Managing soil fertility diversity to enhance resource use efficiencies in smallholder farming systems: a case from Murewa District,Zimbabwe

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.     

Raising the productivity of smallholder farms under semi-arid conditions by use of small doses of manure and nitrogen: a case of participatory research

Participatory on-farm trials were conducted for three seasons to assess the benefits of small rates of manure and nitrogen fertilizer on maize grain yield in semi-arid Tsholotsho, Zimbabwe. Two farmer resource groups conducted trials based on available amounts of manure, 3 t ha⁻¹ (low resource group) and 6 t ha⁻¹ (high resource group). Maize yields varied between 0.15 t ha⁻¹ and 4.28 t ha⁻¹ and both absolute yields and response to manure were strongly related to rainfall received across seasons (P < 0.001). The first two seasons were dry while the third season received above average rainfall. Maize yields within the seasons were strongly related to N applied (R ² = 0.77 in season 1, and R ² = 0.88 and 0.83 in season 3) and other beneficial effects of manure, possibly availability of cations and P. In the 2001–2002 season (total rainfall 478 mm), application of 3 and 6 t ha⁻¹ of manure in combination with N fertilizer increased grain yield by about 0.14 and 0.18 t ha⁻¹, respectively. The trend was similar for the high resource group in 2002–2003 although the season was very dry (334 mm). In 2003–2004, with good rainfall (672 mm), grain yields were high even for the control plots (average 1.2 and 2.7 t ha⁻¹). Maize yields due to manure applications at 3 and 6 t ha⁻¹ were 1.96 and 3.44 t ha⁻¹, respectively. Application of 8.5 kg N ha⁻¹ increased yields to 2.5 t ha⁻¹ with 3 t ha⁻¹ of manure, and to 4.28 t ha⁻¹ with 6 t ha⁻¹ of manure. In this area farmers do not traditionally use either manure or fertilizer on their crops, but they actively participated in this research during three consecutive seasons and were positive about using the outcomes of the research in future. The results showed that there is potential to improve livelihoods of smallholder farmers through the use of small rates of manure and N under semi-arid conditions.     

Multiple benefits of manure: The key to maintenance of soil fertility and restoration of depleted sandy soils on African smallholder farms

Manure is a key nutrient resource on smallholder farms in the tropics, especially on poorly buffered sandy soils, due to its multiple benefits for soil fertility. Farmers preferentially apply manure to fields closest to homesteads (homefields), which are more fertile than fields further away (outfields). A three-year experiment was established on homefields and outfields on sandy and clayey soils to assess the effects of mineral nitrogen (N) fertilizer application in combination with manure or mineral phosphorus (P) on maize yields and soil chemical properties. Significant maize responses to application of N and manure were observed on all fields except the depleted sandy outfield. Large amounts of manure (17 t ha⁻¹ year⁻¹) were required to significantly increase soil organic carbon (SOC), pH, available P, and base saturation, and restore productivity of the depleted sandy outfield. Sole N as ammonium nitrate (100 kg N ha⁻¹) or in combination with single superphosphate led to acidification of the sandy soils, with a decrease of up to 0.8 pH units after three seasons. In a greenhouse experiment, N and calcium (Ca) were identified as deficient in the sandy homefield, while N, P, Ca, and zinc (Zn) were deficient or low on the sandy outfield. The deficiencies of Ca and Zn were alleviated by the addition of manure. This study highlights the essential role of manure in sustaining and replenishing soil fertility on smallholder farms through its multiple effects, although it should be used in combination with N mineral fertilizers due to its low capacity to supply N.     

Recycling of legume residues for nitrogen economy and higher productivity in maize (<Emphasis Type="Italic">Zea mays</Emphasis>)–wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) cropping system

Contribution of legumes towards N economy in cereal-based cropping systems is well-known but there has been a gradual decline in the cultivation of grain legumes, threatening sustainability of maize (Zea mays)–wheat (Triticum aestivum) cropping system in north-western India. A study was made to evaluate and quantify the effect of different grain legumes on productivity, profitability, N economy and soil fertility in maize–wheat cropping system at New Delhi during 2002–2004. Five legumes, viz. blackgram (Vigna mungo), greengram (Vigna radiata), cowpea (Vigna unguiculata), groundnut (Arachis hypogaea) and soybean (Glycine max) were either intercropped with maize or grown in sole cropping, and their residues were incorporated before the following crop of wheat, which was grown with varying rates of N, viz. 0, 40, 80 and 120 kg N ha⁻¹. Maize-equivalent productivity was significantly more with intercropped greengram (16.1–29.9%), cowpea (24.8%) and groundnut (11.1–16.6%) than in sole maize. Land equivalent ratio and other competitive functions were favourably influenced with intercropped maize + greengram and maize + cowpea. Addition of N through legume residues varied from 11.5–38.5 kg ha⁻¹ in intercropped system and 17.5–83.5 kg ha⁻¹ in sole cropping, which improved productivity of following wheat to a variable extent. Nitrogen economy in wheat was 21 kg ha⁻¹ due to residue incorporation of intercropped greengram, cowpea and groundnut; and 49–56 kg N ha⁻¹ of sole cropped greengram and groundnut. Residual soil fertility in terms of organic C and KMnO₄-N showed an improvement under maize-based intercropping systems followed by wheat, and the beneficial effect was more pronounced with sole cropping of legumes due to greater addition of residues. Apparent N balance as well as actual change in KMnO₄-N at the end of study was positive in most intercropped legumes as well as sole cropping systems, with greater improvement noticed under groundnut, soybean and greengram. Net returns were marginal with maize-based intercropping or sole cropping of legumes, but improved considerably with wheat, particularly when greengram, cowpea and groundnut were grown in the previous season. The studies suggested that inclusion of grain legumes, particularly greengram, cowpea and groundnut was beneficial for improving productivity, profitability, N economy and soil fertility in maize–wheat cropping system.     

Conservation tillage,local organic resources and nitrogen fertilizer combinations affect maize productivity,soil structure and nutrient balances in semi-arid Kenya

Smallholder land productivity in drylands can be increased by optimizing locally available resources, through nutrient enhancement and water conservation. In this study, we investigated the effect of tillage system, organic resource and chemical nitrogen fertilizer application on maize productivity in a sandy soil in eastern Kenya over four seasons. The objectives were to (1) determine effects of different tillage-organic resource combinations on soil structure and crop yield, (2) determine optimum organic–inorganic nutrient combinations for arid and semi-arid environments in Kenya and, (3) assess partial nutrient budgets of different soil, water and nutrient management practices using nutrient inflows and outflows. This experiment, initiated in the short rainy season of 2005, was a split plot design with 7 treatments involving combinations of tillage (tied-ridges, conventional tillage and no-till) and organic resource (1 t ha⁻¹ manure + 1 t ha⁻¹ crop residue and; 2 t ha⁻¹ of manure (no crop residue) in the main plots. Chemical nitrogen fertilizer at 0 and 60 kg N ha⁻¹ was used in sub-plots. Although average yield in no-till was by 30–65% lower than in conventional and tied-ridges during the initial two seasons, it achieved 7–40% higher yields than these tillage systems by season four. Combined application of 1 t ha⁻¹ of crop residue and 1 t ha⁻¹ of manure increased maize yield over sole application of manure at 2 t ha⁻¹ by between 17 and 51% depending on the tillage system, for treatments without inorganic N fertilizer. Cumulative nutrients in harvested maize in the four seasons ranged from 77 to 196 kg N ha⁻¹, 12 to 27 kg P ha⁻¹ and 102 to 191 kg K ha⁻¹, representing 23 and 62% of applied N in treatments with and without mineral fertilizer N respectively, 10% of applied P and 35% of applied K. Chemical nitrogen fertilizer application increased maize yields by 17–94%; the increases were significant in the first 3 seasons (P < 0.05). Tillage had significant effect on soil macro- (>2 mm) and micro-aggregates fractions (<250 μm >53 μm: P < 0.05), with aggregation indices following the order no-till > tied-ridges > conventional tillage. Also, combining crop residue and manure increased large macro-aggregates by 1.4–4.0 g 100 g⁻¹ soil above manure only treatments. We conclude that even with modest organic resource application, and depending on the number of seasons of use, conservation tillage systems such as tied-ridges and no-till can be effective in improving crop yield, nutrient uptake and soil structure and that farmers are better off applying 1 t ha⁻¹ each of crop residue and manure rather than sole manure.     

Effects of nutrient cycling on grain yields and potassium balance

Soybean-maize rotation is a profitable cropping system and is used under rain fed conditions in north China. Since crop yields have been reported to decrease when K fertilizers are not used, we analyzed the productivity trends, soil-exchangeable and non-exchangeable K contents, and K balance in a continuous cropping experiment conducted in an area with an alfisol soil in the Liaohe River plain, China. The trial, established in early 1990 and continued till 2007, included 8 combinations of recycled manure and N, P, and K fertilizers. In the unfertilized plot, the yields of soybean and maize were 1,486 and 4,124 kg ha⁻¹ respectively (mean yield over 18 years). The yields of both soybean and maize increased to 2,195 and 7,476 kg ha⁻¹, respectively, in response to the application of inorganic N, P, and K fertilizers. The maximum yields of soybean (2,424 kg ha⁻¹) and maize (7,790 kg ha⁻¹) were obtained in the plots under treatment with N, P, and K fertilizers and recycled manure. K was one of the yield-limiting macronutrients: regular K application was required to make investments in the application of other mineral nutrients profitable. The decrease in the yields of soybean and maize owing to the absence of K application averaged 400 and 780 kg ha⁻¹, respectively. Soybean seed and maize grain yields significantly increased with the application of recycled manure. For both these crops, the variation coefficients of grain were lower with treatments that included recycled manure than without treatment. After 18 years, the soil-exchangeable and non-exchangeable K concentrations decreased; the concentrations in the case of treatments that did not include K fertilizers were not significantly different. Treatment with N, P, and K fertilizers appreciably improved the fertility level of the soil, increased the concentration of soil-exchangeable K, and decreased the non-exchangeable K concentration. In soils under treatment with N, P, and K fertilizers and recycled manure, the soil-exchangeable and non-exchangeable K levels in the 0–20 cm-deep soil layer increased by 34% and 2%, respectively, over the initial levels. Both soil-exchangeable and non-exchangeable K concentrations were the highest with on treatment with N, P, and K fertilizers and recycled manure, followed by treatment with N, P, and K fertilizers. These concentrations were lowest in unfertilized soils; the other treatments yielded intermediate results. The results showed a total removal of K by the crops, and the amount removed exceeded the amount of K added to the soil; in treatments that did not include K fertilizers, a net negative K balance was observed, from 184 to 575 kg ha⁻². The combined use of N, P, and K fertilizers and recycled manure increased the K content of the 0–20 cm-deep soil layer by 125% compared to the increase obtained with the application of N, P, and K fertilizers alone. The results clearly reveal that current mineral fertilizer applications are inadequate; instead, the annual application of recycled manure along with N, P, and K fertilizers could sustain future yields and soil productivity.     

Effects of fertilization on nutrient budget and nitrogen use efficiency of farmland soil under different precipitations in Northeastern China

Based on a consecutive 16-year field trial and meteorological data, the effects of fertilization on the nutrient budget and nitrogen use efficiency in farmland soil under different precipitation years were studied. With no fertilization treatment, the grain yield of maize was 3,520 kg ha⁻¹ (mean yield over 13 years). But the maximum yield increased to 7,470 kg ha⁻¹ when treated with mineral N, P and K fertilizers and recycled manure. The nutrient uptake also increased by twofold to threefold in NPKM treated field compared with that in the control treatment. The highest yields were obtained in years with normal precipitation, despite the different fertilization schemes. The lowest yields were obtained in drought or waterlogging years, which were 44.7–58.5% of the yields in years with normal precipitation. It also appeared that the deficits of N, P and K were greater in the years with proper precipitation than those in arid or flood years, because more production was removed from the field. Soil total N decreased significantly when treated with mineral fertilizer or recycled manure alone. The maximum deficit of soil total N was observed in control treatment (557 kg ha⁻¹) from 1990 to 2005. The N treatment resulted in a significant negative balance of P, due to the high yield of the crop in response to applied N. The application of NP or N to soils resulted in a greater negative K balance than that of the control. The greatest negative balance of total P and available P were obtained under the control and N treatment, and the highest deficit of soil total K and exchangeable K were obtained under NP treatment. We found that the rate of 150 kg N ha⁻¹ year⁻¹ was inadequate for maintaining soil N balance, and amendment of soil with organic source could not stop the loss of soil P and K. The applying rates of 150 kg N ha⁻¹ year⁻¹, 25 kg P ha⁻¹ year⁻¹, and 60 kg K ha⁻¹ year⁻¹ combined with 2–3 t ha⁻¹ organic manure were recommended to maintain soil fertility level. The nitrogen use efficiency (NUE) was greatly improved in the years with proper precipitation and balanced fertilization. Higher NUE and grain yields were achieved under NPK and NPKM treatments in years with normal precipitation. The results clearly demonstrated that both organic and mineral fertilizers were needed to increase crop production, improve NUE and maintain soil fertility level.     

Crop residue,manure and fertilizer in dryland maize under reduced tillage in northern China: I grain yields and nutrient use efficiencies

The rapidly increasing population and associated quest for food and feed in China has led to increased soil cultivation and nitrogen (N) fertilizer use, and as a consequence to increased wind erosion and unbalanced crop nutrition. In the study presented here, we explored the long-term effects of various combinations of maize stover, cattle manure and nitrogen (N) and phosphorus (P) fertilizer applications on maize (Zea mays L.) yield and nutrient and water use efficiencies under reduced tillage practices. In a companion paper, we present the effects on nutrient balances and soil fertility characteristics. The ongoing factorial field trial was conducted at Shouyang Dryland Farming Experimental Station in northern China from 1993 onwards. The incomplete, determinant-optimal design comprised 12 treatments, including a control treatment, in duplicate. Grain yields and N, P, and potassium (K) uptakes and N, P and K use efficiencies were greatly influenced by the amount of rain during the growing season (GSR), and by soil water at sowing (SWS). There were highly significant interactions between GSR and added stover and manure, expressed in complex annual variations in grain yield and N, P and K use efficiencies. Annual mean grain yields ranged from 3,000 kg ha⁻¹ to 10,000 kg ha⁻¹ and treatment mean yields from 4,500 kg ha⁻¹ to 7,000 kg ha⁻¹. Balanced combination of stover (3,000–6,000 kg), manure (1,500–6,000 kg) and N fertilizer (105 kg) gave the highest yield. Stover and manure were important for supplying K, but the effects differed greatly between years. Overall mean N recovery efficiency (NRE) ranged from 28% to 54%, depending on N source. NRE in wet years ranged from 50% to 90%. In conclusion, balanced combinations of stover, manure and NP fertilizer gave the highest yield and NRE. Reduced tillage with adding stover and manure in autumn prior to ploughing is effective in minimizing labor requirement and wind erosion. The potentials of split applications of N fertilizer, targeted to the need of the growing crop (response farming), should be explored to further increase the N use efficiency.     

Impacts of heterogeneity in soil fertility on legume-finger millet productivity,farmers’ targeting and economic benefits

Targeting of integrated management practices for smallholder agriculture in sub-Saharan Africa is necessary due to the great heterogeneity in soil fertility. Experiments were conducted to evaluate the impacts of landscape position and field type on the biomass yield, N accumulation and N₂-fixation by six legumes (cowpea, green gram, groundnut, mucuna, pigeonpea and soyabean) established with and without P during the short rain season of 2005. Residual effects of the legumes on the productivity of finger millet were assessed for two subsequent seasons in 2006 in two villages in Pallisa district, eastern Uganda. Legume biomass and N accumulation differed significantly (P < 0.001) between villages, landscape position, field type and P application rate. Mucuna accumulated the most biomass (4.8–10.9 Mg ha⁻¹) and groundnut the least (1.0–3.4 Mg ha⁻¹) on both good and poor fields in the upper and middle landscape positions. N accumulation and amounts of N₂-fixed by the legumes followed a similar trend as biomass, and was increased significantly by application of P. Grain yields of finger millet were significantly (P < 0.001) higher in the first season after incorporation of legume biomass than in the second season after incorporation. Finger millet also produced significantly more grain in good fields (0.62–2.15 Mg ha⁻¹) compared with poor fields (0.29–1.49 Mg ha⁻¹) across the two villages. Participatory evaluation of options showed that farmers preferred growing groundnut and were not interested in growing pigeonpea and mucuna. They preferentially targeted grain legumes to good fields except for mucuna and pigeonpea which they said they would grow only in poor fields. Benefit-cost ratios indicated that legume-millet rotations without P application were only profitable on good fields in both villages. We suggest that green gram, cowpea and soyabean without P can be targeted to good fields on both upper and middle landscape positions in both villages. All legumes grown with P fertiliser on poor fields provided larger benefits than continuous cropping of millet.     

Effect of planting technique and amendment type on pearl millet yield, nutrient uptake, and water use on degraded land in Niger

Due to increased population pressure and limited availability of fertile land, farmers on desert fringes increasingly rely on marginal land for agricultural production, which they have learned to rehabilitate with different technologies for soils and water conservation. One such method is the indigenous zai technique used in the Sahel. It combines water harvesting and targeted application of organic amendments by the use of small pits dug into the hardened soil. To study the resource use efficiency of this technique, experiments were conducted 1999–2000, on-station at ICRISAT in Niger, and on-farm at two locations on degraded lands. On-station, the effect of application rate of millet straw and cattle manure on millet dry matter production was studied. On-farm, the effects of organic amendment type (millet straw and cattle manure, at the rate of 300 g per plant) and water harvesting (with and without water harvesting) on millet grain yield, dry matter production, and water use were studied. First, the comparison of zai vs. flat planting, both unamended, resulted in a 3- to 4-fold (in one case, even 19-fold) increase in grain yield on-farm in both years, which points to the yield effects of improved water harvesting in the zai alone. Zai improved the water use efficiency by a factor of about 2. The yields increased further with the application of organic amendments. Manure resulted in 2–68 times better grain yields than no amendment and 2–7 times better grain yields than millet straw (higher on the more degraded soils). Millet dry matter produced per unit of manure N or K was higher than that of millet straw, a tendency that was similar for all rates of application. Zai improved nutrient uptake in the range of 43–64% for N, 50–87% for P and 58–66% for K. Zai increased grain yield produced per unit N (8 vs. 5 kg kg⁻¹) and K (10 vs. 6 kg kg⁻¹) compared to flat; so is the effect of cattle manure compared to millet straw (9 vs. 4 kg kg⁻¹, and 14 vs. 3 kg kg⁻¹), respectively, Therefore zai shows a good potential for increasing agronomic efficiency and nutrient use efficiency. Increasing the rate of cattle manure application from 1 to 3 t ha⁻¹ increased the yield by 115% TDM, but increasing the manure application rate further from 3 to 5 t ha⁻¹ only gave an additional 12% yield increase, which shows that optimum application rates are around 3t ha⁻¹.     

Comparative short-term effects of different quality organic resources on maize productivity under two different environments in Zimbabwe

Major challenges for combined use of organic and mineral nutrient sources in smallholder agriculture include variable type and quality of the resources, their limited availability, timing of their relative application and the proportions at which the two should be combined. Short-term nutrient supply capacity of five different quality organic resources ranging from high to low quality, namely Crotalaria juncea, Calliandra calothyrsus, cattle manure, maize stover and Pinus patula sawdust were tested in the field using maize as a test crop. The study was conducted on two contrasting soil types at Makoholi and Domboshawa, which fall under different agro-ecological regions of Zimbabwe. Makoholi is a semi-arid area (<650 mm yr⁻¹) with predominantly coarse sandy soils containing approximately 90 g kg⁻¹ clay while Domboshawa (>750 mm yr⁻¹) soils are sandy-clay loams with 220 g kg⁻¹ clay. Each organic resource treatment was applied at low (2.5 t C ha⁻¹) and high (7.5 t C ha⁻¹) biomass rates at each site. Each plot was sub-divided into two with one half receiving 120 kg N ha⁻¹ against zero in the other. At Makoholi, there was a nine-fold increase in maize grain yield under high application rates of C. juncea over the unfertilized control, which yielded only 0.4 t ha⁻¹. Combinations of mineral N fertilizer with the leguminous resources and manure resulted in between 24% and 104% increase in grain yield against sole fertilizer, implying an increased nutrient recovery by maize under organic–mineral combinations. Maize biomass measured at 2 weeks after crop emergence already showed treatment differences, with biomass yields increasing linearly with soil mineral N availability (R ² = 0.75). This 2-week maize biomass in turn gave a positive linear relationship (R ² = 0.82) with grain yield suggesting that early season soil mineral N availability largely determined final yield. For low quality resources of maize stover and sawdust, application of mineral N fertilizer resulted in at least a seven-fold grain yield increase compared with sole application of the organic resources. Such nutrient combinations resulted in grain harvest indices of between 44% and 48%, up from a mean of 35% for sole application, suggesting the potential of increasing maize productivity from combinations of low quality resources with mineral fertilizer under depleted sandy soils. At Domboshawa, grain yields averaged 7 t ha⁻¹ and did not show any significant treatment differences. This was attributed to relatively high levels of fertility under the sandy-clay loams during this first year of the trial implementation. Differences in N supply by different resources were only revealed in grain and stover uptake. Grain N concentration from the high quality leguminous resources averaged 2% against 1.5% from sawdust treatments. We conclude that early season soil mineral N availability is the primary regulatory factor for maize productivity obtainable under poor sandy soils. Maize biomass at 2 weeks is a potential tool for early season assessment of potential yields under constrained environments. However, the likely impact on system productivity following repeated application of high N-containing organic materials on different soil types remains poorly understood.     

Interactive effects of selected nutrient resources and tied-ridging on plant growth performance in a semi-arid smallholder farming environment in central Zimbabwe

Crop production in sub-Saharan Africa is constrained by numerous factors including frequent droughts and periods of moisture stress, low soil fertility, and restricted access to mineral fertilisers. A 2 year (2005/6 and 2006/7) field study was conducted in Shurugwi district, central Zimbabwe, to determine the effects of different nutrient resources and two tillage practices on the grain yield of maize (Zea mays L.) and soybean (Glycine max (L.) Merr). Six nutrient resource treatments (control, pit-stored manure, leaf litter, anthill soil, mineral fertiliser, mineral fertiliser plus pit-stored manure) were combined with two tillage practices (conventional tillage and post-emergence tied ridging). Basal fertilisation was done with 0 kg ha⁻¹ as control, 240 kg ha⁻¹ PKS fertiliser, 18 t ha⁻¹ manure, 10 t ha⁻¹ manure plus 240 kg ha⁻¹ PKS fertiliser, 35 t ha⁻¹ leaf litter, 52 t ha⁻¹ anthill soil. About 60 kg N/ha was applied to fertiliser only and fertiliser plus manure treatments as top dressing in the form of ammonium nitrate (34.5%N). A split-plot design was used with nutrient resource as the main plot and tillage practice as the subplot, and five farmers’ fields were used as replicates. Grain yield was determined at physiological maturity (140 and 126 days after planting for maize and soybean, respectively) and adjusted to 12.5% moisture content for maize and 11% for soybean. In the first season (2005/06), addition of different nutrient resources under conventional tillage increased (P < 0.05) maize grain yield by 102–450%, with leaf litter and manure plus fertiliser treatments, giving the lowest (551 kg ha⁻¹⁾ and highest (3,032 kg ha⁻¹) increments, respectively, compared to the control. For each treatment, tied-ridging further increased maize grain yield. For example, for leaf litter, tied-ridging further increased grain yield by 96% indicating the importance of integrating nutrient and water management practices in semi-arid areas where moisture stress is frequent. Despite the low rainfall and extended dry spells in the second season, addition of the different nutrient resources still increased yield which was further increased by tied-ridging in most treatments. Besides providing grain, soybean had higher residual effects on the following maize crop compared to Crotalaria gramiana, a green manure. It was concluded that the highest benefits of tied-ridging, in terms of grain yield, were realised when cattle manure was combined with mineral fertiliser, both of which are available to resource-endowed households. Besides marginally increasing yield, leaf litter and anthill which represent resources that can be accessed by very poor households, have a positive effect of the soil chemical environment.     

Smallholder farmer management impacts on particulate and labile carbon fractions of granitic sandy soils in Zimbabwe

Crop production in maize-based smallholder farming systems of Southern Africa is hampered by lack of options for efficiently managing limited and different quality organic nutrient resources. This study examined impacts of farmers’ short- and long-term organic resource allocation patterns on sizes and quality of soil organic matter (SOM) fractions. Farmers’ most- (rich) and least- (poor) productive fields were studied for two seasons under low (450–650 mm yr⁻¹) to high (>750 mm yr⁻¹) rainfall areas in Zimbabwe, on Lixisols with ∼6% clay and 88% sand. Rich fields received 0.5–14 Mg C ha⁻¹ compared with <4 Mg C ha⁻¹ for poor fields, and the differences were reflected in soil particulate organic matter (POM) fractions. Organic inputs were consistent with resource endowments, with well-endowed farmers applying at least five times the amounts used by resource-constrained farmers. Rich fields had 100% more macro-POM (250–2,000 μm diameter) and three times more meso-POM (53–250 μm) than poor fields. Application of high quality (>25 mg N kg⁻¹) materials increased labile C (KMnO₄ oxidizable) in top 60 cm of soil profile, with 1.6 Mg C ha⁻¹ of Crotalaria juncea yielding labile C amounts similar to 6 Mg C ha⁻¹ of manure. Labile C was significantly related to mineralizable N in POM fractions, and apparently to maize yields (P < 0.01). Farmers’ preferential allocation of nutrient resources to already productive fields helps to maintain critical levels of labile SOM necessary to sustain high maize yields.     

Effect of farmer management strategies on spatial variability of soil fertility and crop nutrient uptake in contrasting agro-ecological zones in Zimbabwe

Variability of soil fertility within, and across farms, poses a major challenge for increasing crop productivity in smallholder systems of sub-Saharan Africa. This study assessed the effect of farmers’ resource endowment and nutrient management strategies on variability in soil fertility and plant nutrient uptake between different fields in Gokwe South (ave. rainfall ~650 mm year⁻¹; 16.3 persons km⁻²) and Murewa (ave. rainfall ~850 mm year⁻¹; 44.1 persons km⁻²) districts, Zimbabwe. In Murewa, resource-endowed farmers applied manure (>3.5 t ha⁻¹ year⁻¹) on fields closest to their homesteads (homefields) and none to fields further away (outfields). In Gokwe the manure was not targeted to any particular field, and farmers quickly abandoned outfields and opened up new fields further way from the homestead once fertility had declined, but homefields were continually cultivated. Soil available P was higher in homefields (8–13 mg kg⁻¹) of resource-endowed farmers than on outfields and all fields on resource constrained farms (2–6 mg kg⁻¹) in Murewa. Soil fertility decreased with increasing distance from the homestead in Murewa while the reverse trend occurred in Gokwe South, indicating the impact of different soil fertility management strategies on spatial soil fertility gradients. In both districts, maize showed deficiency of N and P, implying that these were the most limiting nutrients. It was concluded that besides farmers’ access to resources, the direction of soil fertility gradients also depends on agro-ecological conditions which influence resource management strategies.     

Effect of Continuous use of Sodic Irrigation Water with and without Gypsum, Farmyard manure, Pressmud and Fertilizer on Soil Properties and Yields of Rice and Wheat in a Long term Experiment

A long term field experiment was conducted for 8 years during 1994–2001 to evaluate the effect of N, P, K and Zn fertilizer use alone and in combination with gypsum, farmyard manure (FYM) and pressmud on changes in soil properties and yields of rice and wheat under continuous use of sodic irrigation water (residual sodium carbonate (RSC) 8.5 meq l⁻¹, and sodium adsorption ratio (SAR) 8.8 (m mol/l)^1/2 at Bhaini Majra experimental farm of Central Soil Salinity Research Institute, Karnal, India. Continuous use of fertilizer N alone (120 kg ha⁻¹) or in combination with P and K significantly improved rice and wheat yields over control (no fertilizer). Phosphorus applied at the rate of 26 kg P ha⁻¹ each to rice and wheat significantly improved the yields and led to a considerable build up in available soil P. When N alone was applied, available soil P and K declined from the initial level of 14.8 and 275 kg ha⁻¹ to 8.5 and 250 kg ha⁻¹ respectively. Potassium applied at a rate of 42 kg K ha⁻¹ to both crops had no effect on yields. Response of rice to Zinc application occurred since 1997 when DTPA extractable Zn declined to 1.48 kg ha⁻¹ from the initial level of 1.99 kg ha⁻¹. Farmyard manure 10 Mg ha⁻¹, gypsum 5 Mg ha⁻¹ and pressmud 10 Mg ha⁻¹ along with NPK fertilizer use significantly enhanced yields over NPK treatment alone. Continuous cropping with sodic water and inorganic fertilizer use for 8 years slightly decreased the soil pH_e and SAR from the initial value of 8.6 and 29.0 to 8.50 and 18.7 respectively. However, treatments involving the use of gypsum, FYM and pressmud significantly decreased the soil pH and SAR over inorganic fertilizer treatments and control. Nitrogen, phosphorus and zinc uptake were far less than additions made by fertilizer. The actual soil N balance was much lower than the expected balance thereby indicating large losses of N from the soil. There was a negative potassium balance due to greater removal by the crops when compared to K additions. The results suggest that either gypsum or FYM/pressmud along with recommended dose of fertilizers must be used to sustain the productivity of rice – wheat system in areas having sodic ground water for irrigation.     

Effect of the integrated use of legume residue,poultry manure and inorganic fertilizers on maize yield,nutrient uptake and soil properties

Identification of a sustainable integrated soil fertility management option in the tropics will not only salvage the degraded soils but also enhances the attainment of the goal of food security. This study was conducted in 2004 and 2005 on a degraded tropical Alfisol in south western Nigeria to evaluate the effect of legume residue, poultry manure and inorganic fertilizers on maize yield, nutrient uptake and soil properties. The treatments consisted of two rates of poultry manure (0 and 5 t ha⁻¹), three rates of N fertilizer (0, 50 and 100 kg N ha⁻¹ applied as urea), three rates of P fertilizer (0, 30 and 60 kg P ha⁻¹ applied as single super phosphate) and two soybean treatments (with or without incorporation of legume residue) in various combinations as a factorial experiment in Randomized Complete Block design with three replicates. Results showed that poultry manure alone led to significant increase in maize yield (60%) and soil organic matter (45%). In contrast, legume residue incorporation gave significantly lower increase in yield (7%) and soil organic matter (11%). However, the combined application of poultry manure and legume incorporation led to 72% increase in maize yield as opposed to 63 and 10% increase recorded when manure alone or legume alone were incorporated, respectively. Optimal maize yield was achieved when manure application was integrated with P fertilizer application. The interaction of P fertilizer and legume incorporation indicated that soil phosphorus and maize P concentration were significantly increased with the application of the P fertilizer and legume incorporation. Hence, the application of P fertilizer alone is most likely to be economical compared with its integration with legume incorporation.     

Green leaf manuring with prunings of Leucaena leucocephala for nitrogen economy and improved productivity of maize (Zea mays)–wheat (Triticum aestivum) cropping system

Green leaf manuring with prunings of Leucaena leucocephala is regarded as a useful source of N to plants but the actual substitution of N fertilizer, release and recovery of N as well as effects on soil fertility are not adequately studied. The present studies investigated the effect of sole and combined use of Leucaena prunings and urea N fertilizer in different proportions on productivity, profitability, N uptake and balance in maize (Zea mays)–wheat (Triticum aestivum) cropping system at New Delhi during 2002–2003 and 2003–2004. Varying quantities of Leucaena green leaf biomass containing 3.83–4.25% N (18.2–20.5 C:N ratio) were applied to provide 0, 25, 50, 75 and 100% of recommended N (120 kg ha⁻¹) to both maize and wheat before sowing. In general, direct application of urea N increased the productivity of both crops more than Leucaena green leaf manure, but the reverse was true for the residual effect of these sources. The productivity of maize increased progressively with increasing proportions of N through urea fertilizer and was 2.41–2.52 t ha⁻¹ with 60 kg N ha⁻¹ each applied through Leucaena and urea, which was at par with that obtained with 120 kg N ha⁻¹ through urea alone (2.56–2.74 t ha⁻¹). Similarly, wheat yield was also near maximum (4.46–5.11 t ha⁻¹) when equal amounts of N were substituted through Leucaena and urea. Residual effects were obtained on the following crops and were significant when greater quantity of N (>50%) was substituted through Leucaena. Nitrogen uptake and recovery were also maximum with urea N alone, and N recovery was higher in maize (33.4–42.1%) than in wheat (27.3–29.8%). However, recovery of residual N in the following crop was more from Leucaena N alone (8.5–10.3%) than from urea fertilizer (1.7–3.8%). Residual soil fertility in terms of organic C and KMnO₄ oxidizable N showed improvement with addition of Leucaena prunings, which led to a positive N balance at the end of second cropping cycle. Net returns were considerably higher with wheat than with maize, and were comparatively lower with greater proportion of Leucaena because of its higher cost. Nonetheless, it was beneficial to apply Leucaena and urea on equal N basis for higher productivity and sustainability of this cereal-based cropping system.     

Population dynamics of mixed indigenous legume fallows and influence on subsequent maize following mineral P application in smallholder farming systems of Zimbabwe

Developing soil fertility management options for increasing productivity of staple food crops is a challenge in most parts of Sub-Saharan Africa, where soils are constrained by nitrogen (N) and phosphorus (P) deficiencies. A study was conducted to evaluate the response of indigenous legume populations to mineral P application, and subsequently their benefits to maize yield. Mineral P was applied at 26 kg P ha⁻¹ before legume species were sown in mixtures at 120 seeds m⁻² species⁻¹ and left to grow over two rainy seasons (2 years). Application of P increased overall biomass productivity by 20–60% within 6 months, significantly influencing the composition of non-leguminous species. Dinitrogen fixation, as determined by the N-difference method, was increased by 43–140% although legume biomass productivity was apparently limited by nutrients other than P and N. Crotalaria pallida and C. ochroleuca accounted for most of the fixed N. Improved N supply increases the abundance of non-leguminous species, particularly Conyza sumatrensis and Ageratum conyzoides. However, abundance of common weed species, Commelina benghalensis, Richardia scabra and Solanum aculeastrum, declined by up to18%. Application of P did not significantly influence productivity of those legume species that reached maturity within 3 months. There was increased N₂-fixation and biomass productivity of indifallows as influenced by specific legume species responding to P application. Compared with natural (grass) fallows, indigenous legume fallows (indifallows) increased subsequent maize grain yields by ~40%. Overall, 1- and 2-year indifallows gave maize grain yields of >2 and 3 t ha⁻¹, respectively, against <1 t ha⁻¹ under corresponding natural fallows. Two-year indifallows with P notably increased maize yields, but the second year gave low yields regardless of P treatment. Because of their low P requirement, indigenous legume fallows have potential to stimulate maize productivity under some of the most nutrient depleted soils.     

Farmers’ agronomic and social evaluation of productivity,yield and N<Subscript>2</Subscript>-fixation in different cowpea varieties and their subsequent residual N effects on a succeeding maize crop

Cowpea–maize rotations form an important component of the farming systems of smallholder farmers in the forest/savannah transitional agro-ecological zone of Ghana. We evaluated five cowpea varieties for grain yield, N₂-fixation, biomass production, and contribution to productivity of subsequent maize grown in rotation. We further analyzed the interrelationship between these technical dimensions and the social acceptability of these cowpea varieties for farmers. Cowpea grain yield ranged between 1.1 and 1.4 t ha⁻¹ with no significant yield differences among the different varieties. Using the ¹⁵N natural abundance technique, the average proportion of N₂ fixed ranged between 61% for Ayiyi and 77% for Legon prolific. This resulted in average amounts of N₂ fixed in above-ground biomass ranging between 32 and 67 kg N ha⁻¹, respectively. Variation in estimates due to differences in δ¹⁵N among reference plants were larger than differences between cowpea varieties. The amount of soil-derived N ranged from 15 to 20 kg N ha⁻¹. The above-ground net N contribution of the cowpea varieties to the soil (after adjusting for N export in grains) was highest for Legon Prolific (31 kg N ha⁻¹) due to high N₂-fixation and high leaf biomass production. Maize grain yield after cowpea without application of mineral N fertilizer ranged between 0.4 t ha⁻¹ with maize after maize to 1.5 t ha⁻¹ with Legon Prolific. The N fertilizer equivalence values for the cowpea varieties ranged between 18 and 60 kg N ha⁻¹. IT810D-1010 was ranked by the farmers as the most preferred cowpea variety due to its white seed type, short-duration, ease of harvesting and good market value. Despite the high leaf biomass production and high amount of N₂ fixed by Legon Prolific, it was generally the least preferred variety due to lower market price, late maturity, least potential cash income (due to the red mottled seed type) and difficulty in harvesting. Although farmers recognized the contribution of cowpea to soil fertility and yields of subsequent maize, they did not consider this as an important criterion for varietal selection. Soil fertility improvement must be considered as an additional benefit rather than a direct selection criterion when designing more sustainable smallholder farming systems.     

Effects of agronomic practices on the soil carbon storage potential in arable farming in Austria

According to the Kyoto-Protocol for carbon dioxide mitigation the direct human induced sequestration potential of carbon in agricultural soils may in the future be included for calculating net changes in greenhouse gas emissions. Therefore we used long-term experiments on arable land in Austria differing strongly in climate and soil conditions to explore the effects of agronomic practices on changes in soil organic carbon content. Optimal mineral N fertilizer input increased the carbon stocks on an average to 2.1 t ha⁻¹compared with no N fertilization in a 36 years period. Additional farm yard manure application (10 t ha⁻¹ y⁻¹) enhanced carbon storage to about 5.6 t ha⁻¹ after 21 years. Site-specific influences must be considered. Losses of 2.4 t carbon per ha were caused by additional irrigation of sugar beet and maize in a rotation with cereals in a 21 years period. The incorporation of all crop residues resulted in an increase of 3.4 t ha⁻¹ organic carbon in topsoil after 17 years. In the uppermost soil layer (0–10 cm) minimum and reduced tillage treatment enhanced carbon stocks to about 4.7 t ha⁻¹ and 3.2 t ha⁻¹ compared to conventional soil management within a decade. Based on these results, only a limited soil carbon sequestration potential can be inferred: Manuring and incorporation of crop residues are well-proven practices on arable land and therefore no additional human induced carbon sequestration might be achieved. The adoption of minimum tillage on Phaeozems, Chernozems and Kastanozems could, roughly calculated, result in a supplementary carbon storage of about 0.6% of the entire present annual carbon dioxide emission in Austria. However, the storage of carbon in topsoil means only a mid-term sequestration. By changing practices in short-terms, these amounts of carbon might be a source of additional carbon dioxide in the future. This revised version was published online in August 2006 with corrections to the Cover Date.