The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 291

It is assumed under organic standards that soils with a clay content of
> 20% should, under organic management, be able to derive sufficient K
from the mineralization of clay minerals to sustain crop yields. Over the 11
years of this study, it would appear that this was the case, although longer
term studies over a range of soil types are needed to substantiate this claim.
Conclusions
Under experimental conditions, there has been a decline in soil organic
matter levels under stockless organic management. This was significant in
the first 4 years of the trial. If stockless rotations are to become increasingly
widespread within the organic sector, then longer periods of fertility build-
ing and the use of composted green waste would be necessary to prevent the
decline in organic matter levels under stockless organic management.
It would appear that using rock phosphate may be capable of maintain-
ing soil P levels under stockless management with moderate yields and
nutrient offtakes. Similarly, on clay soils, there appears to be no significant
decline in the soil K status.
However, organic farming needs to review the potential impacts of
organic management on soil fertility particularly under intensive all-arable
and horticultural situations. Changes in soil organic matter levels often
occur over a long time period and it is important that rotation design
evolves as our understanding of soil processes develops.
Acknowledgements
The hard work and dedication of all the scientific, advisory and support
staff and students, in particular H. Bulson, S. Millington, C. Stopes and J.
Welsh, who have contributed to this project over its lifetime, are gratefully
acknowledged. This work was funded by the Progressive Farming Trust.
References
David, C., Fabre, B. and Gautronneau, Y. (1996) Towards modelling the conver-
sion of stockless farming to organic farming. On-farm research in South East
of France. In: Kristensen, N.H. and Høgh-Jensen, H. (eds) New Research in Or-
ganic Agriculture . IFOAM, Ökozentrum Imsbach, Tholey-Theley, pp. 23-27.
Elm Farm Research Centre (1993) The Availability of Water Insoluble Phosphorus and
Potassium Sources in Organic Farming Systems . Final Report to MAFF, Contract
CSA 1486. Elm Farm Research Centre, Newbury, Berkshire.
GENSTAT 5.3 (1993) GENSTAT 5, Release 3 Reference Manual . Oxford University
Press, Oxford.