DIFFERENT SPECIES OF PLANTS reach their peak requirements for water at different parts of the season. The peak rate of use and the total seasonal requirement vary with the kind of crop as well as the climate.

Annual crops have a small water requirement while they are still young and small. At the same time, however, the roots are limited to a small volume of soil so that the soil might need frequent but light irrigations. The plants are especially sensitive to shortage of water at this stage of growth, and even short periods of drought might retard growth so seriously that the plant can never catch up to those that did not suffer.

When the plants are larger and have a more extensive root system, their water requirement is higher, but the roots can reach a larger reservoir of moisture and the plant is less sensitive, so that it can withstand short periods of wilting during the heat of the day without damage.

Sensitivity to moisture stress corresponds to the period of most rapid growth. Seeds might germinate rather slowly if the moisture tension is high, but they will still live until the moisture conditions are right. The plant grows most rapidly during the seedling stage and it is most sensitive then to high moisture stress. If not enough moisture is available, the seedling will die or it may be permanently retarded in growth. As the plant gets older and larger, its relative growth rate and its sensitivity to high moisture stress diminish gradually.

Many crops, such as corn and grains, show a second peak of sensitivity at the time they set seeds. Nearly all crops can stand high moisture stress during maturation and ripening without serious damage to yield. But nearly all such crops will be retarded in vegetative growth by high moisture stress.

Sometimes it might be desirable to limit vegetative growth in order to throw the plant into the reproductive stage and encourage a large seed set. Since moderately high moisture stress limits vegetative growth but has little effect on seed production and maturation, an excellent way exists to encourage seeding. It is to withhold water until the soil moisture tension in the root zone approaches the wilting point.

This technique might be quite useful in seed production of alfalfa, clover, and birdsfoot trefoil where there is a tendency for plants getting normal moisture supply to continue to set small amounts of seed over a long period, and at no time is there a large amount of seed to harvest. By withholding water and obtaining moderately high stress, larger amounts of seed are set. Then, if water continues to be withheld, there will be little additional vegetative growth and little additional seed set, thus allowing considerable amounts to be harvested at one time.

Perennial crops require water earlier in the growing season than do the annual crops. The root systems of most perennials are already well established in the spring, so that as soon as conditions are right for growth there will be a relatively high demand for water. The requirement will be somewhat less if the leafy portion does not cover the ground completely, because less transpiring surface is exposed to the sun's radiation. Since the roots are already well established, the plants do not show the same sensitivity to moisture deficiency as do annual crops in the seedling stage. Their well-developed roots enable them to endure lower soil moisture.

The period of maximum use of water generally corresponds to the period of fastest vegetative growth. In the case of fruits and fruit trees, water should be available at low tension during the period of rapid expansion of fruit size. If the fruits are enlarging more or less uniformly through the season, an increase in moisture stress at any time will cause a retardation in growth and result in smaller fruits at harvest. Fruits that remain small during the summer and enlarge in the fall are affected only slightly by moderate stress during the summer but require abundant moisture during enlargement.

Excess water lowers the growth and use of water by most agricultural crops except rice. Plant roots need to be aerated so that the waste carbon dioxide produced by the roots can be dispelled and be replaced by oxygen. Whenever water-logging or compaction stops this process, the rate of moisture uptake is retarded, and the plants might wilt. Plant growth will cease, and the plants will die if the condition continues long enough.

POINTS TO REMEMBER about the use of moisture by plants:

Proper balance of moisture in the root zone of growing plants is necessary for healthy, vigorous growth.

Aeration will limit growth and the uptake of water if soil is too wet. As soil dries out, the plant is placed under increasing stress until the rate that water can move to the plant root and be absorbed is no longer fast enough to prevent retarded growth and decreased vegetative yields.

During dry, hot weather, the limiting stress in plants might occur at relatively low soil moisture tensions of 1 atmosphere or less.

During cool, humid weather, tensions might become 4 atmospheres or higher before plant growth is retarded.

Limiting plant stress is reached at lower soil moisture tensions in soil containing few roots than in soil with large numbers of roots.