Zinc fertilization increases productivity and grain nutritional quality of cowpea (Vigna unguiculata [L.] Walp.) under integrated soil fertility management

Cowpea (Vigna unguiculata [L.] Walp.) is an important but under-studied grain legume which can potentially contribute to improved dietary zinc (Zn) intake in sub-Saharan Africa. In this study, surveys were conducted on smallholder farms in Zimbabwe during 2014/15 to determine the influence of diverse soil fertility management options on cowpea grain productivity and nutrition quality. Guided by the surveys, field experiments were conducted to investigate the influence of Zn fertilizer on the productivity and quality of cowpea under integrated soil fertility management (ISFM). Experiments were conducted on two soil-types, namely, sandy (6% clay) and red clay (57% clay) in 2014/15 and 2015/16 where cowpea was grown in rotation with staple maize (Zea mays L.) and fertilized with combinations of Zn, nitrogen (N), phosphorus (P) and two organic nutrient resources, cattle manure and woodland leaf litter. Cowpea grain yields on surveyed farms ranged from 0.3 to 0.9 t ha−1, with grain Zn concentration ranging from 23.9 to 30.1 mg kg−1. The highest grain Zn concentration was on fields where organic nutrient resources were applied in combination with mineral N and P fertilizers. Within the field experiments, mean grain yields of cowpea increased by between 12 and 18% on both soil types when Zn fertilizer was applied, from a baseline of 1.6 and 1.1 t ha−1 on red clay and sandy soils, respectively. When Zn fertilizer was co-applied with organic nutrient resources, grain Zn concentrations of cowpea reached 42.1 mg kg−1 (red clay) and 44.7 mg kg−1 (sandy) against grain Zn concentrations of 35.9 mg kg−1 and 31.1 mg kg−1 measured in cowpea grown with no Zn fertilizer on red clay and sandy soils, respectively. Agronomic biofortification of legumes is feasible and has the potential to contribute significantly towards increasing dietary Zn intake by humans. A greater increase in grain Zn of cowpea grown on sandy than red clay soils under Zn fertilization illustrates the influence of soil type on Zn uptake, which should be explored further in agronomic biofortification programs

Exploring mechanisms to restore fertility of degraded lixisols for enhanced crop productivity under Smallholder Farmer Management Systems

Croplands in smallholder farming areas of Southern Africa are in a degraded state, with most cereal and legume crops yielding less than 1 t ha-1 of grain. Restoring productivity of these degraded croplands is therefore key to intensifying crop production to meet household food and nutritional security needs. The objective of this study was to evaluate sequences of integrated soil fertility management (ISFM) technology components for rehabilitation of degraded sandy soils and intensification of maize (Zea mays L.)-based cropping systems in Hwedza and Makoni smallholder farming areas in Eastern Zimbabwe. Farmer participatory research approaches, laboratory characterization, remote sensing and Geographic Information Systems were first employed to formulate criteria for assessing degradation of croplands. Second, field experiments were conducted to evaluate alternate sequences of ISFM options based on nitrogen (N)-fixing green manure and grain legumes, low quality organic resources and mineral N and phosphorus (P) fertilizers for rehabilitating the degraded croplands. Third, biophysical data generated from field experimentation were combined with simulation modelling to best-fit the ISFM sequences to different resource categories of farmers. Farmers delineated croplands as productive, moderately productive, degraded and severely degraded based on common indicator weeds, crop performance and soil physical attributes. Overall, laboratory-based soil chemical and biological properties, and most of the weed species closely matched farmer categorization. Spatially, forty percent of the arable land was classified as degraded to severely degraded. Over a 3-year period, rehabilitation of the degraded sandy soils was positively influenced by high quality organic resource application and P fertilization. Indigenous legumes and sunnhemp (Crotalaria juncea L.) planted on otherwise degraded soils, with addition of mineral P fertilizer, led to higher biomass C and N production than under natural fallow. A combination of biomass generated under indigenous legume fallow (indifallow) and sunnhemp fallow, and cattle manure increased soil basal respiration and microbial biomass. Maize grain yields averaged 2.5 t ha-1 under the legume-based sequences compared with 1 t ha-1 under continuous fertilized maize and natural fallow-based options. Over a 4-year period, ISFM sequences based on cattle manure/woodland litter-, NP fertilizer- and legume-based rotations accumulated the highest maize and soyabean grain yields. This was largely explained by a significant accumulation of soil P. These sequences were more productive than farmers’ designated most-and least-productive fields. Cattle manure-based sequences gave the highest cumulative crop yields, while sunnhemp-based sequences attained the highest increase in soil P. Based on costs of seed, fertilizers and labour, the ISFM sequences gave better financial returns than the farmers’ most-and least-productive fields. When assessed over a 49-year period (1962-2011) using Agricultural Production Systems Simulator (APSIM) under agronomic management of different resource categories of farmers, simulated maize yields of the ISFM sequencing options averaged 3.8, 2.4 and 1.8 t ha-1 for resource-endowed, resource-intermediate and resource-constrained households, respectively. The major conclusions from this study were as follows: (i) ethnopedological approaches could aid assessment of soil degradation on smallholder farms to inform decision-making, (ii) degraded croplands are not beyond remedy, and ISFM sequences based on herbaceous legumes are potential entry points to rehabilitate them and (iii) ISFM sequences are a suitable option for intensifying smallholder cropping systems in Southern Africa through increasing crop yields to meet energy and protein requirements of households, and allowing for systematic allocation of the often limited and variable nutrient, seed and labour resources in space and time.,1. The European Union through the African Conservation Tillage Network (ACT) under the Agroecology-based Aggradation–Conservation Agriculture: Tailoring innovations to combat food insecurity in semiarid Africa (ABACO) project to SOFECSA. 2. Rockefeller Foundation 3. Deutscher Akademischer Austausch Dienst (DAAD

Point of no return? Rehabilitating desgraded soils for increased crop productivity on samllholder farmas in eastern Zimbabwe

Soil degradation is a major threat to Southern Africa's agricultural production. Crops show generally weak responses to mineral fertilizers on degraded soils. A three-year study was conducted between 2009 and 2012 on smallholder farms in eastern Zimbabwe to explore entry points for rehabilitating degraded croplands using principles of integrated soil fertility management (ISFM) supported through farmers' local knowledge of soils. Participatory research approaches were first used to investigate farmers' understanding of soil degradation and the commonly used local diagnostic indicators. Farmers' determinants of degraded soils centered on crop performance, indicator weed species and soil physical attributes, and matched laboratory parameters. Overall, physical and chemical properties of the degraded soils were significantly lower than reported values for productive sandy soils in Zimbabwe. Evaluated on ten degraded field sites of corresponding catenary positions and similar slope, the main ISFM options involved nitrogen-fixing herbaceous legumes planted in the first year, with subsequent addition of cattle manure in the second year. In the third year, the influence of the ISFM options on maize productivity and changes in soil biological activity were then evaluated. Phosphorus was applied every year under each sequence. The controls were natural fallow and continuous maize. The treatments were randomly assigned to plots at each of the experimental sites and replicated across farms. Above-ground biomass carbon (C) and nitrogen (N) accumulation was 3038 kg ha− 1 and 203 kg ha− 1, respectively, under 1-year indigenous legume fallow (indifallow) against 518 kg C ha− 1 and 14 kg N ha− 1 under 1-year natural fallow. Two-year indifallow produced approximately three times the biomass N attained under the 2-year natural fallow. When all the treatments were planted to a maize test crop in the third year, herbaceous legume-based sequences showed the highest response to mineral fertilizer N compared with natural fallow-based sequences and continuous fertilized maize. A regression of maize yields against mineral N fertilizer showed a maximum yield of 2.5 t ha− 1 under the herbaceous legume-based sequences against 1 t ha− 1 under continuous fertilized maize and natural fallow-based options following addition of 120 kg ha− 1 of mineral N fertilizer. ‘Green-start’, a Crotalaria juncea L. (sunnhemp)-based sequence, and ‘Indifallow-start 1’, an indigenous legume-based sequence, gave the highest microbial biomass C (MBC) of 243 mg kg− 1 soil compared with 187 mg kg− 1 soil under continuous maize. Microbial biomass N showed a similar trend. Under ‘Green-start’ and ‘Indifallow-start 1’, MBC to organic C ratio averaged 7; about one and half times more than under natural fallow-based sequences and continuous fertilized maize. Consistent with microbial biomass, soil carbon dioxide (CO2) emission under ‘Green-start’ and ‘Indifallow-start 1’ was 22% higher than under natural fallow-based sequences. Continuous maize treatments gave higher metabolic quotients (qCO2) than legume-based sequences, indicating a lower microbial efficiency under the former. We concluded that short-term restoration of productivity of degraded sandy soils should focus on high quality organic resource application and P fertilization to stimulate microbial activity and induce responses to mineral fertilizers. When coupled to P fertilization, herbaceous legume-based ISFM sequences provide a potential entry point for reversing soil degradation and offer opportunities for increasing crop productivity in dominant smallholder farming systems of Zimbabwe and other parts of Southern Africa.Fil: Nezomba, Hatirarami. University of Zimbabwe. Department of Soil Science and Agricultural Engineering; Zimbabwe. University of Zimbabwe. Soil Fertility Consortium for Southern; ZimbabweFil: Mtambanengwe, Florence. University of Zimbabwe. Department of Soil Science and Agricultural Engineering; Zimbabwe. University of Zimbabwe. Soil Fertility Consortium for Southern; ZimbabweFil: Tittonell, Pablo Adrian. Wageningen University and Research Centre; Holanda. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche; ArgentinaFil: Mapfumo, Paul. University of Zimbabwe. Department of Soil Science and Agricultural Engineering; Zimbabwe. University of Zimbabwe. Soil Fertility Consortium for Southern; Zimbabw

Increasing phosphorus rate alters microbial dynamics and soil available P in a Lixisol of Zimbabwe.

Soil phosphorus (P) deficiency is a major challenge to food security in most parts of sub-Saharan Africa, including Zimbabwe, where farmers largely depend on local organic nutrient resources as fertilizer in the production of crops. Soil microorganisms can contribute to synchronous availability of soil P to plants through regulating immobilization and mineralization cycles of soil P pools but their activity may be influenced by antecedent soil P, P fertilizer application regimes and P uptake by plants. Using soils collected from plots where Crotalaria juncea (high quality), Calliandra calothyrsus (medium quality), cattle manure (variable quality), maize stover and Pinus patula sawdust (both low quality) were applied at the rate of 4 t C ha-1 with 16 kg P ha-1 at the start of every season over 16 seasons. A pot study was conducted to evaluate the influence of increasing inorganic P fertilizer rates (26 and 36 kg P ha-1) on soil microbial dynamics, soil P pools, and maize P uptake. Results indicated that nineteen (19) fungal and forty-two (42) bacterial colonies were identified over the study period. Fungi dominated bacteria on day one, with Aspergillus niger showing a 30-98% abundance that depends on organic resource quality. Overall, microbial diversity peaked activity characterized succession on day 29, which coincided with a significant (P<0.05) increase in P availability. Increasing P rate to 26 kg P ha-1 amplified the microbial diverse peak activity under medium-high quality resources while under the control the peak emerged earlier on day 15. Mucor and Bacillus had peak abundances on day 43 and 57, respectively, across treatments regardless of P rates. Treatment and P rate had a significant (P<0.01) effect on microbial P. Bacteria were more responsive to added P than fungi. Increasing P to 36 kg P ha-1 also stimulated an earlier microbial diverse peak activity under maize stover on day 15. Addition of P alone, without supplying complementary nutrients such as N, did not have a positive effect on maize P uptake. Farmers need to co-apply medium-high quality organic resources with high fertilizer P rates to increase microbial diversity, plant available P and maize growth on sandy soils (Lixisols). Our results suggest that there is a need to reconsider existing P fertilizer recommendations, currently pegged at between 26 and 30 kg P ha-1, for maize production on sandy soils as well as develop new fertilizer formulations to intensify crop production in Zimbabwe

Comparative short-term performance of soil water management options for increased productivity of maize-cowpea intercropping in semi-arid Zimbabwe

Rainfall variability poses a great challenge to rainfed cropping in sub-Saharan Africa. We evaluated, over three cropping seasons, conservation agriculture (reduced tillage and mulching) and farmer prioritized conventional tillage- and mulching-based options on seasonal soil water retention, and subsequent productivity of maize (Zea mays L.) and cowpea (Vigna unguiculata Walp L.) in mono- and inter-crops in Eastern Zimbabwe. The experiments were established on sand and clay soils. The first cropping season (2014/15) received evenly distributed rainfall (hereinafter referred to as ‘wetter’), while the two succeeding seasons (2015/16 and 2016/17) had high incidences of intra-seasonal dry spells (hereinafter referred to as ‘drier’). Overall, conventional tillage had 10–31% and 27–40% more moisture than conservation agriculture treatments on sand and clay soils, respectively. Soil moisture was most retained in intercrop under mulch-based conventional tillage. Maize grain yield during the ‘wetter’ season on sand soil was highest and least (P > 0.05) in intercrop under conservation agriculture (2.3 Mg ha−1) and mulch-based conventional tillage (1 Mg ha−1), respectively. On clay soil, intercrop under mulch-based conventional tillage (2.4 Mg ha−1) yielded the best. During the ‘drier’ seasons, intercrop under mulch-based conventional tillage achieved the best maize grain yield on both sand (1.5 Mg ha−1) and clay (1.4 Mg ha−1) soils. Mulching increased maize grain yield by 55–90% during the ‘drier’ seasons, but reduced water use efficiency (WUE) by approximately 15% during the ‘wetter’ season. Over the three seasons, cowpea grain yield did not exceed 1 Mg ha−1 in both mono- and inter-crops. The study revealed contrasting short-term effects of soil water management options on soil moisture retention and intercropping productivity as dictated by seasonal rainfall variability and soil type. These findings point to the need for tillage and mulching typologies across soil types to minimize negative effects of rainfall variability on crop productivity

Conservation Agriculture in Semi-Arid Zimbabwe: A Promising Practice to Improve Finger Millet (<i>Eleusine coracana</i> Gaertn.) Productivity and Soil Water Availability in the Short Term

Increasing within-season dry spells in Southern Africa in recent years have generated growing interest in conservation agriculture (CA) to secure crop yields, especially under rainfed systems. This study aimed to evaluate the effects of CA on finger millet’s (Eleusine coracana (L.) Gaertn) growth, yield and water use efficiency on nutrient-depleted sandy soils. Five treatments, namely (conventional tillage (control), conventional tillage + mulch (partial CA1), reduced tillage only (partial CA2), reduced tillage + mulching (partial CA3) and reduced tillage + mulching + intercropping (full CA)) were evaluated over two consecutive cropping seasons (2015/16 and 2016/17) on-farm in the village of Chidora in Hwedza District, southeast Zimbabwe. All mulched treatments had 15–32% more soil water content over the two growing seasons compared to the control. The higher soil water content under the mulched treatments significantly improved finger millet growth and development during both seasons as evidenced by the lower number of days to emergence (3 days less), greater shoot biomass, higher number of productive tillers and higher number of fingers produced. The full CA treatment achieved the best finger millet grain yield of 1.07 and 1.29 t ha−1 during the 2015/16 and 2016/17 seasons, respectively. Full CA, partial CA3 and partial CA1 increased finger millet grain yield by 70%, 14% and 17% during the 2015/16 cropping season compared to the control. During the 2016/17 cropping season, a similar trend in finger millet grain yield was observed. Full CA was also among the most efficient methods in terms of water utilization (WUE), especially during the 2015/16 season. We concluded that CA, particularly when practiced in full, was more effective at offsetting the water limitations imposed by intra-seasonal dry spells on finger millet and significantly improved productivity

Combinations of in-field moisture conservation and soil fertility management reduce effect of intra-seasonal dry spells on maize under semi-arid conditions

The high frequency of prolonged intra-season dry spells since the turn of the 21st century continues to heighten risk of crop failure in rainfed cropping systems of Southern Africa including Zimbabwe. This study explored the effects of combining in-field moisture conservation techniques and soil fertility management on maize (Zea mays L.) productivity under rainfed conditions in semi-arid eastern Zimbabwe. Treatment combinations were co-designed with farmers through participatory approaches, and tested on-farm on sandy and clayey soils over three consecutive seasons (2015/16–2017/18). Two tillage practices namely conventional (CT) and reduced tillage (RT), with 30 % mulch cover of dried thatching grass (Hyparrhenia filipendula (L.) Stapf) applied either at planting or tasseling or at both stages, were combined with low (35 kg N ha−1, 14 kg P ha−1and 3 t ha−1 of manure) and high (90 kg N ha−1, 26 kg P ha−1 and 7 t ha−1 of manure) fertilizer application rates in a split-split plot design. Intra-seasonal dry spells were more frequent during the first two seasons (i.e. 2015/16 and 2016/17), while the 2017/18 season was rather wet with well-distributed rains. Soil water content varied significantly (p -1 during the 2015/16 and 2016/17, respectively. However, during the wet 2017/18 season, the combination of RT + mulching at planting + high fertilizer rate yielded the best (3.5 t ha-1). On clayey soil, CT + mulching (at both stages) + high fertilizer rate gave the highest yields of 2.4, 2.9 and 3.4 t ha-1 in 2015/16, 2016/17 and 2017/18, respectively. Water use efficiency (WUE) was greatest under CT + mulching (at both stages) + high fertilizer rate especially during seasons that had high occurrence of intra-seasonal dry spells. Overall, the combination CT + mulching (at both stages) + high fertilizer rate increased maize yield by over 200 % and 300 % compared to the non-mulched treatments on sandy and clay soil, respectively, particularly during seasons with high incidences of intra-seasonal dry spells. Consequently, the treatment combination achieved the best economic returns during the drier seasons although income returns were reduced during the wetter season because of considerable yield loss due to waterlogging, particularly on the sandy soil. We thus conclude that mulching at strategic crop growth stages in combination with tillage and judicious addition of organic and inorganic fertilizers is a promising agronomic technique for reducing effects of intra-seasonal dry spells on maize productivity in rainfed smallholder cropping systems. Increasing farmer access to organic and inorganic fertilizers is, however, key to accelerated adoption of such agronomic techniques.</p

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 CO2-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.Keywords: carbon mineralisation, locally available organic resources, nitrogen mineralisation, nutrient-depleted fields, P-fertilisatio

Mulching and Fertilization Effects on Weed Dynamics under Conservation Agriculture-Based Maize Cropping in Zimbabwe

A two-year study was conducted to assess how mulch influences weed dynamics following imposition of different fertilization treatments under three crop establishment options: (i) conventional; (ii) ripping; and (iii) basin, in a two-year maize-legume rotation. Eight treatments were imposed within each crop establishment option and received maize stover mulch applied at 0% or 30% cover before planting  maize (Zea mays) or cowpea (Vigna unguiculata) as test crops. Maize received nitrogen (N) at 35, 90, or 120 kg·ha−1 and phosphorus (P) at 14 or 26 kg·ha−1 applied alone or in combination with 4 or 7 t cattle manure·ha−1, while cowpea received 8 or 17 N·kg·ha−1 and similar P rates to maize. Results indicated that both weed biomass and diversity were influenced more by fertilization than method of crop establishment. On treatments under high fertilizer application rates, or previously planted to cowpea weed biomass ranged between 220 and 400 g·m−2 under mulch and 370–510 g·m−2 (no mulch). Here species richness ranged between 7–16 and was dominated by dicotyledons. This was in contrast to biomass ranges of 75–200 g·m−2 in the low fertilized and control plots, where only one or two grass types dominated. Overall, weed densities were 6% to 51% higher under conventional tillage compared to the two conservation agriculture (CA) options, although the data indicated that mulch significantly (p &lt; 0.05) depressed weed density by up to 70%. We concluded that mulching could be a potential mechanism for reducing weeding labor costs for smallholders and the general environmental and health concerns associated with the use of herbicides in CA systems

Correlations between maize grain yield and soil chemical properties for soils amended with different organic inputs with or without N fertilization for experimental Years 1, 5 and 9.

<p>Correlations between maize grain yield and soil chemical properties for soils amended with different organic inputs with or without N fertilization for experimental Years 1, 5 and 9.</p