Chemistry Reference
In-Depth Information
Table 4
Effects of modest applications of fertiliser on shoot dry matter, water
use and water use efficiency (WUE) for crops of barley at Breda,
Syria,
71
and pearl millet at Sadore and Dosso, Niger (ICRISAT).
(Source: adapted from Gregory).
100
d
n
1
r
2
n
g
|
1
Rainfall
(mm)
Dry matter
(t ha
21
)
Water use
(mm)
WUE
(kg ha
21
mm
21
)
Crop
Season
Fertiliser
Barley
1981/82
324
+
6100
231
26.4
2
4540
231
19.7
1983/84
204
+
2880
176
16.3
2
1340
171
7.8
Millet
1984
260
+
4750
165
28.8
2
2417
163
14.8
1985
380
+
5000
247
20.2
2
3100
270
11.5
1986
440
+
3850
268
14.4
2
1140
211
5.4
d
n
6
h
3
soils.
108
Studies in the Sahel have also concluded that soil fertility is often a
more important factor than rainfall in rangeland and crop productivity, so that
effective management of water cannot be achieved without also managing soil
nutrient constraints.
109
The limiting factors to crop growth at different times
during any particular season could be either water or nutrient availability, or
both. A practical problem to be resolved in many semi-arid regions is how to
afford and apply the optimum amount of fertiliser to produce an economically
viable yield in a given season. For example, an analysis of financial returns
over a 40-year period from fertiliser applications in an area with low and
erratic rainfall in south-eastern Australia found that a very low input of N
fertiliser (5 kg N ha
21
) ensured the greatest economic stability at all sites
examined.
110
Most scientific analysis is conducted with the benefit of
hindsight, but farmers must operate without this benefit, so that conservative
practices tend to dominate.
4.2
Improved Tillage
In addition to managing chemical and biological elements of soil fertility, there
is increasing emphasis on soil physical fertility and health as a means of
improving both root growth and function, and soil ecosystem services. The
practical means by which these ends can be achieved often relies on tillage. In
much conventional agriculture, soil is often inverted (with, for example, a
mouldboard plough or discs), with crop residues and weeds being buried.
There are many reasons why such tillage practices arose, including: (i) the
preparation of a seedbed, allowing uniform germination; (ii) removal of weed
competition early in the crop's growing cycle; (iii) release of nutrients through
mineralisation after oxidation of soil organic matter; (iv) burial of crop
residues, reducing carry-over of diseases and facilitating easier sowing; (v)
