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Chapter 3
Soil fertility management

There has been much research on soil fertility management and enhancement on the basis of inorganic and organic fertilizers (Grant, 1967a, 1967b, 1981, 1995; Tanner and Mugwira, 1984; Mugwira and Mukurumbira, 1984; Mashiringwani, 1983; Piha, 1993; Mugwira and Murwira, 1997; Campbell et al., 1998). The research has involved agronomic research on fertilizer types, application rates and timing of application for the different soil types, rainfall regimes and cropping and farming systems in all the NRs. The principal research organization has been the Agronomy Institute and the Chemistry and Soil Research Institute of the Department of Research and Specialist Services (DRSS). The research has been carried out in cooperation between the research institutes, farmers and the fertilizer companies. Farmers, particularly the large-scale commercial farmers, have also undertaken their own research. The research has resulted in the accumulation of considerable knowledge concerning the different fertilizers, application rates and timing. Chapter 6 discusses this information.

Cattle manure as a source of nutrients

Cattle manure is a major fertilizer input for smallholder farmers on communal lands. The production and management of cattle manure follows a pattern common to all communal lands. Cattle are herded in grazing areas during the day and penned at night in cattle kraals (pens) located at the homestead. Manure that accumulates in the pens is dug out towards the end of the dry season. It is allowed to cure for up to three months and then spread on the fields in September/October, in time for land preparation for the next cropping season. When penned all day, one livestock unit (1 LU = 500 kg live mass) produces about 1.5 tonnes of recoverable manure per year (Rodel, Hopley and Boultwood, 1980). The amount of usable manure that cattle can provide depends on several factors such as the amount of feed, the feeding method (pen rearing, kraaling the animals at night or free range) and the manure collection efficiency. Cattle deposit a large amount of manure on the common grazing lands where it remains uncollected.

Since the early 1960s, several researchers (Grant, 1967a, 1967b, 1970, 1981; Tanner and Mugwira, 1984; Murwira, 1993; Murwira, Swift and Frost, 1995; Mugwira and Murwira, 1997; Nhamo, Murwira and Giller, 2001) have studied the value of cattle manure as a fertilizer compared with mineral fertilizers. The studies have included experiments comparing the ability of manure and inorganic fertilizers to restore and maintain soil productivity in nutrient-depleted or fallowed sands on communal lands. They have also investigated the best combination of manure and N fertilizer, the use of lime, manure and N and P fertilizers to restore the productivity of depleted sands on some communal lands (Grant, 1970, 1981). The general conclusion from these studies is that manure applied alone produces low crop yields and needs supplementing with inorganic fertilizers, particularly N, for optimal yields.

In 1962, the Agronomy Section of the Grasslands Experiment Station initiated a medium-term experiment to evaluate changes on sandy soils under continuous maize cultivation. The experiment involved annual applications of N, lime and manure treatments to maize grown continuously on granitic sand, a situation generally encountered on communal lands in NRs III and IV (Grant, 1967a, 1967b). On the basis of these trial results and other experiments, Grant (1981) observed that manure application to granitic sands overcomes or prevents deficiencies of micronutrients, including S, Mg, Zn and B, and enhances soil available N, P and K.

Quality and effectiveness of manure as a fertilizer

The quality and effectiveness of cattle manure on plant growth and crop yield has been the subject of extensive studies.

Mugwira and Murwira (1997) established that cattle manure from communal lands contains an average of 1.04 percent N, 0.15 percent P and 0.78 percent K compared with 1.87 percent N, 0.58 percent P and 0.78 percent K for manure from feedlots. According to Tanner and Mugwira (1984), cattle manure applied to fields in the communal areas has an N content that ranges from 0.5 to 1.4 percent of dry matter.

The N release is low and spread over time. Manure applications result in increases in pH, waterholding capacity, hydraulic conductivity and infiltration rates, and a decrease in bulk densities (Grant, 1967a, 1970; Murwira, Swift and Frost, 1995). Manure also has the long-term effect of raising the level of organic matter where applied in large quantities. The main determinants of the effectiveness of cattle manure as a fertilizer are its N content, location of N in organic fractions and its release, as well as the application rate.

Grant (1967b) notes that P, calcium (Ca), Mg, Fe, Zn and Cu contents are lower in manure from communal lands. However, applying manure on wetland soils may be inadequate because of its low P content. It would be necessary to apply supplementary P fertilizer. According to Mugwira and Mukurumbira (1984), the low nutrient content of the manure contributes to its limited effectiveness in improving plant growth and crop yield.

Grant (1967b) submits that, given that the manures are generally of low quality with a lot of sand and maize stover, the benefits from fertilizing with manure stem more from the bases released than from the supply of N and P. Grant (1967a) suggests that manure would not be an adequate source of N for high-yielding maize crops because of its inability to supply continuously large amounts of readily available N. Mugwira (1985) found that the supplementation of manure with mineral fertilizer applied separately was generally effective in enhancing the effectiveness of manures on communal lands.

Improving the effectiveness of manure and other options

Several trials have examined ways to improve the effectiveness of manure as a source of plant nutrients. The general observation is that farmers can use available organic matter (crop residues, weed biomass, and animal manure) more efficiently than at present. Mugwira and Mukurumbira (1984) suggested that cocomposting of manure with N and P nutrient sources in cattle pens would improve the low-nutrient manure. Suggested approaches for improving the manure include: (i) improving pastures by planting legumes in order to improve the dung quality; (ii) proper management of manures in cattle pens in order to decrease nutrient loss; (iii) storing manure in pits in order to minimize drying and leaching during hot and rainy days; and (iv) anaerobic treatment of the manure.

In 1996, the DRSS and the ACFD, with funding from the Rockefeller Foundation under the Soil Fertility Network, conducted a survey to assess traditional farmer manure-management practices on some communal lands. The management technologies identified as suitable for promotion included: the use of crop residues to absorb nutrients from urine; pit storage of manure combined with the use of crop residues in summer and winter to reduce drying and leaching in hot and wet periods; anaerobic treatment of manure; and the above-mentioned improvement of pastures by planting legumes that lead to better dung quality (Nzuma, Murwira and Mpepereki, 1998).

The ACFD has initiated a collaborative project that is using effective micro-organisms (EMs) that have the ability to fix N from the air and solubilize P, making it more available to plants. The micro-organisms can be used to inoculate composts and manure products.

Green manure

Since 1992, the Rockefeller Foundation, through the Soil Fertility Network, has funded on-farm green manure trials by the Farming Systems Research Unit (FRU) of the DRSS on several communal lands. The experiments have involved both food crops (cowpeas and soybeans) and forage legumes (velvet beans and sunhemp) intercropped or rotated with maize to improve soil fertility, reduce striga (witch weed) infestation and improve maize yield. The experiments have generally shown that green manures improve soil fertility and maize yields (Chibudu, 1998; Chivinge, Kasembe and Mariga, 2001; Hikwa and Waddington, 1998; Mapfumo and Giller, 2001; Muza, 2003). However, the use of green manures to improve the soil condition and fertility has declined to insignificant levels because of economic changes and fertilizer use.

The European Union, through the Institute of Environmental Studies at the University of Zimbabwe (UZ) and with the participation of the Soil Science and Agricultural Engineering Department at the UZ and the Southern African Development Community / International Centre for Research in Agroforestry (SADC/ICRAF), has supported on-farm trials on pigeon pea intercropped with maize and cowpea as a traditional legume. The farmers prefer a pigeon-pea crop that matures at the same time as most crops in order to avoid extra protection measures (Mapfumo et al., 2000).

Fertilizer-based soil management packages

A fertilizer-based soil management package has been developed and promoted for variable rainfall regimes in communal areas in all NRs. The Rockefeller Foundation funded this initiative through the Soil Fertility Network with the participation of the Soil Fertility Network and the Department of Soil Science and Agricultural Engineering. The package includes application of mineral fertilizers at different rates at different growth stages of the plant, rather than at recommended application rates applied mainly at planting. An example is the split application of Compound D rather than the application of one dose at planting (Piha, Pangenyama and Tapfuma, 1995).

Previous research showed that soils of the marginal arid and semi-arid lands are generally deficient in N and that N supplementation is necessary in order to increase crop yields. However, as most small-scale farmers do not have adequate working capital to buy mineral fertilizers, biological nitrogen fixation (BNF) is considered an option (Mugabe, 1994). In Zimbabwe, the roots of non-leguminous crops are not colonized by N-fixing organisms and the Department of Crop Sciences of the UZ is experimenting with the use of bacterial inoculants to increase the N-fixing abilities of cereals. The work involves the use of gene-gun technology to transfer N-fixing genes to cereals. Mugabe (1994) remarks that while the potential of BNF to promote sustainable utilization of marginal lands and increasing crop yields at small-scale farms is recognized in Zimbabwe, policies on BNF research are yet to be developed. There is also limited government funding.

Other research has focused on mycorrhizal inoculation. Mycorrhiza is a fungal strain on plant roots that assists the plant to extract P and other micronutrients from the soil. The mycorrhiza used for the research was Glomus spp., which enhanced mycorrhizal inoculation and increased the dry weight of cowpea by 100 percent.

The soils of communal lands are generally low in P. Experiments have been carried out using a technique for improving the P content of manure (van Straaten, 1999; van Straaten and Fernandes, 1995). The work involves the pelleting of dust from phosphate rock deposits for easy handling in adding to the manure. This technique is also used for producing phosphate rock as a fertilizer for direct application and for incorporation in fertilizer mixtures. The technique is considered cheaper than that used to produce double and triple superphosphates.

Another practice that conserves soil and water and enhances soil fertility is conservation farming. This practice, refined and promoted by Oldrieve (1993), advocates minimum tillage and permanent surface cover by crop residues. The ACFD and a number of non-governmental organizations (NGOs) are promoting the practice in several communal areas.

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