Nutrient disequilibria
in agroecosystems
Concepts and Case studies
Edited by E.M.A. Smaling, O. Oenema and L.O. Fresco
© CAB International 1999
The
stocks and flows of nutrients into and out of farming systems are discussed
in the fourteen separate studies that make up this book. The first six
chapters are conceptual in nature, comparing, for example, nutrient flows
between natural and agricultural systems and plant-nutrient availability
as a function of total nutrients and the processes that influence this
ratio. These chapters are followed by case studies of nutrient flows in,
for example, mixed farming systems in Mozambique and Kenya, agriculture-aquaculture
in The Philippines, marine fisheries in Norway, plantation forestry in
South Africa, fertilizer use in annual cropping systems in China and ecological
agriculture in Switzerland. The final section of the book provides recommendations
in the form of 'Ten Commandments' for integrated nutrient management.
Taking China as an example, the problem is seen as one of unbalanced nutrition rather than of negative nutrient balances, although both go hand in hand. Use of mineral fertilizers gradually became more widespread in the fifties after thousands of years of total dependence on organic fertilizers. Following a period of huge N application, P fertilizer was introduced as the soils ran out of their natural supply. The same situation has now arisen for K. Nutrient balances for N and P gradually shifted from negative to positive but, with inadequate supply of K and a large percentage of straw removed from the system, the balance for K is now highly negative. Soil K depletion continues and crop yield response to K is increasing significantly.
In Kenya, for instance, the nutrient balance is very much affected by the local economic situation. Farms on which food crops are predominant show a trend of strong nutrient mining because low crop prices deter farmers from buying fertilizers. The nutrient balance improves with higher cash flows because mineral fertilizers become more affordable. The overall conclusion, however, is that nutrients are being mined in smallholder agriculture in Kenya. This also holds true for Mozambique and without doubt for the rest of Africa.
Among the 'Ten Commandments', the editors call for the promotion of 'Integrated Nutrient Management' at political and farm household levels in an effort to reverse the trend of worsening nutrient imbalance. They also make suggestions of how this could be done. They point out that "large areas suffer from either severe nutrient depletion or environmental pollution as a result of over use" and that part of the problem is that for too long soil has been regarded as "a good without price".
Nutrient disequilibria in Agrosystems: Concepts and case studies is published by CABI Publishing, CAB International, Wallingford, Oxon OX10 8DE, UK, ISBN 0 85199 268 4 and sold for £60.