Increasing fertilizer access and use is an essential component for improving crop production and food security in sub-Saharan Africa (SSA). However, given the heterogeneous nature of smallholder ...farms, fertilizer application needs to be tailored to specific farming conditions to increase yield, profitability, and nutrient use efficiency. The site-specific nutrient management (SSNM) approach initially developed in the 1990 s for generating field-specific fertilizer recommendations for rice in Asia, has also been introduced to rice, maize and cassava cropping systems in SSA. The SSNM approach has been shown to increase yield, profitability, and nutrient use efficiency. Yield gains of rice and maize with SSNM in SSA were on average 24% and 69% when compared to the farmer practice, respectively, or 11% and 4% when compared to local blanket fertilizer recommendations. However, there is need for more extensive field evaluation to quantify the broader benefits of the SSNM approach in diverse farming systems and environments. Especially for rice, the SSNM approach should be expanded to rainfed systems, which are dominant in SSA and further developed to take into account soil texture and soil water availability. Digital decision support tools such as RiceAdvice and Nutrient Expert can enable wider dissemination of locally relevant SSNM recommendations to reach large numbers of farmers at scale. One of the major limitations of the currently available SSNM decision support tools is the requirement of acquiring a significant amount of farm-specific information needed to formulate SSNM recommendations. The scaling potential of SSNM will be greatly enhanced by integration with other agronomic advisory platforms and seamless integration of digital soil, climate and crop information to improve predictions of SSNM recommendations with reduced need for on-farm data collection. Uncertainty should also be included in future solutions, primarily to also better account for varying prices and economic outcomes.
•The SSNM approach was adapted for rice, maize and cassava production systems in SSA.•The SSNM approach increased yield, profitability, and nutrient use efficiency.•SSNM had 24% and 69% higher yield than the farmer practice for rice and maize.•Rice and maize yields were 11% and 4% higher in SSNM than local recommendations.•Integration with other digital platforms and databases can improve SSNM tools.
An improved understanding of soil fertility variability and farmers’ resource use strategies is required for targeting soil fertility improving technologies to different niches within farms. We ...measured the variability of soil fertility with distance from homesteads on smallholder farms of different socio-economic groups on two soil types, a granite sand and a red clay, in Murewa, northeast Zimbabwe. Soil organic matter, available P and CEC decreased with distance from homestead on most farms. Soil available P was particularly responsive to management, irrespective of soil type, as it was more concentrated on the plots closest to homesteads on wealthy farms (8–13
mg
kg
−1), compared with plots further from homesteads and all plots on poor farms (2–6
mg
kg
−1). There was a large gap in amounts of mineral fertilizers used by the wealthiest farmers (>100
kg N and >15
kg P per farm; 39
kg
N
ha
−1 and 7
kg
P
ha
−1) and the poorest farmers (<20
kg N and <10
kg P per farm; 19
kg
N
ha
−1 and 4
kg
P
ha
−1). The wealthy farmers who owned cattle also used large amounts of manure, which provided at least 90
kg N and 25
kg P per farm per year (36
kg
N
ha
−1 and 10
kg
P
ha
−1). The poor farmers used little or no organic sources of nutrients. The wealthiest farmers distributed mineral fertilizers evenly across their farms, but preferentially targeted manure to the plots closest to the homesteads, which received about 70
kg N and 18
kg P per plot (76
kg
N
ha
−1 and 21
kg
P
ha
−1) from manure compared with 23
kg N and 9
kg P per plot on the mid-fields (26
kg
N
ha
−1 and 10
kg
P
ha
−1), and 10
kg N and 1
kg P per plot (and ha
−1) on the outfields. Crop allocation on the homefields was most diversified on the wealthiest farms where maize was allocated 41% of the area followed by grain legumes (24%) and paprika (21%). Maize was allocated at least 83% of the homefields on farms with less access to resources. All the farmers invariably applied nutrients to maize but little to groundnut. Maize grain yields were largest on the homefields on the wealthy farms (2.7–5.0
t
ha
−1), but poor across all fields on the poor farms (0.3–1.9
t
ha
−1). Groundnut grain yields showed little difference between farms and plots. N and P partial balances were largest on the wealthy farms, although these fluctuated from season to season (−20 to +80
kg N per farm and 15–30
kg P per farm; average 21
kg
N
ha
−1 and 8
kg
P
ha
−1). The partial balances on the wealthy farms were largest on the homefield (20–30
kg N and 13
kg P per plot; >26
kg
N
ha
−1 and >13
kg
P
ha
−1), but decreased to 10–20
N and 6–9
kg P per plot (<20
kg
N
ha
−1 and 13
kg
P
ha
−1) in mid-fields and −7 to +10
kg N and −1 to +1
kg P per plot (<10
kg
N
ha
−1 and <2
kg
P
ha
−1) in the outfields. N and P balances differed little across plots on the poor farms (−2 to +4
kg per plot; −5 to +4
kg
ha
−1) due to limited nutrients applied and small off-take from small harvests. This study highlights the need to consider soil fertility gradients and the crop and nutrient management patterns creating them when designing options to improve resource use efficiency on smallholder farms.
Nutrient limitation is a major constraint in crop production in sub-Saharan Africa (SSA). Here, we propose a generic and simple equilibrium model to estimate minimum input requirements of nitrogen, ...phosphorus and potassium for target yields in cereal crops under highly efficient management. The model was combined with Global Yield Gap Atlas data to explore minimum input requirements for self-sufficiency in 2050 for maize in nine countries in SSA. We estimate that yields have to increase from the current ca. 20% of water-limited yield potential to approximately 50–75% of the potential depending on the scenario investigated. Minimum nutrient input requirements must rise disproportionately more, with N input increasing 9-fold or 15-fold, because current production largely relies on soil nutrient mining, which cannot be sustained into the future.
•A new method to assess minimum nutrient input requirements for cereals.•Minimum nutrient requirements now in the Global Yield Gap Atlas.•To meet food self-sufficiency by 2050, sub-Saharan Africa maize output must increase upto 4 fold.•Producing sufficient maize on current farmland requires upto 15-fold rise in N input.•Increased N inputs should be matched with management to raise N use efficiency and curb soil degradation in sub-Saharan Africa.
Little is known about productivity of smallholder maize–pigeonpea intercropping systems in sub-Saharan Africa. We conducted a survey of 277 farm households in Northern Tanzania to assess ...socio-economic factors, field management characteristics, and their association with productivity of maize–pigeonpea intercrops. On each farm, crop assessments were focused on a field that the farmer identified as most important for food supply. Variables associated with yields were evaluated using linear regression and regression classification. Biomass production ranged between 1.0 and 16.6 for maize, and between 0.2 and 11.9 t ha
−1
for pigeonpea (at maize harvest). The corresponding grain yields ranged between 0.1 and 9.5 for maize, and between 0.1 and 2.1 t ha
−1
for pigeonpea. Plant density at harvest, number of years the field had been cultivated, slope, weeding, soil fertility class, fertiliser and manure use were significantly associated with variation in maize grain yield, with interactions among the factors. Fields on flat and gentle slopes with plant density above 24,000 ha
−1
had 28% extra yields when fertiliser was applied, while less than 24,000 plants ha
−1
yielded 16% extra yield when manure was applied. Plant density at harvest was the key factor associated with pigeonpea yield; fields with densities above 24,000 plants ha
−1
yielded an average of 1.4 t ha
−1
, while less than 24,000 plants ha
−1
yielded 0.5 t ha
−1
. We conclude that performance of intercrops can be enhanced through application of organic and inorganic nutrient sources, and agronomic interventions including weeding, implementing soil conservation measures on steep slopes and optimising plant density.
Soil fertility varies markedly within and between African smallholder farms, both as a consequence of inherent factors and differential management. Fields closest to homesteads (homefields) typically ...receive most nutrients and are more fertile than outlying fields (outfields), with implications for crop production and nutrient use efficiencies. Maize yields following application of 100
kg
N
ha
−1 and different rates and sources of P were assessed on homefields and outfields of smallholder farms in Zimbabwe. Soil organic carbon, available P and exchangeable bases were greater on the homefields than outfields. In each of three experimental seasons, maize yields in homefield control plots were greater than in the outfields of farms on a granitic sandy and a red-clay soil. Application of mineral N significantly increased maize yields on homefields in the first season (2.1–3.0
t
ha
−1 on the clay soil and 1.0–1.5
t
ha
−1 on the sandy soil) but the effects of N alone were not significant on the outfields due to other yield-limiting factors. Greatest yields of about 6
t
ha
−1 were achieved on the clayey homefield with 100
kg
N
ha
−1 and 30
kg
P
ha
−1 applied as single super phosphate (SSP). Manure application gave greater yields (3–4
t
ha
−1) than SSP (2–3
t
ha
−1) in the sandy homefield and in the clayey outfield. Maize did not respond significantly to N, dolomitic lime, manure and P on the sandy outfield in the first and second seasons. In the third season, manure application (∼17
t
manure
ha
−1
year
−1) on the sandy outfield did result in a significant response in grain yields. Apparent P recovery in the first season was 55–65% when P was applied at 10
kg
ha
−1 on the clayey homefield (SSP), clayey outfield (SSP and manure) and sandy homefield (manure) with apparent P recovery less than 40% when P was applied at 30
kg
ha
−1. On the sandy outfield, P recovery was initially poor (<20%), but increased in the successive seasons with manure application. In a second experiment, less than 60
kg
N
ha
−1 was required to attain at least 90% of the maximum yields of 2–3
t
ha
−1 on the sandy homefield and clayey outfield. N use efficiency varied from >50
kg
grain
kg
−1
N on the infields, to less than 5
kg
grain
kg
−1
N on the sandy outfields. Apparent N recovery efficiency by maize was greatest at small N application rates with P applied. We conclude that blanket fertilizer recommendations are of limited relevance for heterogeneous smallholder farms. Targeted application of mineral fertilizers and manure according to soil type and past management of fields is imperative for improving crop yields and nutrient use efficiencies.
African farming systems are highly heterogeneous: between agroecological and socioeconomic environments, in the wide variability in farmers’ resource endowments and in farm management. This means ...that single solutions (or ‘silver bullets’) for improving farm productivity do not exist. Yet to date few approaches to understand constraints and explore options for change have tackled the bewildering complexity of African farming systems. In this paper we describe the Nutrient Use in Animal and Cropping systems – Efficiencies and Scales (NUANCES) framework. NUANCES offers a structured approach to unravel and understand the complexity of African farming to identify what we term ‘best-fit’ technologies – technologies targeted to specific types of farmers and to specific niches within their farms. The NUANCES framework is not ‘just another computer model’! We combine the tools of systems analysis and experimentation, detailed field observations and surveys, incorporate expert knowledge (local knowledge and results of research), generate databases, and apply simulation models to analyse performance of farms, and the impacts of introducing new technologies. We have analysed and described complexity of farming systems, their external drivers and some of the mechanisms that result in (in)efficient use of scarce resources. Studying sites across sub-Saharan Africa has provided insights in the trajectories of change in farming systems in response to population growth, economic conditions and climate variability (cycles of drier and wetter years) and climate change. In regions where human population is dense and land scarce, farm typologies have proven useful to target technologies between farmers of different production objectives and resource endowment (notably in terms of land, labour and capacity for investment). In such regions we could categorise types of fields on the basis of their responsiveness to soil improving technologies along soil fertility gradients, relying on local indicators to differentiate those that may be managed through ‘maintenance fertilization’ from fields that are highly-responsive to fertilizers and fields that require rehabilitation before yields can improved. Where human population pressure on the land is less intense, farm and field types are harder to discern, without clear patterns. Nutrient cycling through livestock is in principle not efficient for increasing food production due to increased nutrient losses, but is attractive for farmers due to the multiple functions of livestock. We identified trade-offs between income generation, soil conservation and community agreements through optimising concurrent objectives at farm and village levels. These examples show that future analyses must focus at farm and farming system level and not at the level of individual fields to achieve appropriate targeting of technologies – both between locations and between farms at any given location. The approach for integrated assessment described here can be used
ex ante to explore the potential of best-fit technologies and the ways they can be best combined at farm level. The dynamic and integrated nature of the framework allows the impact of changes in external drivers such as climate change or development policy to be analysed. Fundamental questions for integrated analysis relate to the site-specific knowledge and the simplification of processes required to integrate and move from one level to the next.
The effects of cattle manure and inorganic N‐fertilizer application on soil organic carbon (SOC), bulk density, macro‐aggregate stability and aggregate protected carbon were determined on clay and ...sandy soils of the Murewa smallholder farming area, Zimbabwe. Maize was grown in four fields termed homefields (HFs) and outfields (OFs) because of spatial variability induced by management practices and with the following fertility treatments: control (no fertility amelioration), 5, 15 and 25 t/ha cattle manure + 100 kg/ha N applied annually for seven consecutive years. The addition of cattle manure resulted in significant (P < 0.01) increases in SOC, macro‐aggregate stability and aggregate protected carbon in clay soils from at least the 5 t/ha cattle manure rate and was comparable between HFs and OFs on clay soils. Aggregate protected carbon in clay soils was significantly higher from the 15 and 25 t/ha cattle manure rates compared to the 5 t/ha cattle manure treatment. In contrast, only SOC was significantly (P < 0.05) increased with the addition of cattle manure on the sandy soils, while bulk density, macro‐aggregate stability and aggregate protected carbon were not significantly changed. Bulk density was also not significantly (P > 0.05) different on the clay soils. A significant and positive linear relationship (r2 = 0.85) was found between SOC and macro‐aggregate stability, while an r2 value of 0.82 was obtained between SOC and aggregate protected carbon on the clay soils. However, no regressions were performed on data from the sandy soils because of the lack of significant changes in soil physical properties. Application of cattle manure and inorganic N‐fertilizer significantly increased (P < 0.05) maize grain yield on both soil types. Results show that inorganic N‐fertilizer combined with cattle manure at 5–15 t/ha per yr is necessary to increase maize yields and SOC on sandy soils in Murewa, while at least 15 t/ha per yr cattle manure is required on the clay soils to improve physical properties in addition to maize yields and SOC.
In communal areas of NE Zimbabwe, feed resources are collectively managed, with herds grazing on grasslands during the rainy season and mainly on crop residues during the dry season, which creates ...interactions between farmers and competition for organic resources. Addition of crop residues or animal manure is needed to sustain agricultural production on inherently poor soils. Objectives of this study were to assess the effect of village-level interactions on carbon and nutrient flows, and to explore their impact on the long-term productivity of different farm types under climate variability. Crop and cattle management data collected in Murewa Communal area, NE Zimbabwe was used together with a dynamic farm-scale simulation model (NUANCES-FARMSIM) to simulate village-level interactions. Simulations showed that grasslands support most cattle feed intake (c. 75%), and that crop residues produced by non-cattle farmers sustain about 30% of the dry season feed intake. Removal of crop residues (0.3–0.4
t
C
ha
−1
yr
−1) from fields of non-cattle farmers resulted in a long-term decrease in crop yields. No-access to crop residues of non-cattle farmers increased soil C modestly and improved yields in the long-term, but not enough to meet household energy requirements. Harvest of grain and removal of most crop residues by grazing cattle caused a long-term decline in soil C stocks for all farm types. The smallest decrease (−0.5
t
C
ha
−1) was observed for most fertile fields of cattle farmers, who manure their fields. Cattle farmers needed to access 4–10
ha of grassland to apply 3
t of manure ha
−1
yr
−1. Rainfall variability intensifies crop–livestock interactions increasing competition for biomass to feed livestock (short-term effect) or to rehabilitate soils (long-term effect). Prolonged dry seasons and low availability of crop residues may lead to cattle losses, with negative impact in turn on availability of draught power, affecting area under cultivation in consecutive seasons until farmers re-stock. Increasing mineral fertiliser use concurrently with keeping crop residues in fertile fields and allocating manure to poor fields appears to be a promising strategy to boost crop and cattle productivity at village level. The likelihood of this scenario being implemented depends on availability of fertilisers and decision of farmers to invest in rehabilitating soils to obtain benefits in the long-term. Adaptation options cannot be blind to what occurs beyond field and farm level, because otherwise recommendations from research and development do not fit the local conditions and farmers tend to ignore them.
Smallholder farms in sub-Saharan African exhibit substantial heterogeneity in soil fertility, and nutrient resource allocation strategies that address this variability are required to increase ...nutrient use efficiencies. We applied the Field-scale resource Interactions, use Efficiencies and Long-term soil fertility Development (FIELD) model to explore consequences of various manure and fertilizer application strategies on crop productivity and soil organic carbon (SOC) dynamics on farms varying in resource endowment in a case study village in Murewa District, Zimbabwe. FIELD simulated a rapid decline in SOC and maize yields when native woodlands were cleared for maize cultivation without fertilizer inputs coupled with removal of crop residues. Applications of 10 t manure ha
−1
year
−1
for 10 years were required to restore maize productivity to the yields attainable under native woodland. Long-term application of manure at 5 and 3 t ha
−1
resulted in SOC contents comparable to zones of high and medium soil fertility observed on farms of wealthy cattle owners. Targeting manure application to restore SOC to 50–60% of contents under native woodlands was sufficient to increase productivity to 90% of attainable yields. Short-term increases in crop productivity achieved by reallocating manure to less fertile fields were short-lived on sandy soils. Preventing degradation of the soils under intensive cultivation is difficult, particularly in low input farming systems, and attention should be paid to judicious use of the limited nutrient resources to maintain a degree of soil fertility that supports good crop response to fertilizer application.
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