Introduction:
During their life cycle crops need an appropriate amount
of water to survive and produce. All crops have different
requirements for water (#1).
The amount of available water for the crops depend on the
amount of rainfall and of the soil capacity to retain moisture
(#2). Most farmers depend on rainfall for their crops. They
do not have any means to control the factor of water. In such
a case, the amount of rainfall, its distribution during the
year, and the soil capacity to retain it, will determine which
crops can be successfully produced and also during what time
of the year they can be planted.
Farmers who have access to an irrigation system can control
the amount of available soil moisture (#3). They can also
plant during any time the year if other climatic factors permit.
It is important that the farmer knows the critical periods
when the crops require most of the water. In general crops
will require water during the following development periods
(#4):
-
Germination
-
Flowering stage
-
Grain formation
-
Tuber formation
-
Fruit formation
Concept #1:
Crops' water requirement.
Water is the chief component of living tissues (#5). The
crops we will be growing in the production unit are no exception.
Water is involved in the transfer of nutrient elements from
the soil into the roots and upward throughout the plants.
It supplies hydrogen as a component of organic materials formed
during photosynthesis. Also, the transpiration of water cools
the plant under high temperature conditions.
Historically, yields are drastically reduced more by drought
than by too much water (#6-7). This is discouraging to the
farmer working in hot, dry climates, for his crops generally
transpire more than do those in wetter climates. The result
is that where water is scarce and expensive, plants require
more water to produce a given yield than their counterparts
in areas where water is plentiful. Rainfall has its greatest
value when it falls during the crops growing season. The critical
period for moisture for most crops occurs just before or after
flowering. In corn, for example, rainfall ten days after tasseling
and silking has an almost dominant effect on yield. Plant's
water needs are at no time more important than when it starts
to reproduce.
Other factors, such as soil fertility, disease, and insect
damage affect the plant's ability to make efficient use of
water. Plants grown in fertile soil use more water than those
on infertile soils because they produce more of everything.
They make, in other words, more efficient use of the water
they receive per unit of production.
Different crops require different amounts of water (#8).
This requirement is a strong determinant in what type of crop
is best suited to our soils and climate. In general, the deeper
the root of the crop, the greater its need for water (#9).
Corn, for example, needs only half as much water as soybeans
to produce the same amount of dry matter.
Learning Activity #1: Group Discussion:
Go through the list of crops made during the first lesson.
Compare to the list from table 1. How much water do they require?
Which crops really need irrigation or can be grown during
the rainy season? Discuss possibilities with the group.
Concept #2:
Is rainfall enough for my crops?
Most farmers will grow their crops on rain-fed land with
the exception of some that will have available irrigation
during a short season (#10). With an irregular rainy season
every year it is very hard to plan for the crops water needs.
The farmers just hope that it will rain and his crop survive
and produce at least for his family to eat. This does not
mean there is nothing the farmer can accomplish through soil
management to increase the available water for his crops.
The main key to raising the amount of available water is to
learn to manage rainfall to get the most out of it. Here are
three basic steps that a farmer can do to raise the amount
of available water for his crops.
-
Increase the rate at which water enters the soil surface,
thus reducing water runoff. It is not the amount of rain
that falls but how much you get into your soil that counts.
When possible, protect the soil surface from the beating
action of rain that breaks soil particles. This requires
leaving the past crop residues on the surface of the soil
(#11-12).
-
Get more production for each centimeter of water used
by the crop through proper fertilization, timely operations,
and other production practices. Plant the crop across
the slope or on the contour (#13). Normal land preparation
(seedbed preparation, planting, cultivating) will leave
ridges that hold some water and reduce runoff (#14).
-
Early and appropriate control of weeds to prevent them
from competing with the crops for water (#15). Cultivate
to control weeds and break the crust that forms after
rainfall. Cultivation will not have a lasting effect but
will increase the rate at which rainfall enters during
the first part of the next rain.
The use of narrow rows under certain conditions contributes
to a more efficient water utilization (#16). Narrow rows,
high populations, and large leaf area shade the soil and reduce
soil evaporation. But greater exposure of leaves to the sun
increases the loss from leaves. A large canopy is desirable
because more water moves through the plant, thus contributing
to crop yield rather than being lost through soil surface
evaporation.
Learning Activity #2: Group Discussion:
With the help of table 1 determine which crops can be grown
during the rainy season. Discuss with the group the possibilities.
Show what happens to water intake and movement when: 1).
Soil has high organic matter; 2) clay soil; 3) sandy soil;
4) dry soil; 5) moist soil; etc. (#17).
Concept #3
Irrigation
Farmers with access to an irrigation system can control the
available moisture in the soil (#18). They can plant crops
at different times during the year when irrigation is the
only limiting factor. Irrigation for crops requires large
amounts of water, and the supply must be reliable. Small streams
and farm ponds are poor sources because they can supply the
least water when it is most needed during drought periods.
Depending on the natural rainfall and water capacity of the
soil, attention should be directed to the stage of crop development
in determining the time to irrigate (#19). When water is applied
at the proper time, a limited number of irrigations gives
as good results as a greater number. Water should be applied
when about 50 to 60% of the available soil moisture has been
consumed. Some people with experience have confidence in estimating
the 50 % available moisture level by squeezing the soil in
their hand. A sandy soil will not form a ball; loams and silt
loams form a ball that crumbles easily; clay loams form a
ball which is pliable but shows cracks.
Irrigation to protect crops from drought and to boost yields
does not necessarily insure additional profit from crop production.
Water must be applied at the right time, in the right amount
and at the proper rate. Also, adding water is no guarantee
that yields will be higher. Irrigation must be combined with
cropping practices which will assure making the most of the
water available for the crop, including high fertility, well
timed operations, growing adapted varieties, good insect and
weed control and a good disease prevention program.
There are several types of irrigation systems. The most common
system used by small farmers is the surface systems. Sprinklers
and drip irrigation systems are very expensive and most farmers
will not have access to them.
Surface systems normally require less investment in equipment.
However, more labor is usually necessary and water requirements
are greater. Reuse systems in conjunction with surface irrigation
will greatly improve the efficiency of water usage.
While flood irrigation is usually the lowest cost method
where water is relatively inexpensive, it is likewise the
least efficient in terms of water usage. Furrow irrigation
is another variation of flood irrigation (#20). The water
is confined to narrow furrows rather than wide border checks.
Furrows may be either straight, zig-zag, or contour. |
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