The Environmental Effects of Conventional and Organic/Biological Farming systems. II. Soil Ecology, Soil Fertility and Nutrient Cycles

Abstract

The inputs characteristic of conventional and organic/biological farming systems are examined, and their physical, chemical and biological impacts on the ecology and fertility of agricultural soils, and on nutrient cycles within these systems, are evaluated. Inorganic fertilisers applied in conventional systems may not preserve soil structure, can cause wide fluctuations in the pH and ion concentrations of the soil solution, and can substantially reduce some soil faunal populations, especially earthworms. Organic manuring practices characteristic of organic systems tend to maintain soil structure, are less disruptive of the soil chemical environment, encourage populations of beneficial soil fauna and contribute to the control of microbial pathogens. Where inorganic fertilisation practices fail to maintain soil organic matter levels, and therefore soil structure, they inhibit crop rooting and reduce the water retention capacity of the soil. Organic manuring practices generally facilitate crop rooting, improve water retention capacity and result in a more even distribution of nutrients in the soil profile. The retention of macronutrients and availability of micronutrients are enhanced by organic manuring practices. The application of highly soluble inorganic fertilisers, generally results in higher losses of macronutrients, can cause nutrient imbalances and disrupt crop uptake of all nutrients and can damage crop roots. Earthworms, which contribute significantly to soil fertility, are generally found at higher population densities in organically manured soils. Inorganic fertilisation practices which impair soil structure can limit the cycling of some crop nutrients, and accelerate the loss of others. Chemical impacts of inorganic fertilisation tend to enhance nutrient losses from the soil, whereas organic manuring generally promotes a more efficient cycling. The slower nutrient release rate of organic manures, while promoting more efficient cycling and use, can limit availability to the crop. Greater emphasis is placed on achieving efficient nutrient cycling in organic farming systems. The balancing of crop and livestock enterprises, and attention to waste handling and management on organic farms, reflects this. Nutrient cycling efficiencies within farming systems have important implications for water resources. Addition of phosphorus to surface waters causes eutrophication. Losses of phosphorus tend to be larger from conventional than organic systems, due to soil erosion effects and disposal or accidental loss of livestock wastes. Nitrate contamination of groundwaters in many areas is closely linked to agricultural practices, but the relative impacts of conventional and organic systems are difficult to evaluate. However, the application of large quantities of inorganic nitrogen fertilisers in conventional systems generally leads to a greater availability, and therefore potential for loss, of nitrogen, than do the organic and biological sources used in organic systems. The organic approach of maximising nutrient cycling is more conserving of non-renewable energy and mineral resources than is the conventional approach based on linear throughputs of nutrients. However, the potential for enhancing nutrient cycling between the human population and agriculture as a whole is seriously limited by infrastructural and technical considerations. Recommendations for research are made, to promote a more sustainable and resource efficient approach to the maintenance of fertility in agricultural soils.