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Measuring the water storage capacity of soils and the amount of water in soils are vitally important in determining how much water to apply by irrigation. Again, field capacity is the maximum amount of water that a soil can hold against the force of gravity. Water applied to soil beyond field capacity will be lost to deep percolation.
Determining the amount of moisture available to plants in an unsaturated soil is important in calculating the amount of water needed to bring the soil to field capacity. Soils vary in the amount of water they can hold at field capacity. Factors that cause the variation include depth and texture of soil and the percentage of moisture in the soil at the permanent wilting point. Some methods used to determine volumes of water available to crops are considered in the following sections.
One of the commonest methods of determining soil moisture content is the oven-dry method. It consists of taking a soil sample of approximately 200 grams, determining its exact weight, and drying the sample in an oven at a temperature of 105 to 110 centigrade for 24 hours, then weighing the sample and determining the moisture loss by subtracting the oven-dry weight from the moist weight.
Moisture content is expressed as a percentage of the oven-dry weight of the soil. For example, if a 212-gram moist soil sample weighs 197 grams after drying, the percentage of moisture is calculated by dividing 197 into 15, which gives 7.6 percent. Always subtract the weight of the container from both the moist and dry-weight determinations.
The electrical properties of conductance or resistance can be used to indicate the moisture content of soils. The electrical properties of soils change when moisture content changes. Porous blocks of gypsum containing electrical elements are placed in the soil. The moisture content of the blocks change as the soil moisture content changes.
It has been determined that gypsum (plaster of parts) blocks tend to achieve moisture equilibrium with soil. As the moisture increases, the amount of gypsum in solution increases and the resistance between electrical elements in the block decreases.
Materials such as fiber glass and nylon have also been used for making blocks.
Gypsum blocks operate best at tensions between 1 and 15 atmospheres, while nylon blocks are more sensitive and function best at tensions less than two atmospheres. Because of their volubility, gypsum blocks deteriorate in one to three seasons. Gypsum blocks are less sensitive than nylon and fiber glass blocks to soil salts.
To use gypsum blocks, dig a hole to the deepest depth from which you want moisture data. At each desired interval from the bottom up, bury a gypsum and bring its leads to the surface. Bury a number of blocks at different depths in one location.
It is customary to use a color code for the leads if a series of readings are being recorded at each location. For example, the deep leads might carry a red marker, medium leads a white marker, and shallow leads a blue marker.
Having all red leads indicate the moisture content 4 feet deep greatly simplifies recording.
Small transistorized meters for reading electrical properties are available from commercial outlets, as are gypsum blocks with electrodes and leads installed. Gypsum blocks very widely.
Other still more scientific methods using radio isotopes or tensiometers, available in developed countries, are not discussed here.
The oven-dry method is likely the only method measuring soil moisture available to Peace Corps Volunteers in developing countries.
A common method, used by farmers and irrigation technicians alike, is the "feel or physical appearance method." This is a fairly accurate method of measuring soil moisture in the field by taking a soil sample with a soil tube or auger at various depths.
The soil auger is usually nothing more than a carpenter's auger with the screw point and side cutting edges removed. It is light and easy to carry. In soils containing fine gravel, it is frequently difficult, and sometimes impossible, to obtain samples with a soil auger. With a soil tube, it is sometimes possible to cut through gravel layers and still obtain satisfactory samples. The tubes are designed so that (1) they can be pushed into the soil with a minimum of effort, (2) the soil will readily enter the tube, and (3) the tube can be easily extracted from the soil. A portion of the tube is cut away so the soil sample can be inspected when it is taken up.
After the texture of the soil has been determined, the soil sample is first "ribboned" between the thumb and forefinger. If a fairly good ribbon is extruded, soil moisture is usually above 50 percent in the heavier soils. Soils with a very small percentage of clay will not form a continuous ribbon, and the "ball" method should be used. Table 4-1 describes the ball forming method and the percentages of moisture generally left in the soil. Table 4-2 shows water holding capacity of soils.
Table 4-1. Guide for judging how much soil moisture is available for crops
|
Available soil moisture |
Feel or appearance of soil | ||
|
remaining |
Light texture |
Medium texture |
Heavy texture |
|
0 to 25 percent |
Dry, loose, flows through fingers. |
Powdery dry, sometimes slightly crusted but easily broken down into powdery condition. |
Hard, baked, cracked, some times has loose crumbs on surface. |
|
25 to 50 percent |
Appears to be dry, will not form a ball.* from pressure. |
Somewhat crumbly but holds together |
Somewhat pliable, will ball under pressure.* |
|
50 to 75 percent |
Tends to ball under pressure, but seldom holds together. slick slightly with pressure. |
Forms a ball somewhat plastic, will sometimes |
Forms a ball, ribbons out between thumb and forefinger. |
|
75 percent to field capacity |
Forms weak ball, breaks |
Forms a ball, is very pliable, |
Easily ribbons out between |
| |
easily, will not slick. in clay. |
slicks readily if relatively high |
fingers, has slick feeling. |
|
At field capacity (100 percent) |
Upon squeezing, no free water appears on soil, but wet outline of ball is left on hand. |
Upon squeezing, no free water appears on soil, but wet outline of ball is left on hand. |
Upon squeezing, no free water appears on soil, but wet outline of ball is left on hand. |
|
Saturated |
Water appears on ball and hand. |
Water appears on ball and hand. |
Water appears on ball and hand. |
* Ball is formed by squeezing a handful of soil very
firmly.
(S.C.S. Inf. Bull No. 199)
Persons with some experience in irrigation soon become aware that the wetter the soil, the deeper one sinks into the mud. This observation has been used to indicate how well the soil is irrigated.
Inserting a shovel into the soil gives a better indication of soil moisture. A still better method is to use a steel rod about one-half inch in diameter. By pushing the rod into the soil, the depth of wetting can be determined.
These methods, while not highly accurate, are most useful where accurately calibrated instruments are unavailable.
Growth of most crops produced under irrigation is stimulated by moderate quantities of soil moisture and retarded by either excessive or deficient amounts of moisture. Air is essential to satisfactory crop growth; hence, excessive filling of the soil pore space with water drives out the air and inhibits plant growth.
On the other hand, soils with deficient amounts of water hold it so tightly that plants must exert extra energy to obtain it. If the rate of intake by the plant is not high enough to maintain turgidity of the leaves, wilting will follow. When soil moisture content is somewhere between these two extremes, plants grow most rapidly.
Light green in alfalfa generally indicates an adequate moisture supply and satisfactory growth. Among root crops such as sugar beets, a need for water is generally noted by temporary wilting during the warm part of the day. Grain crops such as maize also will wilt temporarily when moisture is in short supply. In fruit crop production, it is not practical to wait for wilting to detect moisture requirements.
Plant roots will not grow into a dry soil, nor will they grow in or into a water logged soil; rice and a few other crops are exceptions. Application of excessive amounts of water inhibits root growth and activity, so plants develop a yellowish appearance and are unthrifty and slow growing.
Table 4-2. Water holding properties of soils
|
Soil type |
Average moisture holding (field) capacity in mm per m |
Depth of avail- able water per unit depth of soil mm/m |
Amount of water (mm) needed to restore root zone (1 m) when 50% level is reached* |
|
Clay |
250 |
160-300 |
125 |
|
Clay loam |
220 |
100-180 |
110 |
|
Silt loam |
185 |
60-130 |
90 |
|
Silt |
150 |
50-115 |
75 |
|
Sandy loam |
125 |
40-110 |
60 |
|
Fine sand |
85 |
20- 40 |
40 |
* This is probably more than can be applied at one time because lower levels of root zone will not be as dry as upper levels where more roots are withdrawing moisture. Also uniform soil texture is assumed over entire depth, such uniformity seldom exists except in deep alluvial soils along streams.
A particularly critical stage for plants is when seed is germinating and for a short time thereafter as roots develop some depth. To handle this problem at seeding time requires that one of two conditions exist:
1. Seeding is done at a time of the year when normal rainfall will be sufficient to provide moisture for germination. Seeds must be in contact with very moist soil at the surface.
2. If seed must be planted during a normally dry time then the soil should be very well irrigated before or immediately after planting. This problem is very severe where furrow irrigation is used and seed is planted on the ridge. Usually not enough water can be applied to have the top center of the ridge sufficiently moistened by lateral and upward movement of water to the top of the ridge by capillary action.
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