The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 282

also represents a major component of the N input. Main outputs are
through crop and animal sales.
Discussion
The limited data compiled suggest that the soil P index is reduced in soils in
organic farming systems, while the levels of available K in soils are less
affected. However, this may result from the samples used. Samples from
the organic farms were those submitted for analysis rather than sampled
routinely, and would have included grass as well as arable fields. Studies
have shown that levels of available P and K in soil have both increased
(Clark et al ., 1998) and decreased (Penfold et al ., 1995) on conversion to
organic farming. Loes and Ogaard (1997) found reductions in available P
and K in soils with previously high levels following conversion to organic
farming as a result of the reduction in net imports. Clark et al . (1998)
showed that most changes in soil P and K could be predicted from nutrient
budgets for the systems.
The nutrient budgets for the model farms show both surpluses and
deficits for P and K. Management of nutrients within a stockless rotation is
more difficult than in the other systems considered, the import of compost
is essential to maintain crop production, and some use of approved K fertil-
izers may be necessary. On the lowland dairy farm, on which the model
is based, rock phosphate is used occasionally, which will address the small P
deficit. Within the farms, there may also be large differences in nutrient sta-
tus over time, with soil nutrient status rising in areas which receive manure,
etc., more frequently and declining on fields that receive less inputs or have
larger nutrient offtakes (Bacon et al ., 1990). A more detailed look at spatial
flows would indicate areas of enrichment and depletion on the farm allow-
ing better allocation of P and K. Where P and K supplies are limited, it may
be necessary to target them to the most responsive crops. Temporal flows of
nutrients also need to be considered as fertilizers are often applied to the ley
phase of the rotation and are later made available to subsequent crops.
Loes and Ogaard (1997) suggested that organic farms may maintain
lower soil nutrient levels as an adaptation to lower inputs. Mäder et al .
(1999) demonstrated that plant nutrient deficiencies were not seen, and
hypothesized that the larger microbial biomass in soils farmed organically
may have been responsible for controlling the supply of P and K to the soil
solution. The less intensive nature of production under organic standards
may mean that lower soil P and K levels are acceptable. With careful farm
husbandry, P and K can also be managed sustainably in organic farming
systems, so that except where large reserves have been built up in soils,
organic farmers do not mine the fertility of the soil but maintain economi-
cally sustainable production levels.