Zimbabwe, it is essential to improve understanding of the functioning of SOM under smallholder management. There is little available information on the degree to which overall farm management affects the various functions of the component parts of SOM. Smallholder farmers in Zimbabwe clearly recognize the importance of soil fertility and conservation (Mugwira and Shumba, 1986; Murwira and Mukamuri, 1998; Mapfumo and Giller; 2001), which is almost entirely dependant on locally available resources. There is general knowledge that application of organic residues improves the physical conditions of soil although information as to why this happens is still scanty (Sanchez et al., 1989). Farmer manipulation of the soil resource base differs in intensity partly because of the limited availability of either nutrient source or a result of differences in farmers’ conceptualization of soil fertility management. The issue of resource allocation influences production goals and is of direct relevance to soil fertility management. It therefore calls for a more direct intervention to improve soil status while at the same time strengthening farmer knowledge and skills.
Smallholder farmers have, over time, found widespread use of locally available forms of organic nutrient sources such as livestock manures, woodland litter, green manures, composted materials, household waste and crop residues (Campbell et al., 1998; Mapfumo and Giller, 2001). However, the negative nutrient balances, estimated at -22 kg N ha-1,-2.5 kg P ha-1 and -15 kg K ha-1 from arable lands (Smaling et al., 1997), are suggestive of management practices that are purely extractive with inadequate nutrient inputs to balance the system (Grant, 1995). The socio-economic boundaries within which farmers operate are inundated with numerous constraints. For example, crop residues have alternative uses as dry season livestock feed and/ or livestock bedding during the rainy months in Zimbabwe (Murwira, 1993; Nzuma et al., 1998). Manure application is a preserve of cattle owners, while composting is often too labour intensive. Overall, the quantities of organic nutrient inputs available to farmers are limited (Mapfumo and Giller, 2001).
Farmers’ options for soil fertility replenishment may be grouped into three broad categories. These include (i) use of mineral fertilizers, (ii) crop sequences and intercrops with N2-fixing grain legumes, green manures and trees, and (iii)
livestock manure, crop residues and other forms of organic nutrient sources from within and around the farm (Mugwira and Murwira, 1997; Buresh and Giller, 1998). In addition to supplying nutrients and improving soil physical properties, organic inputs can also lead to the formation of SOM (Woomer and Swift, 1994). However, SOM formation can be controlled to some extent by the quality, quantity and management of these organic residues.
2.6.1 Livestock manure
Livestock manure, cattle manure in particular, is a traditional source of plant nutrients and can be rated as one of cheapest sources of organic fertilizer in many smallholder farming systems (Mugwira, 1984; Mugwira and Murwira, 1997). Manure application to soil results in increases in soil pH, infiltration rate, water holding capacity and decreased bulk densities (Grant, 1967; Murwira, 1993). Apart from supplying N, several studies have demonstrated the importance of manure as a major source micro-nutrients (Grant, 1967; Nhamo, 2001). The quality of livestock manure is very variable due to variation in animal diets and manure management before field application. Feeds rich in secondary compounds like lignin and polyphenols are more likely to contribute to increasing SOM levels following application of manure from such feeds. Decomposition studies of some communal area cattle manures by Murwira (1993) and Nyamangara et al. (1999) showed that N release is slow and spread over time. This therefore means that its application may not necessarily benefit the growing crop, but may have advantageous residual effects in the medium- to long-term. However, use of manure is a benefit of livestock owners. Soil C content under manure management is likely to be variable due to differences in the chemical composition of the manures, rate and mode of application and the frequency with which the manure is applied to a particular field.
2.6.2 Green manures
The multiple role of leguminous crops in the smallholder farming systems ranks them highly on the soil fertility agenda (Sanchez, 1995; Giller et al., 1998; Mapfumo, 2000). Apart from supplying plant nutrients, legume green manure can be produced and utilized in situ. Application of legume green manure in arable
farming systems provides a large amount of readily decomposable C and a ready supply of N for soil microorganisms. In Zimbabwe, leguminous species such as
Tephrosia, Crotalaria and Mucuna were recommended as N supply sources to
maize as early as the 1950s (Rattray and Ellis, 1952; Gilbert, 1998). Studies have shown that between 40-80% of applied organic N from high quality residues is added to soil and mineralized to available forms (Haggar et al., 1993; Palm, 1995).
In cereal cultivation, N contributions from high quality organic resources such as green manures were estimated to reach high levels of up to 250 kg N ha-1 yr-1 (Giller, 2001). However, information on potential contributions of green manures to the different SOM fractions is still scanty. Although green manures are most beneficial in providing nutrients in the short-term, an option more likely to be appealing to most smallholder farmers, they are likely to have a minimal role in SOM build-up (Palm et al., 2001).
2.6.3 Intercrops and rotations
The practice of legume-cereal intercrops by smallholder farmers is common in Zimbabwe although there is a general lack of awareness on the beneficial role in soil fertility amelioration of this farming method. Groundnut (Arachis hypogaea) has often been the common legume intercropped or grown in rotation with maize (Shumba, 1983; Waddington et al., 1998), although other legumes like cowpea (Vigna unguiculata) (Nhamo et al., 2003) are widely intercropped with maize. Inclusion of legumes in cropping systems may result in erosion control (Chikowo, 2004), smothering of weeds (Mapfumo et al., 2005), soil moisture conservation (Mapfumo and Giller, 2001) and biological nitrogen fixation (Giller, 2001), although the net benefits may vary significantly between seasons.
Maximum land utilization is probably the main reason why farmers intercrop. However, because of phenological differences, there is bound to be competition for nutrients, moisture and light in such cropping systems and SOM build-up may only be dependent on prevailing organic matter management strategies. Of significance to the soil C pool may be the inputs from incorporation of the crop residues because the supply of herbaceous material from intercrops in the form of senesced material is unlikely to be sufficient to impact on SOM. Maize yield increases have been realized under management systems that included incorporation of groundnut stover into soil (Mukurumbira, 1985). However, it is a common practice to remove the uprooted crop residues from the field, so there may be little or no C contribution by the legume to soil.
2.6.4 Crop residues
Crop residues, by-products that result from the cultivation of cereals, legumes, root crops and tubers among other things, perform various functions in smallholder farming systems (Palm et al., 2001). When left in the field after harvest, they play an important role in nutrient cycling, erosion control, livestock feed, water conservation and maintenance of favourable soil physical properties (Powell and Unger, 1998). Livestock also play a role in converting low quality crop residues into somewhat richer nutrient sources. The magnitude of the beneficial effects of crop residues to SOM pools depends on the quantity and quality of the residues, the subsequent crop to be grown and soil management. Organic residues with different chemical compositions vary in their C and N mineralization potentials and decomposition patterns (Mafongoya and Nair, 1997). Maize, by virtue of its being the staple food crop in Zimbabwe, is grown widely and thus, the maize stover is a readily available residue in most smallholder farming communities. Maize stover
has a high C:N ratio implying that its nutrient release pattern is be slow, and with proper management, the residues could play an important role in SOM formation (Campbell et al., 1998; Mapfumo, 2000; Palm et al., 2000).
2.6.5 Woodland litter
The use of woodland litter as a soil fertility amendment is a traditional practice in most communal farming areas of Zimbabwe and has of late gained momentum in the wake of increasing fertilizer costs and dwindling livestock populations. While there is considerable information on the chemical composition and decomposition patterns of litter from temperate areas (Jensen, 1974; Aber et al., 1990), knowledge of litter as a source of plant nutrients, amounts collected and application rates is scanty. Annual woodland litterfall may be as much as 5 t ha-1 and according to Nyathi and Campbell (1993), measured annual litter collection in Masvingo, southern Zimbabwe, ranged from 0.2 to 1.2 t household-1. The same survey showed that those smallholder farmers who owned cattle realized the importance of woodland litter by putting the collected litter in cattle pens during the dry season to enhance manure quality. Possibilities of building SOM in this type of management are likely given the heterogeneous nature of the litter quality which usually comprise of both herbaceous and tree litter (Mtambanengwe and Kirchmann, 1995; Mtambanengwe, 2000). However, the exact proportion of decomposing woodland litter incorporated into SOM pool is not known unlike high quality organic fertilizers.
2.6.6 Household waste and compost
Of all the organic amendments used by smallholder farmers, household waste is perhaps the most heterogeneous in terms of quality and composition. Proportions of paper, cooked food, plastics, metals and ash are normally very high in pits used
for discarding household wastes. Eventually, only decomposable material can be applied to arable land, thus sorting or separation of ‘biological waste’ from the other materials is inevitable (Kirchmann, 1994). Household waste is perhaps one of the commonly used organic amendments, particularly in homefields and vegetable gardens, as virtually all households, resource poor and rich farmers have something to throw away (Campbell et al., 1998). On the other hand, compost material, because it is labour intensive, is applied to smaller field portions. Both high and low quality materials find their way in compost heaps. There is no information available as to what contributions compost manure and household waste have on SOM build-up.
2.6.7 Termitarium soil
Termitaria soil is also an important nutrient source in Zimbabwean smallholder agriculture because of their high fertility status (Nyamapfene, 1986). Large portions of the Zimbabwean landscape are dotted with mounds of different species of termites. As such, access to termitaria soil for application to arable lands encompasses a large group of farmers. It is perhaps the feeding habits of the termites that determine the quality of the termitaria soil. Wood, grass or litter- feeding termites may consume large proportions of organic matter in their surroundings and the non-digested part of this material is accumulated in mounds and gallery walls (Dangerfield, 1990). Addition of termitarium soil to arable lands has been reported to increase the calcium, magnesium and top-soil clay contents. Farmers have reported residual effects lasting up to five years (Carter and Murwira, 1995), although the sustainability of their use is questionable given the amount of labour required to get the termitarium soil to the fields.
2.6.8 Mineral fertilizers
Mineral fertilizer use in Zimbabwe is widespread in the sub-humid and high potential areas. The most common mineral fertilizer forms used are compound D (7%N:14%P2O4:7%H2O) and ammonium nitrate (34.5% N). Often, the nutrient
source is purchased in amounts inadequate to replace those nutrients lost annually in harvested produce (Smaling et al., 1997). The recommended rates of top-dressing N fertilizer in Zimbabwe varies between 51 to 69 kg N ha-1 for Natural regions II to IV, but studies have shown that farmers apply less than a quarter of that (Grant, 1981; Campbell et al., 1998). In Integrated soil fertility management, several studies have shown the complementarities by combining mineral N fertilizers with different quality organic resources (Palm et al., 2001; Nyamangara
et al., 2005). However, still scanty is information on the actual effect of this
management practice on SOM dynamics in different soils.
2.6.9 Ash
Virtually every household in communal Zimbabwe has access to ash. Ash is generated from firewood, a source of energy in cooking food and heating. Use of ash for soil fertility management is widespread (Kanyoka, 2004) but the method of use varies within and between households. Its composition also varies with source although calcium is the most abundant element in wood ash (Stromgaard, 1984; Araki, 1993). Under smallholder management, the quality of ash is compromised by a high sand and charcoal content making up ~80% of total weight (F. Mtambanengwe, unpublished data). Use of ash is usually in combination with household wastes and is commonly applied around the homestead. Because of the negligible quantities generated, information on its likely contribution to SOM has previously not been quantified.