In conventional systems four frequently used elements, nitrogen, phosphorus, potassium and calcium are often applied as synthetic fertilizers in relatively heavy concentrations that frequently exceed crop requirements. This can cause soil imbalances in two ways: (1) by increasing or decreasing availability of some elements essential for crop growth and also by changing soil pH, and (2) by increasing productivity over the short term; but in decreasing productivity over the longer term due to imbalances and deficiencies for some other essential elements that are not replaced. For example, high levels of phosphorous fertilization can lead to a deficiency of both zinc and iron causing adverse effects on plant growth. Organic systems use organic fertilizers such as manures, compost, crop residues, legumes, rock phosphate and rock potash, containing minor and trace elements as well as moderate amounts of the primary elements.
In general, organic soils contain superior average and balanced levels of nutrients, which have indirect, beneficial effects for pest, disease and weed management (Lampkin 1999). For example, of the nine farms studied by Berry et al. (2003) seven had a positive N budget, six had a positive P budget and three had a positive K budget on the organic part of the farm compared to the conventional part. Derrick and Dumaresq (1999) found that soil in an organic farm contained higher concentrations of exchangeable potassium, calcium, sodium and lower concentrations of exchangeable molybdenum. Joo et al. (2001) found that available phosphorus values were 986 and 935 mg/kg in organic and conventional farm soils, respectively. Average total phosphorus values were 2,973 mg/kg in the organic fields and 1,830 mg/kg in the conventional fields. Oehl et al. (2002) reported that after 21 years of organic management an adequate level of available phosphorus was maintained. Wells et al. (2000) also reported that after 3'/2 years of vegetable cropping, available phosphorus increased on the organically managed field. Fumigation extractable carbon and nitrogen, mineralizable N, arginine ammonification and substrate-induced respiration were significantly higher in organic and low input than in conventional systems (Gunapala and Scow 1998). However, the results of some studies contrast with these findings (e.g. Derrick and Dumaresq 1999; Loes and Ogaard 1997; Haraldsen et al. 2000).
Organic farming encourages the reduction of agrochemicals and promotes soil conservation principles (Saha et al. 2007). Those production systems are associated with positively enhanced soil physical, chemical and biological characteristics (Brown et al. 2000). Organically managed soils do not contain readily soluble nutrients except K, and normally have slow-release properties. They are more fertile with higher total N, total P, humic acid, exchangeable nutrient cations, water-holding capacity and micro-bial biomass, than conventionally managed soil (Wells et al. 2000). When organic fertilizers are incorporated into the soil, a greater reliance is placed on chemical and biological processes to release nutrients in plant available forms in soil solution (Stockdale et al. 2002); in other words, 'feeding the soil not the plant'.
Conventional farming systems are often associated with nutrient leaching from arable lands and ground water pollution (Hansen et al. 2000). Application of farm yard manures, legumes, compost and other organic fertilizers in organic farming systems causes lower nutrient input and less nutrient leaching than conventionally managed fields (Hansen et al. 2000; Kirchmann and Bergstrom 2001; Vetterli et al. 2003). Similarly, phosphate pollution in surface and ground water could be less in organic agriculture due to the absence of any highly soluble phosphate fertilization (Vetterli et al. 2003).
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