The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 288

were no applications of animal residues or other green waste composts. No
applications of potassium fertilizers were made over the lifetime of the trial.
The objective of the experiment was to establish if stockless organic
rotations were agronomically viable. The effect on soil fertility was assessed
by measuring soil organic matter levels, extractable phosphorus and
potassium using the standard ADAS methods (MAFF, 1986). Other
parameters measured included crop yields, expressed at a standard 15%
moisture content, nutrient offtakes, weed biomass and pest and disease
incidence.
In the period 1996-1998, crop yields were measured and the nutrient
content for these years was assumed to be an average of the previous
8 years' data.
Statistical analysis (analysis of variance ANOVA) was conducted using
Genstat 5.3 and the graphs were produced using 'Excel 97'.
Results
There was a significant decline in soil organic matter levels over time across
all three rotations. The most notable change occurred in the first 4 years of
the trial, cycle 1, as the management changed from a grassland to an arable
system. Between cycle 1 and 2, the soil organic matter percentage fell from
3.02 to 2.58%. The decline between cycle 2 and 3 was less dramatic, with
soil organic matter levels falling to 2.42% at the end of the trial. There was
no notable difference between rotations. Changes in soil organic matter
levels over time are presented in Fig. 4.9.1.
There were no differences between the courses of the rotations
although there was a slight decline between course one, the fertility building
course, and course four, the furthest point from the green manure crop, as
would be expected.
There were no significant changes in the available phosphorus over
time, although slow-release rock phosphate had been applied to the
green manure crops when soil analysis revealed a deficiency. The available
P was not significantly affected by the course of the rotation but there were
differences between the rotations. Available P in rotation C, at 14.02 mg P
kg −1 , was significantly lower compared with rotations A (15.42 mg P kg −1 )
and B (15.68 mg P kg −1 ).
Levels of potassium were not affected over the 11 years of the trial,
despite no application of supplementary fertilizers or livestock residues.
There was no difference in soil potassium levels between rotations
A (142.6 mg K kg −1 ) and C (138.0 mg K kg −1 ). However, there was a
significant effect between rotation A and rotation B, in which the available
K levels were 128.3 mg K kg −1 . These results are supported by the crop
offtake data, presented in Table 4.9.2.