2.7.1 Compost and inorganic fertilisers
Integrated nutrient supply into the soil from compost and inorganic fertilisers though not well understood improves compost use efficiency by increasing mineralisation of compost-N, particularly in soils with low indigenous inorganic-N content (Han et al., 2004). Integrating readily available sources of inorganic-N with compost reduces the amount of compost applied. Reduction in quantity of compost applied also reduces labour costs associated with making the compost (in case of smallholder farmers), transportation and application of the compost. According to Sikora and Enkiri, (2001), if compost is applied to agricultural land at the N requirement of grain crops (40–100 kg N ha-1), application rates approach 40–100 Mg ha-1compost.
Nutrient integration from inorganic and organic sources can reduce the accumulation of non-nutrient constituents in soils mainly when bio-solids and sewage sludge compost
are used as organic amendments. According to Palm et al., (1997), integrated use of organic and inorganic sources influences nutrient availability in the following ways;
i. by the nutrients added,
ii. through mineralisation – immobilisation patterns, iii. as an energy source for microbial activities, iv. as precursors to soil organic matter(SOM),
v. by reducing phosphorus (P) sorption of the soil.
The strength of the integrated nutrient input-system lies in its ability to meet the short- term as well as long-term nutrient requirements of crops through the fast-releasing fertiliser nutrient pool and the slow releasing organic nutrient pool, respectively
(Sharma et al., 2008).
According to Han et al., (2004), there is a decrease in net N mineralisation during the first days of urea-compost (composted sawdust and manure) blending due to the growing microbial biomass as a result of the organic-C and inorganic-N in the compost. Despite that observation, Han et al., (2004) noted net mineralisation at the end of their 6 weeks incubation experiment ranging from 7.6% to 14.5% on three different soils incubated at 25 ± 0.5 °C. Similarly Goyal et al., (1999) reported increased microbial biomass C as a result of blending organic amendments with inorganic fertilisers from 147 mg kg-1 in unfertilised soil to 423 mg kg-1in soils amended with wheat straw and inorganic fertilisers (urea and single super phosphate). The amendments provided readily decomposable organic matter in addition to increasing root biomass and root exudates due to greater crop growth. It is expected therefore that compost-fertiliser blends will have high efficiency in terms of supplying enough nutrients for crop growth. Sikora and Enkiri (2000) in an 87-day growth chamber incubation study (fescue grass at 25°C) amended sandy loam soils with different combinations of biosolid compost and NH4NO3fertiliser of 0/100%, 16.7/83.3%, 33/65% and 50/50% (supplying 100 mg kg-1 total N). They noted linear yield increment (9.1, 10.5 and11.4 g pot-1) in blends with increasing percentages of biosolid compost N and decreasing percent fertiliser. Blends containing 33 or 50% compost produced yields greater than 100% NH4NO3. Sikora and Enkiri (2000) obtained comparable results for N uptake. They concluded that benefits from biosolids compost are seen when NH4NO3 contribution in the blends is 67% or
less than the recommended for optimal growth. The limit set by Sikora and Enkiri (2000) on the nutrient blending can be disputed as availability and uptake of nutrients for crop growth depends on a number of factors including the initial conditions of the soil, length of the growth season and the physiology of the crop.
In a 6 year field study on loamy sand soils, Eichler-Lobermann et al., (2007) concluded that organic (cattle manure and manure biowaste compost) and inorganic fertilisation (triple-superphosphate) resulted in high contents of water soluble P and double lactase soluble P in the soil. Mixing organic and inorganic fertilisation in spring resulted in high spring wheat yield of up to 7.7 t ha-1 as compared to autumn wheat yield of 7.3 t ha-1. Similarly, nutrient uptake of up to 150 kg ha-1total P was observed for spring wheat as compared to 143 kg ha-1 in autumn (Eichler-Lobermann et al., 2007). They suggested that availability of P depends on a number of factors that include pH, organic matter content and soil moisture. Eichler-Lobermann et al., (2007) did not explore the effect that these factors may have had on the availability of P in the soil and plant uptake in the experiment.
2.7.2 Compost and effluent nutrient integration
The concept of nutrient blending from organic and inorganic sources can advance the idea of sustainable agriculture with less input of inorganic fertilisers and higher build-up of organic matter in the soil through organic amendments. Much as the synergistic effects have been observed of compost on inorganic fertilisers and vice versa (Sharma et al., 2008), inorganic (chemical) fertilisers still remain expensive to most smallholder, urban and peri-urban farmers in developing countries because of the escalating energy costs associated with its production, cost of transportation and above all, according to Herring and Fantel, (1993) andHilton et al., (2010)P-fertilisers may run out of supply as presently known reserves of rock phosphates may be depleted within 50 years.
Sikora and Enkiri (2000), integrated bio-solid compost and NH4NO3in a growth cabinet experiment, Eichler-Lobermann et al., (2007) integrated organic (cattle manure and manure biowaste compost) and inorganic fertilisation, Goyal et al., (1999) combined wheat straw, urea and single super phosphate while Han et al., (2004), integrated urea and compost (composted sawdust and manure). A seemingly viable option is to blend compost with STSE for irrigation of crops but no research has been done on this type of
nutrient integration. STSE is a product of sewage treatment that is either disposed in rivers or into the soil. Disposal of effluent in most developing countries is regarded as an economically viable option as it reduces the cost of treating sewage (Friedel et al., 2000). According to Katanda et al., (2007), sewage effluent contains larger proportions of plant nutrients such as N, P, organic matter as well as heavy metals. Heavy metals are non-biodegradable and can persist in the environment long enough to diminish soil quality and to be taken up by plants into the food chain (Katanda et al., 2007; Chakrabarti and Chakrabarti, 1988). However, once treated, STSE contains a lower proportion of heavy metals as most of the heavy metal end up in sludge hence the use of STSE for irrigation of crops (Emongor and Ramolemana, 2004).
The issues raised above can be attributed to among other factors, the effluent loading (application) rates, the sewage treatment process and the concentration of the heavy metals in the effluent. Usage of sewage effluent as the sole source of nutrients for crops results into larger loading rates that will lead into accumulation or leaching of heavy metals and plant nutrients in the soil.
In optimising nutrient potential from compost through irrigation with treated effluent, various questions will arise, one of which will be how to combine organic amendments with readily available nutrient sources e.g. effluent and organic amendments to optimise nutrient availability from the later. The proportion of organic amendments (compost) to the quantity of STSE is a research question that has to be answered so as not to deprive the crops of nutrients and water. Synchronisation of available and nutrient uptake is another significant issue that often affects integration of nutrient sources.
The contribution of effluent-N to the N cycle has been presented in Figure 2-9. It also shows the processes that N undergoes for plant and microbial availability. Effluent-N will contribute to the N cycle through either the NH4+-N or NO3--N pools; in the NO3--N pool it will be immediately available for plant uptake or leached while in the NH4+-N pool, immobilisation, nitrification or plant uptake by higher plants will be expected to take place.
Figure 2-9 Modified N cycle showing the contribution of effluent to the N-cycle (adapted from Johnson et al., (2005)).
Optimising nutrient potential without proper estimation of the rates of mineralisation of the compost due to the readily available nutrients from treated effluent can result in the accumulation of nutrients into the soil and leaching in case of excess precipitation or irrigation. Determination of N mineralisation rates is essential in estimating how much of the organic amendments to apply in order to supply the right amount of nutrients for optimising crop production. In optimising nutrient potential from compost by irrigating compost amended soils with treated effluent, the study of nutrient dynamics due to the conjunctive supply of nutrients becomes necessary for efficient sustainable management of the system. Excess NO3--N which can come from the optimised nutrients of compost and treated effluent can be lost by denitrification and can lead to undesirable N-outputs from the soil e.g. in the form of gases (NOx and N2O) which are known to increase the greenhouse effect (Fonseca et al., 2007b). Residual effects, percolation and subsequent accumulation should also be considered in terms of heavy metals. Though it is expected that the treated effluent is likely to contain less heavy metals, subsequent impacts of the
these metals can trigger a chain reaction in the soil that can affect availability of other essential nutrients.
Compost blending with effluent (treated) can be regarded as a lifeline to most urban dwellers in developing countries who practise urban or peri-urban agriculture. According to FAO (1998) "urban" agriculture implies growing crops, raising small livestock or milk cows within the city for own-consumption or sale while "peri-urban" agriculture refers to farm units close to towns which operate intensive, semi - or fully commercial farms to grow vegetables and other horticultural crops, raise chickens and other livestock and produce milk and eggs.