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

Soil pH and lime requirement for high potential communal areas of Zimbabwe

A research paper on the administering of soil nutrition in Zimbabwe's rural areas.A steady increase in the use of acidifying nitrogenous fertilisers by Zimbabwe’s communal area farmers since 1980 has not been matched by liming the soils to correct soil pH. A study was conducted in 1995 to assess the pH status of soils based on soil samples submitted to the Chemistry and Soil Research Institute of the Department of Research and Specialist Services by the communal area farmers in 1982-84 and 1992-94 so as to assess the lime requirement of the soils. Results showed that there was a gradual acidification of the soils. During the 10- year period the proportion of soils with pH of 5,0 or less increased from 42% to 77%. The soil pH results implied potential problems of crop production which included low fertiliser effectiveness in 77% of the soils with pH of 5,0 or less, Al toxicity and P deficiency in 43% of the soils with pH of 4,5 or less, and micronutrient deficiency (e.g. Mo)

Effects of organic and inorganic nitrogen fertilizer on maize (Zea mays L.) nitrogen uptake and nitrate leaching measured in field lysimeters

A research paper on the effects of organic and inorganic nitrogen fertilizer on maize crops in Zimbabwe.Maize (Zea mays L.) production in the smallholder farming areas of Zimbabwe is based on both organic and inorganic nutrient sources. A study was conducted to determine the effect of cattle manure, N fertiliser, and their combinations on nitrate concentration in leachate leaving the root zone and to establish N fertilisation levels which minimise N leaching. Maize was grown for two seasons (1996/97 and 1997/98) in field lysimeters repacked with a coarse-grained sandy soil. Average leachate volumes over the two seasons were similar between different treatments, and ranged from 388 to 418 mm yr1. Nitrogen fertiliser, especially the high rate (120 kg N ha1), and manure plus N fertiliser combinations resulted in high nitrate leachate concentrations (up to 37 mg N L-1) and nitrate losses (up to 56 kg N ha-1 yr1) which represent both environmental and economic concerns. Nitrate leaching from manure only treatments was relatively low (average less than 23 kg N ha-1 yr1), and plant availability in these treatments tended to be higher in the second season. It was concluded that N leaching at high inorganic N fertiliser rates posed a serious economic and environmental risk when all the fertiliser was applied at planting. It was also concluded that the risk of N leaching from aerobically composted cattle manure was low in the short term

Soil organic carbon dynamics of improved fallow-maize rotation systems under conventional and no-tillage in Central Zimbabwe

Fallowing increases soil organic carbon (SOC) during the fallowing phase. However, this benefit is lost quickly during the cropping phase. The objective of this study was to evaluate SOC dynamics of an improved fallow-maize rotation under no-tillage (NT) and conventional tillage (CT) from time of fallow termination, through the next two cropping seasons. The treatments studied were improved fallows of Acacia angustissima (A. angustissima) and Sesbania sesban (S. sesban), natural fallow and continuous maize. Our hypothesis is that fallowing maintained higher SOC and lower soil bulk densities through the cropping phase when compared with continuous maize system and that NT maintained higher SOC when compared with CT. Soil organic carbon was significantly greater under fallows than under continuous maize from fallow termination to the end of the second cropping season. Soil organic carbon for the 0¿5 cm depths was 11.0, 10.0, 9.4 and 6.6 g kg¿1 for A. angustissima, S. sesban, natural fallow and continuous maize, respectively at fallow termination. After two cropping seasons SOC for the same depth was 8.0, 7.0, 6.1, 5.9 g kg¿1 under CT and 9.1, 9.0, 8.0, 6.0 g kg¿1 under NT for A. angustissima, S. sesban, natural fallow and continuous maize, respectively. Total SOC stocks were also higher under fallows when compared with continuous maize at fallow termination and after two cropping seasons. Soil bulk densities were lower under fallows when compared with continuous maize during the period of study. We concluded that fallows maintained greater SOC and NT sequestered more SOC than CT. Acacia angustissima was the better tree legume fallow for SOC sequestration when compared with S. sesban or natural fallow because it maintained higher SOC and lower bulk densities after two seasons of maize cropping

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 strategie

Long-term nitrate and phosphate loading of river water in the Upper Manyame Catchment, Zimbabwe

Urbanisation and agriculture represent a dramatic example of human interference in catchment hydrology. The impact of agricultural, domestic, industrial and municipal activities on river flow and water quality within the Upper Manyame Catchment Area (UMCA) was assessed using 7-year nitrate, phosphate and water flow rate data, collected by the Environmental Management Agency (EMA). Water samples for nitrate and phosphate analysis were collected at 8 points along the Manyame (2 points), Marimba (2 points) and Mukuvisi (4 points) rivers, and runoff volume was recorded at the mouth of each river. Annual runoff of each river was closely related to rainfall amount, with the lowest runoff being recorded during drought years. High nitrate and phosphate concentrations were recorded directly downstream of residential, municipal and industrial areas suggesting that these were the major sources of the pollutants found in the river water. For example, phosphate concentration at 2 sites along Mukuvisi River (downstream of domestic and industrial areas) exceeded the statutory limit (0.5 mg/ℓ) for ‘safe’ or good quality water (‘blue’ category) according to the Zimbabwe Water (Waste and Effluent Disposal) Regulations, and ranged from 0.78 mg/ℓ during the dry season to 2.23 mg/ℓ during the wet season. In the Marimba River phosphate concentration at Site 4 (downstream of domestic, industrial and sewage processing plant) also exceeded the safe water quality standard by 4–6 times. Although Marimba River contributes the lowest proportion of runoff (relative to the other two rivers sampled) entering Lake Chivero, it contributed the highest nitrate (114 840 kg/yr) and phosphate (84 324 kg/yr) loading. It was concluded that anthropogenic activities within the UMCA were the major sources of nitrate and phosphate pollution in the three rivers and pose a serious threat to the ecological sustainability of the rivers and lakes downstream, and to the economic wellbeing of nearby cities which rely on the water for potable use

Effect of Inorganic and Organic Fertilizer Application on Nitrate Leaching in Wetland Soil Under Field Tomato (Lycopersicon esculentum ) and Leaf Rape (Brassica napus )

The implications of increased application of N inputs to agricultural systems in Africa for nitrate leaching are still only partially understood in Africa. A lysimeter experiment was carried out on a loamy sandy soil in central Zimbabwe in order to determine the effect of cattle manure and mineral N application on nitrate leaching. A cluster of zero-tension (free flowing) lysimeters was established, and leachates and soil samples were analyzed for nitrate N concentration and mineral N content, respectively. Increasing the application rates from 100 kg N fertilizer + 15 Mg manure to 200 kg N fertilizer + 30 Mg manure ha−1 increased NO3–N leaching by 60 %. Applied N lost in leachate increased by 6 and 19 % for the tomato and rape crops, respectively, when N fertilizer and manure application rate was doubled. Higher mineral N fertilizer and cattle manure applications increase total N lost in leachate. The pollution of groundwater with nitrate in leaf rape cropping in Zimbabwe is potentially higher than that found in the production of tomato for the crop rotation in the current study

Effect of manure quality on nitrate leaching and groundwater pollution in wetland soil under field tomato (Lycopersicon esculentum, Mill var. Heinz) rape (Brassica napus, L var. Giant)

Recent decades have seen an increase in groundwater pollution thought to be a consequence of increasing intensity of land use, primarily through greater use of high N analysis materials as fertilizers. A two-season lysimeter experiment was carried out in a wetland in central Zimbabwe in order to determine the effect of cattle manure quality on (1) NO3–N concentration in leachate and nitrate leaching (2) dry matter accumulation and uptake of N by tomato and rape crops grown in wetland conditions. Two cattle manure quality types based on N content were used in the experiment. The manure collected from a kraal of the smallholder wetland community was classified as high quality manure (high N, 1.36 % N) while that collected from the adjacent commercial farming area was classified as low quality manure (low N, 0.51 % N). The two manure types were applied in rates of 0, 15, 30 Mg ha−1. The treatments were arranged in a randomized complete block design with four replicates. When 15 and 30 Mg high and low N manure ha−1 were applied, the concentration of NO3–N in leachate exceeded the recommended 10 mg L−1 concentration in portable water by 15–104 and 53–174 % respectively. The substitution of 15 and 30 Mg of high N manure with 15 and 30 Mg ha−1 of low N manure reduced total N lost through leaching by 10–43 and 22–69 % respectively. Ground water contamination by nitrate overload can be considerably reduced by application of low N manure to vegetable crops

Nitrous oxide emissions from European agriculture; an analysis of variability and drivers of emissions from field experiments

Nitrous oxide emissions from a network of agricultural experiments in Europe and Zimbabwe were used to explore the relative importance of site and management controls of emissions. At each site, a selection of management interventions were compared within replicated experimental designs in plot based experiments. Arable experiments were conducted at Beano in Italy, El Encin in Spain, Foulum in Denmark, Logården in Sweden, Maulde in Belgium, Paulinenaue in Germany, Harare in Zimbabwe and Tulloch in the UK. Grassland experiments were conducted at Crichton, Nafferton and Peaknaze in the UK, Gödöllö in Hungary, Rzecin in Poland, Zarnekow in Germany and Theix in France. Nitrous oxide emissions were measured at each site over a period of at least two years using static chambers. Emissions varied widely between sites and as a result of manipulation treatments. Average site emissions (throughout the study period) varied between 0.04 and 21.21 kg N2O-N ha−1 yr−1, with the largest fluxes and variability associated with the grassland sites. Total nitrogen addition was found to be the single most important determinant of emissions, accounting for 15% of the variance (using linear regression) in the data from the arable sites (p < 0.0001), and 77% in the grassland sites. The annual emissions from arable sites were significantly greater than those that would be predicted by IPCC default emission factors. Variability in N2O within sites that occurred as a result of manipulation treatments was greater than that resulting from site to site and year to year variation, highlighting the importance of management interventions in contributing to greenhouse gas mitigation

Factors affecting the choice of conservation agriculture practices adopted by smallholder cotton farmers in Zimbabwe

Conservation agriculture (CA) has been widely promoted in Zimbabwe as an antidote to non-viable agricultural production and continual land degradation. However, the adoption process had been quite slow and has not yet entered into the exponential uptake phase. This study aimed at identifying factors that influence the level of adoption of CA components. A cluster analysis from results of a household survey administered to 146 households in Muzvezve II, Kadoma District, Zimbabwe identified five dominant CA strategies (clusters) practiced by cotton growing farmers. A multinomial logit model revealed that the choice of CA components adopted is positively influenced by farmer’s age, formal education, access to extension services, labour, animal draught power availability and land size. The empirical results suggests that, to promote adoption of a complete package of CA policies that increase access to formal education and extension of CA should make strategic intervention through innovative methods of farmer to farmer extension services. Promotion of longer-term and effective CA can only be accomplished through targeting young educated farmers. It is of paramount importance as well to address the main factors leading to non-adoption and slow adoption such as labour and animal draught power availability