Small Scale Irrigation Systems (Peace Corps) Section 3. Water and plants (introduction...) Transpiration Root systems Available moisture for plant use

Small Scale Irrigation Systems (Peace Corps)

Section 3. Water and plants

Plant growth comes primarily from converting sunlight, carbon dioxide, and water to carbohydrates, proteins, and cellulose. Other nutrient elements needed in lesser amounts include nitrogen, particularly to form proteins, and a wide range of minerals. Water is needed for plant growth for two purposes:

· It enters directly into the chemical formation of various constituents of the plant.

· It serves as a transport mechanism by which nutrients move from the soil to parts of the plant where growth is occurring or it moves chemicals formed by the plant to various plant locations as growth occurs.

Water and carbon dioxide are combined into various carbohydrates by photosynthesis and sunlight furnishes the energy required. The amount of water required for this chemical process is relatively low compared with the amount required to transport other nutrients from the soil to growing parts of the plant. The water required to transport nutrients moves upward in the plant through long capillary tubes (xylem) and is evaporated and transpired through the leaves.

The rate that water evaporates depends on the type of plant. It is very low for desert plants and much higher for most crop plants that grow rapidly.

As water transpires, it causes negative pressure at the roots which then take additional water and soluble nutrients into the plant. Extremely large areas of roots are required to absorb the required water.

Dittmer, measuring the area and length of roots and root hairs of a single four-month-old rye plant, found a total root and root hair length of 11,200 km growing 90 km/day with a total surface area of 639 m².

Transpiration

A large supply of water is necessary for all the activities of plants such as photosynthesis and growth. A young leaf often contains up to 90 percent water. The water content of a leaf at any specific time represents only a fraction of the water reaching the leaf during the growing season. The large water requirement is necessary because water is constantly lost by evaporation from the cells of all aerial parts, especially the leaves.

The leaf has a remarkable structure for capturing carbon dioxide from the air. The undersides of most leaves have a profusion of small openings called stomates. The opening and closing of the stomates is controlled by adjacent guard cells, which are activated by light.

The greater part of water absorbed by plants is lost by transpiration. A vigorous sunflower plant has been estimated to transpire approximately 200 kilograms of water in a 100-day growing season. An individual corn (maize) plant has been calculated to remove 204 liters of water from the soil in a season, 90 times the amount it needed for all other purposes. A mature apple tree may transpire 360 liters of water per day. One hectare containing 88 trees thus would transpire 300 tonnes of water in a midsummer month.

Several external factors greatly influence transpiration rates: radiant energy, air movement, humidity and temperature, and soil conditions. Radiant energy is a dominant factor since the stomata of most plants are open in light. An intensity curve shows a maximum rate of transpiration more or less corresponding with light intensity. Absorption of radiant energy is largely within the visible light spectrum although some of the longer wave lengths (infrared) are also absorbed. Radiant energy absorbed that is not used for photosynthesis is transformed into heat and becomes a factor in water vaporization.

Humidity conditions in the atmosphere are usually expressed as relative humidity. Relative humidity refers to the amount of moisture in the atmosphere at a given temperature compared to the total amount of moisture the atmosphere could hold at the stated temperature. The higher the temperature, the more moisture atmosphere can hold. Thus, if the sub-stomata! air chamber has higher relative humidity than the outer atmosphere, there will be a diffusion gradient (difference in pressure) and water vapor will be lost to the outer atmosphere. A rise in temperature, moderate air currents, and leaf movement also increase transpiration.

Sizes and shapes of leaves affect the amount of water transpired. In general, the modified leaves of such desert plants as cactus lose less water than the leaves of a temperate-region plant such as the sunflower. Water loss is also regulated by the stomata in that when they are completely closed, transpiration is stopped; however, a decrease in diameter of SO to 75 percent apparently has only a slight effect on rate of transpiration

It is apparent that water movement in plants is affected by the water loss from transpiration and the amount of water absorbed from the soil. Some factors that affect the amount of water absorbed from the soil are the extent of the plant root system, the amount of water in the soil, and the concentration of solutes in the soil water.

Root systems

The roots of plants contact the soil and the soil solution and, therefore, absorb water and salts. The plant root system may be viewed as a large probing network that exploits the soil's water and salt resources. The roots also transport water and salts and anchor plants in the soil. There are structurally two main types of root systems--tap root and fibrous root systems. The tap root system consists of a primary or central root that grows more rapidly than any branch roots. Branch roots arise from the central root. In some plants such as carrots, radishes, and beets, the diameter of the tap root may exceed that of the stem.

The fibrous root system has no central axis and most of the branches grow to approximately the same length and diameter. This type of root system may be constituted of relatively thin roots as in annual plants or some parts may grow to larger diameters as in most trees.

Numerous studies of the distribution of root systems have shown that the extent and mass of root development is generally greater than previously supposed. Root growth habits vary with plant species and reflect the influence of soil and climatic factors. In arid regions, alfalfa roots of two-month-old plants have been recorded five feet deep. In general, the more extensive the root system, the greater its absorption capacity.

The depth from which roots of plants can remove moisture from the soil varies with the type of crop. But most annual crops develop root systems that draw most of their water from about the top half meter of soil. Some perennials like alfalfa and trees have deeper root systems and can use more subsoil moisture. Annual plants, such as sorghum, which are commonly grown in semi-arid regions without irrigation, have deeper root systems than such crops as most vegetables and maize (corn) which are commonly grown in more humid climates. Table 3-1 shows the effective rooting depths of some common crops.

Table 3-1. Effective depths for plant roots, cm

Plant roots	Depth, cm
Onion, lettuce	30
Pasture, potato, bean, cabbage, spinach, strawberry	60
Sweet corn, table beet, peas, squash, carrot, eggplant, peppers	90
Sugar-beet, sweet potato, cotton, citrus, lima bean, artichoke	120
Melon, flax, maize, small grains	150
Alfalfa, asparagus, noncitrus orchard, grapes, hops, grains other than maize, sudangrass, sorghum, tomato	180

Available moisture for plant use

Soil water may be classified as unavailable, available, and gravitational or superfluous. If water is applied to a soil until all pore space is filled, the soil is said to be "saturated." About half of the moisture in saturated soil will be lost to gravity. Usually the gravitational water will drain away within about 24 hours. Except for rather slight losses to evaporation from the soil surface and continuing drainage to gravity, the soil moisture remaining indicates the "field capacity" of that soil.

In practice, field capacity is usually determined two days after an irrigation. A soil will come to field capacity more quickly when an active crop is growing than when no roots are removing water from the soil.

Field capacity can be measured by determining moisture content of soil after an irrigation that was heavy enough to ensure thorough wetting of the soil. Observing the decrease in moisture by making moisture determinations at different times after irrigation is valuable in understanding and properly interpreting the moisture-holding characteristics of a soil.

If there are plants growing on the soil, the moisture level continues to drop until it reaches the "permanent wilting point" (p.w.p.). Soil moisture content near the wilting point is not readily available to the plant. Hence the term "readily available moisture" has been used to refer to that portion of the available moisture that is most easily extracted by the plants, approximately 75 percent of the available moisture. After that, the plants cannot absorb water from the soil quickly enough to replace water lost by transpiration.

Formerly, it was often thought that plants thrive equally well regardless of the moisture level, as long as the level was between field capacity and permanent wilting. It is more logical, however, to assume that if water is abundant enough to be easily absorbed from the soil that plants should thrive better. This is supported by research findings.

Maximum crop yields are obtained when the moisture during the critical growing season is maintained near the upper level of the available soil moisture profile. But when a crop is watered too frequently, even with light irrigations, part of the soil will be so constantly saturated that the crop will suffer from poor aeration.

The soil moisture content when plants permanently wilt is called the permanent wilting point or the wilting coefficient. The permanent wilting point is at the lower end of the available moisture range. A plant will wilt when it can no longer extract enough moisture from the soil to meet its needs. Wilting depends upon the rate the plant uses water, the depth of the root zone, and the water holding capacity of the soil. Crop growth should not be retarded by lack of available soil moisture.

Among the root crops, sugar beets readily indicate need for water by temporary wilting, particularly during the warmest part of the day. Withholding irrigation until the crop definitely shows a need for water is likely to retard growth. It is essential to maintain readily available water in the soil for crops to grow satisfactorily.

Plant roots will not grow into a dry soil, nor will they grow in or into a water logged soil except for rice and a few other crops. Application of excessive amounts of water inhibits root growth and activity, primarily because oxygen becomes unavailable to the plant roots. Plants become yellowish, unthrifty, and slow growing.