Chemistry Reference
In-Depth Information
d
n
1
r
2
n
g
|
5
Figure 8
Soil C contents of three aggregate size fractions (macroaggregates (.250
mm), microaggregates (53-250 mm), and silt and clay (,53 mm)) after 3
years of 4 Mg litter-C ha
21
yr
21
input (no input, Tithonia diversifolia,
Calliandra calothyrsus, and Zea mays) in a maize cropping system in
Central Kenya. Error bars represent the standard error of the mean of each
aggregate size fraction. Significant differences in total soil carbon between
treatments are denoted by different letters.
36
The overall aim of ISFM is maximal AE of the inputs used and thus
maximal crop yields and crop residue production for a given amount of inputs
along the linear part of a standard fertiliser response curve. Within the ISFM
framework, crop residues are proposed to be recycled in situ or returned to the
field in the form of manure after these have been fed to livestock. In principle,
higher crop productivity should generate a larger amount of crop residues and
thus have a positive effect on SOM stocks. The often-reported higher SOM
contents in plots near the homestead compared with those further away are
related to higher crop productivity on those plots combined with a preferential
allocation of organic waste to these plots.
33
An interesting research issue, relevant for ISFM and related to the local
adaptation component of ISFM, is whether fertiliser N-AE is higher on soils
with a high soil fertility status, such as the homestead fields, compared with
soils with lower soil fertility status. In plots where any of the above constraints
limit crop growth, a higher SOM content may enhance the demand by the crop
for N and consequently increase the fertiliser N use efficiency. On the other
hand, SOM also release available N that may be better synchronised with the
demand for N by the plant than fertiliser N and consequently a larger SOM
pool
may
result
in
lower
use
efficiencies
of
the
applied
fertiliser
N.
A
