Use of Moisture by Plants

Sterling A. Taylor.

The growth of plants reflects the rate at which energy is used to remove water from the soil to supply their needs.

Most of the water that plants take up from the soil passes out into the atmosphere as transpired water. Consequently the rate of transpiration determines more than anything else how fast water must be taken up.

Plants wilt whenever the rate of transpiration exceeds the rate at which water enters their roots. A wilted plant does not grow. If something happens to reduce the rate of transpiration such as an increase in the relative humidity of the air or a drop in leaf temperature the rate of uptake might exceed the rate of transpiration (or depletion), and the plant will regain turgidity fully distended tissues and grow again.

Weather conditions govern almost entirely the transpiration rates for turgid plants, which need more water on hot, dry, windy days than on humid, cool, or calm days. Likewise plants have greater need for water in hot, dry climates than in cool or humid climates. The needs also are greater in regions where solar radiation is high than in places where much of the sun's energy is intercepted and absorbed in the atmosphere.

The transpirational needs of crop plants are small on cool, cloudy, calm days, when the atmosphere is nearly saturated, but they might need more than one-half inch of water a day on hot, windy days when relative humidity is low.

The total water requirement for the crops is the sum of the daily requirements for every day of the season. It drops as low as 6 to 8 inches for short-season crops in humid climates. It may be 30 to 40 inches for long-season crops in and climates.

Solar radiation strongly influences the amount of evaporation and transpiration that takes place. It is therefore a good indication of the rate at which plants require water.

The amount of solar energy arriving at the earth's surface averaged 542 calories a square centimeter a day at Albuquerque, N. Mex., in 1954; it was 288 at Astoria, Oreg. The maximum at Astoria was 585 calories a day in May; at Albuquerque it was 777 a day in June. The minimum at Albuquerque was 295 calories per square centimeter a day in December. At Astoria the minimum was 79.

Solar radiation is high throughout the dry regions of Washington, Oregon, Idaho, Utah, Nevada, California, Arizona, New Mexico, Texas, Oklahoma, Wyoming, and Kansas, where it reaches average values above 700 calories a square centimeter a day for one or more months in the year. The rate of water use is highest there.

In the humid localities of New York, Maine, Minnesota, Pennsylvania, Maryland, Rhode Island, and Massachusetts, the mean monthly solar radiations did not exceed 600 calories a square centimeter a day in 1954. The rate of water use by plants was lowest in those States.

The amount of solar radiation and rate of water use by plants were intermediate at places where measurements were made in Florida, North Carolina, Georgia, Indiana, Arkansas, Louisiana, North Dakota, and Nebraska.

The areas of high solar radiation usually have higher temperatures and lower average relative humidities. Much more soil water therefore is required for a given plant there than in areas of lower solar radiation. Local winds and other conditions can modify these generalizations.

WATER CAN BE STORED in the soil in limited amounts. The soil itself limits the amount that can be stored for future plant use. Some farmers supply large excesses of water to the soil during irrigation in the false belief that all the water that enters the soil will be held there until it is evaporated or used by the plant.

Excess water actually may do serious damage when it carries nutrients below the root zone or raises the water table in low-lying areas to a point where drainage may become necessary to keep the land productive. The term "field capacity" is used sometimes to express the amount of water that remains in the soil moisture reservoir after the applied surplus has leaked away.

Part of the excess water drains out rather fast in soils that have good internal drainage. It enters the water table if one exists, or it might enter dry soil and eventually create a new water table.

The movement of water into dry soil from irrigation or rain continues uniformly in all directions until it strikes a wormhole, root channel, or a change in texture or structure. Then its movement is retarded.

Soils of coarse texture or structure that are overlain by finer soil will retard the movement of water and tend to increase the amount of water retained in the finer overlying soil.

An underlying layer of fine soil may have the same effect if the water transmission rate is appreciably less in the fine material. The movement of moisture in soil does not stop when one stops applying water to the surface. Rather, the movement continues for hours or days.

Plants cannot remove all the water retained in the soil.

Lyman J. Briggs and H. L. Shantz, of the Department of Agriculture, found that about the same amount of water remained in a given soil at the end of a long, dry summer in the desert regions of Utah, regardless of the species and the size of the many kinds of plants that grew there. Different soils, however, contained different amounts of water at the end of the summer and also when plants growing in them wilted permanently.

In speaking of the availability of water to plants, they said that the permanent wilting of plants represented a point the permanent wilting point on a curve that relates time and the amount of water left in the soil. This point is reached when the forces opposing the further removal of soil moisture exceeds the forces exerted by the plant.

FORCES OF MOISTURE retention in soil increase as the soil dries out the energy that must be expended to remove each additional increment of water from the soil increases; the moisture stress increases as the soil dries out.

When the soil is near field capacity, the forces that retain water in the soil increase only a little with each increment of water removed. The forces of retention increase progressively more rapidly as water is removed. By the time the soil moisture has been reduced to the wilting point, the forces of retention are increasing very rapidly as moisture is removed. This fact led F. J. Viehmeyer and A. H. Hendrickson, of California, to suggest that water could be used with equal facility by plants in the moisture range from field capacity to permanent wilting point.

The concept that water is retained by forces in the soil and withdrawn when plants exert greater force was expressed by Willard Gardner, of Utah.

He showed that such terms as "field capacity" (amount of water in the soil when free drainage becomes negligible) and "permanent wilting point" or "wilting coefficient" (the amount of water in the soil when plants wilt and fail to recover unless water is added) are points on a continuous curve that relates soil moisture content to the energy required to remove water from the soil.

Therefore the facility with which plants can get water must increase as the energy required to remove water increases. Evidence to support this idea has accumulated until there is little doubt that the energy of retention affects the ease with which plants can take up water.

Plants frequently can get water fast enough to supply all their needs even though the energy required to remove the water might increase. When that is the case, plant growth may not be retarded or diminished by the lower availability of water. For some deep-rooted crops, such as alfalfa, or orchards, the surface soil might be reduced to a moisture content near the wilting point before the plant is unable to get water fast enough from greater depths to supply its needs.

Retardation in growth often occurs in dry regions where water is needed by plants at a faster rate, even though the moisture content of the surface soil is much higher than permanent wilting. Growth in the dry regions of Utah is progressively retarded as the average force of moisture retention in the soil comprising the root zone increases beyond 1 atmosphere. Field capacity corresponds to about one-third atmosphere and the wilting point to about 12 atmospheres in the same region.