Chemistry Reference
In-Depth Information
minimum than regular tillage) than with Napier barriers (larger for regular
than minimum tillage) (Figure 6).
4.4 Organic Matter and Rehabilitation of Non-Responsive Soils
The nature of non-responsiveness can consist of chemical (e.g. soil acidity-
related constraints), physical (e.g. the occurrence of hard pans at shallow
depth), or biological (e.g. presence of a large Striga hermonthica (Delile) Benth.
seed bank in maize-based systems) constraints, or a combination of all these.
In areas where a large proportion of the soils occur in a non-responsive status
and where soils are not irreversibly degraded (e.g. in cases where all topsoil has
been lost due to erosion), rehabilitation of such soils is a necessity for
sustainable agricultural intensification. Obviously, organic resources are not a
cure to all the above constraints but, under certain circumstances, application
of organic inputs, often at high seasonal or yearly rates, is an option to
rehabilitate non-responsive soils and induce substantial increases in fertiliser
AE (Path C in Figure 1).
In Zimbabwe, for instance, applying farmyard manure for 3 years to
sandy soils at relatively high rates enabled a clear response to fertiliser where
such response was not visible before rehabilitation (see Figure 7). Application
of single super phosphate (SSP) fertiliser or manure with 100 kg N ha
21
on the
sandy homefield increased maize yields in the first season, but optimum
responses were attained at P application rates of 10 kg ha
21
. The response of
maize to 100 kg N ha
21
at the different rates of P in the first season was very
poor and not significant on the sandy outfield, with maximum yields less than
1 tonne ha
21
, irrespective of the source of P. The lack of response to nutrients
added and dolomitic lime observed on the sandy outfield, which was acidic and
low in all nutrients, was unexpected and indicates that this field was deficient in
other nutrients besides N, phosphorus (P), calcium (Ca), and magnesium (Mg).
Yields on the sandy outfield were marginally improved in the second season of
manure application. Only in the third season did manure significantly increase
yields, against decreases in yields for the sole N treatment. The yields remained
markedly small on the sandy outfield with all SSP treatments (Figure 7).
In a set of medium-to-long term agroforestry trials in West Africa, topsoil
Ca content, effective cation exchange capacity and pH were substantially
higher under Senna siamea than under Leucaena leucocephala, Gliricidia
sepium, or the no-tree control plots in sites with a clay accumulation soil
horizon (Bt horizon) rich in exchangeable Ca.
31
This was shown to be largely
related to the recovery of Ca from the subsoil under Senna trees. At one of the
sites in Benin Republic, after 6 years of continuous cropping, topsoil
degradation related to high soil acidity and low base saturation levels, resulted
in zero maize yields even in presence of fertiliser.
32
Integration of hedgerow
trees, especially Senna siamea, resulted in maize yields approaching 2 tonne
ha
21
, indicating that hedgerow prunings can counteract soil acidity-related
constraints and restore soil responsiveness.
d
n
1
r
2
n
g
|
5
