Agriculture Reference
In-Depth Information
until it reaches the so-called permanent wilting point (PWP), where most
plants are no longer able to extract water from the soil and the water potential
is too strong. At PWP, plants start to wilt, irrespective of the time of day.
Water at PWP is retained by the soil with a force of 1500 kPa (15 bars, ~15
atmospheres).
Several methods are available to determine plant needs and the amount
of irrigation that must be applied. Simple approaches involve digging a small
hole with a shovel to see if there is moisture at a certain depth or checking
whether the annual weeds in the orchard are wilting. In some cases, growers
use rain gauges (pluviometers) to determine the amount of rain and calculate
how much irrigation water to apply knowing the crop's total water needs on
a weekly basis. Total crop water needs depend upon the surface area covered
by the crop canopy and the potential evaporation. In certain banana-growing
areas in Honduras, irrigation needs are estimated based on research data
showing that the plants need about 2000 mm of water falling on the ground,
~40 mm/week, supplied with three applications of 13 mm each time. The
amount applied is reduced by the amount of rainfall recorded in the rain
gauge in 2-day periods. For example, if it rained 6 mm they would only irrigate
to apply 7 mm.
A more accurate method is to use tensiometers (Fig. 4.5c). Tensiometers
are pieces of pipe with a porous tip where water can fl ow in and out. The
pipe is fi lled with water and at the other end there is a manometer, which will
measure the tension by which water is retained by the soil. The manometer is
calibrated in centibars (cbars) or hundredths of a bar, a bar being equivalent to
1 atmosphere. In a saturated soil, there will be practically no tension and the
manometer should indicate 0 cbars; at 10-25 cbars there is a good amount of
water available, while at >25 cbars, water is becoming more dii cult for the
plant to extract and at 75-80 cbars irrigation is essential.
Tensiometers are usually installed at a number of locations spread
throughout the orchard and at dif erent depths to represent the root-zone
conditions. If the tensiometer with the porous tip 30 cm deep shows high
tension, irrigation is necessary, while if the one with the tip at 60 cm shows
tension it means more water has to be applied to reach that depth. The 30
cm depth indicates when and the 60 cm tip indicates the amount of water
required. For avocados, irrigation should be carried out at 40-50 cbars
tension. In basin or furrows irrigation, 50-60 cbars would indicate the need
to irrigate, and with drip irrigation 10-15 cbars would be the threshold.
In the last few years more sophisticated remote indicators or sensors have
become available. Some sensors measure the electrical resistance of gypsum
or other blocks whose resistance varies according to the soil water content.
Other instruments measure the time an electromagnetic impulse takes to
travel through the soil (TDR - time domain refl ectometry), and that varies
according to soil moisture content, and the neutron emitter that receives the
refl ected emissions, which also vary according to soil moisture.
