The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 283

The nutrient budgets for the model farms show surpluses for nitrogen.
While N budgets are usually positive on organic farms (Nguyen et al .,
1995), the N surplus commonly is lower than in comparable conventional
systems (Halberg et al ., 1995). Calculated N surpluses were greater than
comparable conventional systems on three pairs of farms in New Zealand
(Nguyen et al ., 1995). However, low P and S availability in the soils on the
organic farms may have limited N fixation, so that it was overestimated in
the calculation; crop uptake of N in arable crops may also have been limited
by other nutrients. The positive N surplus on organic farms may imply that
considerable nitrate leaching is likely. However, nitrate leaching losses mea-
sured directly have been found to be significantly lower on organic than
similar conventional farms (Eltun and Fugleberg, 1996; Berg et al ., 1997).
Leaching losses of between 7 and 52 kg N ha −1 were measured at all
the points in a rotation by Philipps and Stopes (1995), with an estimated
average of ~20 kg N ha −1 year −1 . However, the differences seen between
organic and conventional systems are much reduced when leaching is
expressed relative to crop production levels (Philipps and Stopes, 1995).
Nitrate leaching losses from organic systems cannot be related directly to
the magnitude of the N surplus. Management of the grass-clover leys, such
as timing of incorporation, weather conditions and N demands of the
following crop, need to be considered. Losses of N from manure can also be
important; measures to place manure heaps on paved surfaces and under
cover can help reduce N losses (Dewes, 1995).
Conclusions
Farm gate budgets are useful in indicating whether the system is balanced
at the whole farm scale. The information can also be used for understand-
ing fluxes at a regional level and as a policy tool. For the farmer, a more
detailed approach may be required, as average N, P and K balances may
mask important differences between fields. More complete information will
allow the farmer to optimize management practices and maintain or
improve the internal cycling of nutrients. Further work is necessary to
understand whether organic farms can be balanced in terms of N, P and K
and to allow surpluses and deficits to be interpreted correctly. Additionally,
work is required to allow surpluses and deficits to be interpreted alongside
measurements of soil nutrient pools.
The number of organic farms in the UK has increased dramatically
(Lampkin and Measures, 1999). Limited information exists on the
comparative sustainability of organic and conventional agriculture. Direct
comparisons between systems are difficult, due to differences in factors
such as soils, weeds, pests, diseases, climate and enterprises (Fowler et al .,
1993). In addition, the methods of measurement, data recorded and