Agriculture Reference
In-Depth Information
Thus p is the average fraction of total available water that can be depleted
before moisture stress occurs. Rooting depths and p values for alliums are
given in Table 6.3. These rooting depths and p values are low compared with
those of other crop species, a consequence of their shallow rooting habit and
low root length per unit volume of soil (see Chapter 2). p depends on the
potential crop evapotranspiration rate, ET C . p decreases as ET C
increases, and
increases with a decreasing ET C (Eqn 6.13).
p = p Table6.3 + 0.04(5 - ET C ) (Eqn 6.13)
The soil texture also has an effect, with p values being 5-10% lower for
clay soils and 5-10% higher for sandy soils. Volumes of total available water,
TAW and readily available water, RAW, for fully grown bulb onion crops on a
typical range of soils are contrasted with values for maize in Table 6.4. To avoid
yield restrictions, water depletion should not fall below RAW, and it can be seen
how this requires more frequent irrigation for onions than maize and on sand
rather than on silt soils.
Below RAW - i.e. when the soil water deficit is greater than p (root zone
water depletion, D r > RAW) - transpiration can be assumed to decrease linearly
towards zero at permanent wilting as K S , the stress adjustment factor,
decreases from unity towards zero (see Fig. 6.13); therefore:
K S = (TAW-D r )/(TAW-RAW) = (TAW-D r) /[(1-p)TAW] (Eqn 6.14)
D r = root zone water depletion mm, estimated from the daily soil water
balance
TAW = total available water mm
K S from Eqn 6.14 above can be used in Eqn 6.11 to compute the
evapotranspiration adjusted for water stress, ET Cadj .
For maximum growth rate, irrigation should be applied before D r falls
below RAW, but the amount of water applied should not exceed D r , in order to
avoid leaching nutrients from the root zone.
EVAPOTRANSPIRATION AND YIELD Bulb yields increase linearly with evapo-
transpiration (see Fig. 6.14a). The yield response factor, K Y , the relative
Table 6.4. Values of Total Available Water (TAW) and Readily Available Water (RAW) (mm)
for a bulb onion and a maize crop on contrasting soil textural types (from Allen et al. , 1998).
Depletion
Rooting
fraction
Loamy Sand
Silt
Silty clay
depth
for RAW
p a
Z r (m)
TAW
RAW
TAW
RAW
TAW
RAW
Onion
0.4
0.30
36
11
68
20
48
14
Maize
1.2
0.55
108
59
204
112
144
79
a The p value applies to the standard climatic conditions given in Table 6.3.
 
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