Chemistry Reference
In-Depth Information
workers have commented on the small proportion of water potentially
available to crops that is actually transpired in some environments.
71,100
For
example, transpiration from barley crops in northern Syria was only a small
component (,35%) of total water use.
101
Similarly, in Niger, where rainfall
frequently occurs as intensive showers, transpiration was normally less than
evaporation from the soil surface,
102
and on sloping land (angle 2-3%) in
farmers' fields transpiration was as low as 6% of rainfall.
103
Such findings have
led many to conclude that the efficiency with which water is used to produce
crops could be significantly improved in many rain-fed environments.
100,104
Several potential agronomic management options exist for reducing
evaporation directly from soil, and increasing the supply of water to crops
(Table 3). However, the success or otherwise of a particular management
practice in increasing the efficiency with which water is used will depend on a
combination of soil and climatic conditions.
105
The physical factors that
appear to be important in determining the success of a management option
include the moisture characteristic curve and hydraulic conductivity of the soil,
the amount of crop cover and the distribution of roots, the quantity and
temporal distribution of rain, and the potential rate of evaporation.
105
Future
increases in water-use efficiency in rain-fed systems will come, to a
considerable extent, from capturing rainfall and then retaining it in soils until
it is needed by crops.
106
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4.1.3 Nutrient-Water Interactions
It has been appreciated for a long time that there is an interaction between the
efficiencies which water and nutrients are used, not least because the
availability and mobility of nutrients in soils depends on water. In rain-fed
production systems this interaction is a major constraint to the efficient use of
inputs, because rainfall is not known in advance. Where soil nutrients are
deficient for maximum growth of crops, application of fertilisers and manures
may not only result in increased growth but also in increased water use
efficiency (WUE, defined as the quantity of crop dry matter produced per unit
of water used). This effect of modest applications of fertiliser has been well
documented in several studies
71,107
and is illustrated in Table 4. Fertiliser use
may increase slightly the total amount of water used (e.g. barley in Syria;
71
maize in the UK),
107
but the principal effect is to increase early canopy growth
so that it shades the surface and thereby reduces evaporation from the soil
surface as a proportion of the total water that is evaporated. However, the
beneficial effect of fertiliser in increasing growth and reducing evaporation
from the soil surface is not universal, and is dependent on the wetness of the
soil surface and the evaporative demand.
100
In semi-arid production systems, the efficiency of N and P fertilisers depends
on the amount of water available to the crop so that the response to N, in
particular, is variable and limited in dry years. Typically, crop response to N
increases with increasing rainfall while response to P decreases on P-deficient
