is dominated by smallholder farming, generally characterized by little use of inputs on soils with low fertility (Santpoort, 2020; Ten Berge et al., 2019). As a result, maize yields are often limited by multiple nutrient deficiencies, which can be addressed by the use of mineral and organic fertilisers (Goredema‐matongera et al., 2021; Ten Berge et al., 2019; Vanlauwe et al., 2015). It has been recognized decades ago that soils which have been cropped with little or no inputs for prolonged periods lack not only the macronutrients but also micronutrients to sustain crop growth (Kang and Osiname, 1985; Rodel and Hopley, 1972). Nevertheless, soil fertility and crop nutrition research in SSA has mainly focused on macronutrients, i.e. nitrogen (N), phosphorus (P) and potassium (K) (Kihara et al., 2017; Stoorvogel et al., 1993; Vanlauwe et al., 2015). Research on micronutrient deficiencies in crops has received less frequent attention (Mutsaers et al., 2017).
With regard to maize, studies on yield response to micronutrient fertilisation in SSA have often been conducted for only limited sets of locations with either a positive or absent yield response (Abbas et al., 2007; Abunyewa and Mercer-Quarshie, 2003; Chiezey, 2014; Chilimba et al., 1999; Eteng et al., 2014; Njoroge et al., 2018; Osiname et al., 1973; Shehu et al., 2018; Yerokun and Chirwa, 2014). Other studies have focused on the effect of micronutrient fertilisation on yields at the regional or global scale in order to understand where micronutrients may be yield-limiting. In 1990, Sillanpää (1990) published the results of 190 single-micronutrient omission field trials distributed over 15 countries. It was found that among all micronutrients, zinc (Zn) was most of the time yield-limiting, with a positive yield response to Zn fertilisation for 49% of all locations. More recently, maize nutrient omission trials including treatments with a mixture of secondary nutrients and micronutrients have been conducted across various countries in SSA. Kihara et al. (2017, 2016) concluded that application of secondary and micronutrients (calcium (Ca), magnesium (Mg), sulphur (S), boron (B) and zinc (Zn)) increased maize yields in several SSA countries by 0.8 Mg ha-1 on average, an increase of 25% compared to application of NPK alone. Similar results were found by Wortmann et al. (2019), who reported a mean increase in maize yields between 20 and 30% when S, Zn and B were fertilised. These studies suggest that secondary and micronutrient deficiencies limit maize yields across SSA. On the other hand, Rurinda et al. (2020) concluded that the overall maize yield response to secondary and micronutrients (S, Ca, Mg, Zn, and B) was small, i.e. between 0 and 0.3 Mg ha-1, across all studied sites in Nigeria, Tanzania and Ethiopia.
The aforementioned studies by Kihara et al. (2017) and Wortmann et al. (2019) suggest that deficiencies of secondary and micronutrients hamper maize yields across SSA. Since mixtures of secondary and micronutrients were used in these studies, it remains unclear which particular micronutrients are deficient at which locations. Furthermore, using mixtures of nutrients makes it challenging to identify soil properties that explain particular nutrient limitations for maize growth (Kihara et al., 2017). Such analyses are however indispensable for extending existing science-based fertiliser recommendation schemes that currently include only NPK, with secondary and micronutrients (Rurinda et al., 2020; Sattari et al., 2014).
Apart from yield quantity (Abbas et al., 2007; Abunyewa and Mercer-Quarshie, 2003; Kihara et al., 2017; Manzeke et al., 2014; Njoroge et al., 2017; Sillanpää, 1990), Zn is also relevant for human health and insufficient intake can result in severe health issues. More than 17.3% of the global population are prone to insufficient Zn intake (Kiran et al., 2022) and 50% of all children in SSA are estimated to be at risk of Zn deficiency (Black et al., 2008). The risk of human Zn deficiency is considered high especially in Eastern and Southern African countries (Joy et al., 2014). Micronutrient deficiencies in humans are widespread in regions where crops are grown in soils with low micronutrient levels, as soil availability determines plant uptake and therefore micronutrient concentrations in the edible parts of plants (Cakmak, 2004; Dimkpa and Bindraban, 2016; Gashu et al., 2021; Manzeke et al., 2012). Berkhout et al. (2019) indeed found significant relations between soil concentrations of micronutrients such as Zn and Cu in SSA, and prevalence of child mortality, stunting, wasting and underweight, which are typical health problems associated with micronutrient deficiencies. However, current assessments of possible micronutrient deficiencies among humans are based on standard food composition tables and consequently do not take into account variability in soil properties and associated soil Zn availability, which can significantly affect grain Zn concentrations, and subsequent Zn intake by humans (Gashu et al., 2021; Manzeke et al., 2012). It has been shown that increasing soil Zn availability through fertilisation is a feasible strategy to increase grain Zn concentrations, and thereby reduce the risk for human Zn deficiency (Cakmak, 2008; de Valença et al., 2017; Joy et al., 2015; Manzeke et al., 2012), also known as agronomic biofortification (Kiran et al., 2022). Next to soil Zn availability, the genetic variation among cultivated maize varieties has great implications on the Zn uptake from the soil, and the translocation of Zn to the edible parts (Brkic et al., 2004; Oikeh et al., 2007). Knowledge on the effect of soil properties and maize variety on total Zn uptake and associated grain Zn concentrations, and how these factors affect the effectiveness of agronomic biofortification, can enhance target-based intervention programs to combat human Zn deficiencies.
Soil Zn availability for plant uptake decreases with increasing pH, due to precipitation and increased adsorption to reactive surfaces such as soil organic matter and metal (hydr)oxides (Alloway, 2009; Van Eynde et al., 2022). With increased amounts of soil organic matter, the availability of Zn may decrease due to increased adsorption (Van Eynde et al., 2022), or increase due to soil organic matter mineralization (Tella et al., 2016) or formation of soluble organic Zn complexes (Hernandez-Soriano et al., 2013). Different chemical extractions have been formulated to evaluate soil Zn availability for plant uptake, the associated yield response to Zn fertilisation (Chilimba et al., 1999; Duffner et al., 2013; Lindsay and Norvell, 1978; Mertens and Smolders, 2013) and Zn concentrations in the edible plant parts (Kihara et al., 2020; Manzeke et al., 2012). For example, a soil test with diethylenetriaminepentaacetic acid (DTPA) as chelating agent is widely used for near-neutral and calcareous soils (Lindsay and Norvell, 1978), while others have used acidic soil extracts such as HCl or Mehlich-3 (M3) for more acidic soils (Alloway, 2009; Mehlich, 1984; Mertens and Smolders, 2013). The DTPA and M3 soil extracts are currently most often used for Zn fertiliser recommendations and critical extractable soil Zn levels have been derived below which a positive maize yield response to Zn-fertilisation can be expected. Based on field and greenhouse experiments, these critical soil Zn levels range from 1-2.5 mg kg-1 Zn-M3 (Chilimba et al., 1999; Cuesta et al., 2020; Wendt, 1995), or 0.5–1 mg kg-1 Zn-DTPA (Chilimba et al., 1999; Cuesta et al., 2020; Lindsay and Norvell, 1978). Alternatively, weak salt extractions such as 0.01 M CaCl2 have been used for measuring soil available Zn (Houba et al., 2000), assuming that these extractions approximate more the directly available pool for plant uptake (Duffner et al., 2013; Menzies et al., 2007). Validation of soil extracts such as DTPA, M3 or 0.01 M CaCl2 as diagnostic criteria for Zn availability to field-grown maize, however, is limited.
Therefore, this study aims to test whether soil properties can be used to predict where Zn availability is limiting maize yields (quantity) and grain Zn concentrations (quality), and whether the application of Zn fertilisers increases yield quantity and/or quality. Using Zn fertiliser omission trials in several African countries, we aimed to test the following hypothesis, namely that crop yield, Zn uptake and grain Zn, and their response to Zn fertilisation, can be predicted based on soil parameters that have been shown before to predict Zn in the soil solution: pH, soil organic matter, the Zn quantity, and perhaps metal (hydr)oxides (Van Eynde et al., 2022). Findings from this study will help to understand under which circumstances Zn fertilisation can increase maize yields, as well as grain Zn concentrations in SSA.