Hatirarami Nezomba

Assessing the potential benefits of organic and mineral fertiliser combinations on legume productivity under smallholder management in Zimbabwe

Productivity of grain legumes on sandy soils of southern Africa is critically limited by marginal fertilisation. Effects of co-applying phosphorus (P)-based mineral fertilisers and organic nutrient resources to cowpea (Vigna unguiculata (L.) Walp.) and soyabean (Glycine max L.) were investigated on smallholder farms in eastern Zimbabwe over two years. Over 70% of the surveyed farmers grew cowpea without fertilisation. Fertilisation of legumes with one or more nutrient resources increased shoot biomass productivity by between 20% and 300% relative to the non-fertilised control. Fertilised soyabean and cowpea yielded 2.2 t grain ha−1 and 2.5 t grain ha−1, respectively, translating to more than double the yields of unfertilised controls. In contrast, sole application of either mineral P-containing fertilisers or organic nutrient resources yielded less than 1 t ha−1 legume grain. The effects of combined organic and mineral fertilisation were also reflected in increased C〇2-carbon evolution from soils following growth of the legumes. Under the same soils, net nitrogen (N) mineralisation was highest where cattle manure was co-applied with an NP-containing fertiliser, with at least 85 mg N kg−1 soil released within six weeks. Co-application of organic and NP-containing fertilisers significantly enhance legume grain yields and residual soil N availability, but most smallholder farmers do not currently use this fertilisation strategy.

Nitrogen fixation and biomass productivity of indigenous legumes for fertility restoration of abandoned soils in smallholder farming systems

Abstract Most legume-based soil fertility technologies often fail to make the desired impact on nutrient-depleted soils partly due to low N2-fixation rates and poor biomass productivity. A study was conducted in the 2004/05 and 2005/06 rainfall seasons to evaluate biomass productivity and N2-fixation of indigenous legumes on nutrient-depleted fields under low (450- 650 mm yr−1) to high (> 800 mm yr−1) rainfall areas of Zimbabwe. Legume species, mostly of Crotalaria, Indigofera and Tephrosia genera, were sown in mixtures on disturbed soil. Indigenous legume fallows (indifallows) produced 2–5 t biomass ha−1 under low and 5–15 t biomass ha−1 under high rainfall. They significantly (P<0.05) out-yielded natural fallows by 84% and a sunnhemp (Crotalaria juncea) green manure fallow by 32% over a growth period of six months. Cumulatively, indifallows produced more biomass (~29 t ha−1) than natural fallows (~ 8 t ha−1) over two seasons. Indigenous legumes derived 61–90% of their N from the atmosphere with amounts fixed ranging from 2–57 kg N ha−1 under medium rainfall conditions to 1–173 kg N ha−1 under high rainfall. Application of P increased indifallow biomass productivity by 15% under low rainfall, and N2-fixation by 32% and 18% under low and high rainfall, respectively. These results demonstrated that indigenous legumes generate sufficiently high biomass and fix nitrogen on nutrient-depleted fields where most conventional green manure and grain legume cultivars often fail to establish.

Sequencing integrated soil fertility management options for sustainable crop intensification by different categories of smallholder farmers in Zimbabwe

SUMMARY Research has proved that integrated soil fertility management (ISFM) can increase crop yields at the field and farm scales. However, its uptake by smallholder farmers in Africa is often constrained by lack of technical guidelines on effective starting points and how the different ISFM options can be combined to increase crop productivity on a sustainable basis. A 4-year study was conducted on sandy soils (<10% clay) on smallholder farms in eastern Zimbabwe to assess how sequencing of different ISFM options may lead to incremental gains in soil productivity, enhanced efficiency of resource use, and increase crop yields at field scale. The sequences were primarily based on low-quality organic resources, nitrogen-fixing green manure and grain legumes, and mineral fertilizers. To enable comparison of legume and maize grain yields among treatments, yields were converted to energy (kilocalories) and protein (kg) equivalents. In the first year, ‘Manure-start’, a cattle manure-based sequence, yielded 3.4 t ha −1 of maize grain compared with 2.5 and 0.4 t ha −1 under a woodland litter-based sequence (‘Litter-start’) and continuous unfertilized maize control, respectively. The ‘Manure-start’ produced 12 × 10 6 kilocalories (kcal); significantly (p < 0.05) out-yielding ‘Litter start’ and a fertilizer-based sequence (‘Fertilizer-start’) by 50%. A soyabean-based sequence, ‘Soyastart’, gave the highest protein production of 720 kg against <450 kg for the other sequencing treatments. In the second year, the sequences yielded an average of 5.7 t ha −1 of maize grain, producing over 19 × 10 6 kcal and 400 kg of protein. Consequently, the sequences significantly out-performed farmers’ designated poor fields by fivefold. In the third year, ‘Soya-start’ gave the highest maize grain yield of 3.7 t ha −1 ; translating to 1.5 and 3 times more calories than under farmers’ designated rich and poor fields, respectively. In the fourth year, ‘Fertilizer-start’ produced the highest calories and protein of 14 × 106 kcal and 340 kg, respectively. Cumulatively over 4 years, ‘Manure-start’ and ‘Soya-start’ gave the highest calories and protein, out-performing farmers’ designated rich and poor fields. Sunnhemp (Crotalaria juncea L.)based sequences, ‘Green-start’ and ‘Fertilizer-start’, recorded the highest gains in plant available soil P of 4m g kg −1 over the 4-year period. Assessment of P agronomic efficiencies showed significantly more benefits under the ISFM-based sequences than under farmers’ designated rich and poor fields. Based on costs of seed, nutrients and labour, ‘Soya-start’ gave the best net present value over the 4 years, while ‘Fertilizer-start’ was financially the least attractive. Overall, the ISFM-based sequences were more profitable than fields designated as rich and poor by farmers. We concluded that ISFM-based sequences can provide options for farm-level intensification by different categories of smallholder farmers in Southern Africa.

Indigenous legumes biomass quality and influence on C and N mineralization under indigenous legume fallow systems

Non-cultivated N2-fixing indigenous legumes can be harnessed to enhance soil fertility replenishment of smallholder farms. Understanding N release patterns of biomass generated by such legumes is key in managing N availability to crops. Nitrogen and C mineralization patterns of indigenous legume species, mainly ofTephrosia andCrotalaria genera, and of soils sampled at termination of 1- and 2-year indigenous legume fallows (indifallows)were investigated in leaching tube incubations under laboratory conditions. With the exception ofTephrosia longipes Meisn (2.4%) andCrotalaria cylindrostachys Welw.ex Baker (1.8%), all indigenous legumes had >2.5% N. Total polyphenols and lignin were <4% and 15%, respectively, for all species.Crotalaria pallida (L.) andEriosema ellipticum Welw.ex Baker mineralized >50% of the added N in the first 30 days of incubation. Similar to mixed plant biomass from natural weed fallow,C. Cylindrostachys immobilized N during the 155-day incubation period. Indifallow fallow biomass reached peak N mineralization 55 days after most legumes had leveled off. Carbon release by legume species closely followedN release patterns,with mostCrotalaria species releasing >500 mg CO2-C kg−1 soil. Soils sampled at termination of fallows reached peak N mineralization in the first 21 days of incubation, with indifallows mineralizing significantly (P<0.05) more N than natural fallows. Application of mineral P fertilizer to indifallows and natural fallows increased C and N mineralization relative to control treatments. It was concluded that (i) indigenous legumes generate biomass of high quality within a single growing season, (ii) the slow N release of biomass generated under indifallow systems suggests that such fallows can potentially be manipulated to enhance N availability to crops, and (iii) N and C mineralization of organic materials in sandy soils is likely controlled by availability of P to the soil microbial pool.

Why organic resources and current fertilizer formulations in Southern Africa cannot sustain maize productivity: Evidence from a long-term experiment in Zimbabwe.

Sustainability of maize-based cropping systems is a major challenge for southern Africa, yet the demand for maize as staple food and animal feed in the region continues to increase. A study was conducted on a sandy clay loam (220 g clay kg-1 soil) at Domboshawa in Zimbabwe to investigate the long-term effects of organic resource quality and application rate, and nitrogen (N) fertilization on soil chemical properties and maize (Zea mays L.) productivity. Crotalaria juncea (high quality), Calliandra calothyrsus (medium quality), cattle manure (variable quality), maize stover and Pinus patula sawdust (both low quality) were incorporated into soil at 4.0 t C ha-1 (high rate) and 1.2 t C ha-1 (low rate) at the start of each cropping season for nine consecutive years. At both high and low application rates, each of the five organic resources was applied in combination with or without mineral nitrogen (N) fertilizer at 120 kg N ha-1. The nine-year period saw maize grain yields declining by 22% to 84% across treatments despite increases in soil organic carbon, total N and available P from 6% to 80%. Crotalaria, Calliandra and manure led to a less steep yield decline. Exchangeable calcium (Ca), magnesium (Mg) and potassium (K), and soil pH explained much of the variation in yield patterns observed under the different organic resource applications. Maize grain yield was positively correlated with exchangeable Ca (r = 0.51), Mg (r = 0.62) and K (r = 0.53), and soil pH (r = 0.49), but negatively correlated with other soil properties over the 9-year period. We concluded that declining soil exchangeable basic cations were the underlying causes of decreasing maize productivity, and was aggravated by use of low rates of organic resource inputs, particularly with N fertilization. Current nutrient management and fertilizer recommendations that emphasize inorganic N, P and K significantly undervalue the role played by organic resources in sustainability of maize cropping systems in southern Africa.