Agriculture Reference
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from the drains is compared with SON measured in 2 M KCl soil extracts.
(The comparison is semi-quantitative because the DON and SON were not
sampled at the same times.) The data show that the DON pool is equivalent
to only 2-10% of the SON pool, increasing in the order nil N plot < N
fertilizer plots < FYM plot. However, ~10% of the N leached from drains is
organic, and significant amounts of DON are leached from plots receiving
FYM.
Thus the amount of N that is likely to be leached in organic forms
appears to be much less than that contained in soluble organic forms within
the soil. This raises the question of how much SON is mineralized to
ammonium and nitrate prior to leaching or whether it is taken up directly
by crops (Murphy et al ., 1999b). Its contribution to N loss from arable
agriculture would seem to be small, except from manured land.
We have also measured the dynamics of mineral N and SON through
an agricultural year in the plough layer (0-23 cm) of the Ley-Arable Exper-
iment at Woburn Farm (Fig. 4.2). SON is not as dynamic as mineral N, but
it changes through the year. In particular, the quantity of SON increases
during periods of plant uptake and mineralization-immobilization. How-
ever, we have no conclusive evidence of a causal link. Under continuous
arable cultivation, the size of the SON pool is relatively constant at
~15-20 kg SON-N ha −1 , decreasing with leaching in early winter and
increasing markedly during the period of rapid root growth in spring
(Fig. 4.2b). Under ploughed-out grass, the SON pool size is larger at
~20-25 kg SON-N ha −1 and the changes are larger than under continuous
arable, but occur at the same times of the year and in the same way
(Fig. 4.2a). The dynamics of the SON and mineral N pool in the plough
layer were reflected in sub-soil down to 90 cm (data not shown). The
period of rapid plant growth both above and below ground appears to play
an important role in SON dynamics. In this soil at least, there is a relatively
constant pool of SON (related to SOM content and soil texture) and a
more dynamic pool of SON which reflects current plant dynamics. We are
studying the chemical composition of these pools.
Improving the Efficiency of Use of P and K Fertilizers
Field experiments around the world, applying P fertilizer with manure on
contrasting weakly and strongly P-fixing soils, are testing the hypothesis
that, if P can be kept in organic forms, fixation is reduced and availability
and crop yields can be increased. Results are encouraging (Greenland,
1997; Sanchez et al ., 1997). Table 4.4 shows yields of maize from the 23-
year-old Kabete Experiment at the Kenyan Agricultural Research Institute
(KARI). As with the Broadbalk Experiment, at the beginning of the Kabete
Experiment, NP fertilizer gave as good if not better yields than those
 
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