In experiments with soil columns in cylinders, Mr. Russel found at Lincoln that the major effects of mulches on evaporation persist about 2 days after a rain. Evaporation declined about 0.1 inch during the first 24 hours after wetting the soil columns. More than 2 tons an acre of residue did not improve the control of evaporation.

The surface of unmulched soil seems to dry faster after a rain than does the soil surface under the mulch. If another rain wets the mulched soil before it dries out, more moisture from the first rain will be saved under the mulch than under the bare soil condition. But if the period between rains is long enough to allow evaporation to proceed until the surfaces of both mulched and bare soils are dry, the evidence points toward slightly higher total evaporation from the mulched soil.

Mulches of straw or other residues sometimes can reduce losses from evaporation. In North Carolina, for example, mulches applied at the time of the last cultivation increased yields of corn 21 bushels an acre in eight experiments conducted under droughty conditions and 5.4 bushels an acre in 10 experiments conducted under normal or abnormal moisture conditions. A part of the increase in the dry years was due to the effect of the mulch in controlling evaporation of moisture from the soil. The value of the mulch in controlling evaporation may depend on the length of the dry period. Very likely the mulches provide control of evaporation in short droughts, but mulched soil may lose as much water by evaporation as bare soil during long droughts.

CROPS AND CROPPING SYSTEMS differ as the soil and environment change. Maximum efficiency in the utilization of all available moisture is usually the goal in selecting the crop or cropping system. This guide is particularly important in regions of limited precipitation. In order to realize maximum utilization of moisture in dry areas, selection of a crop from the standpoint of its seasonal moisture requirements, timeliness of seeding, and cultivating at the proper time are important.

Crops best adapted to dry land are those that make maximum growth when climatic conditions are not too severe. The general displacement of spring wheat by winter wheat in the dryland areas where both are adapted is an example. Winter wheat is usually mature before the hottest part of the summer. Spring wheat, being somewhat later, often completes its development under much more severe conditions of temperature and transpiration.

Grasses and legumes have not been able to compete on an economic basis with wheat and other grain crops on cultivated dryland soils. Legumes and grasses leave the soil in a dry condition and have a high water requirement, which limits their use and adaptation to dry-farming systems. For example, Arthur C. Dillman found in western South Dakota that 430 pounds of water were needed to produce 1 pound of dry matter of spring wheat. Alfalfa required 798 pounds of water to produce 1 pound of dry matter.

The amount of water available to a particular crop may be augmented somewhat by growing it after a crop that does not exhaust the water supply of the soil. Row crops, such as corn and potatoes, usually do not utilize all of the available soil moisture; crops that follow therefore are favored by this residual moisture. Grasses and legumes, as we said, leave the soil dry to great depths; crops that follow them often suffer for lack of moisture.

Findings at several places in the Great Plains show that the yield of the first grain crop following a legume or grass is depressed well below the second or third grain crop after the sod crop. In most instances, this decline occurs on land fallowed for a year following the plowing of the sod crop. At North Platte, Nebr., and St. Anthony, Idaho, legumes such as sweetclover or alfalfa caused greater declines than grass. Over-stimulation due to higher soil nitrogen often occurs after legumes, and grain yields are depressed when moisture becomes the limiting factor before the crop matures.

Grasses help greatly to keep the soil in place and make it receptive to water. Because of their fibrous root system, grasses use all available water within their rooting depth. In experiments at North Platte and Havre, Mont., yields of grass tend to increase until 2 or 3 years after the date of their establishment and then decrease with increasing age of the sod. These declines have been due to depletion of subsoil moisture and thinning stands.

Deep-rooted crops make effective use of stored moisture in deep soils. Alfalfa and coastal Bermuda-grass are examples. If the nutrient status of lower depths is adequate, they can utilize soil moisture at depths of 1 feet or more. If the storage capacity of the soil can be recharged in seasons of heavy rainfall, these crops resist drought on deep soils.

Sorghums and cotton can withstand dry periods and then resume rapid growth when it rains. They yield more if their growth is not interrupted by drought, but they are not total failures in years when dry spells occur. Sorghum has a greater ability to withstand drought than corn. This is attributed to a better developed absorbing system as compared to the transpiring system. The primary root area of the two crops is similar, but the secondary root area of sorghum is much larger than that of corn.

One way you can reduce water losses by transpiration is to seed fewer plants to the acre. In the drier areas, that is commonly done by planting crops in wider rows. Lowering the number of plants in the row is a second possibility. The net effect is to give each plant access to more moisture. The yield from each plant is increased, but usually the acre yield is less. A reduction in yield often is acceptable in that a more thickly planted crop might fail to mature.

Research done in the southern High Plains between 1945 and 1954 disclosed that plant population and not row spacing determines the yield of grain sorghum. Regardless of plant population, narrower rows (20 inch) tended to yield more than wider rows (40 inch) in years of above-normal precipitation. The reverse was true in the drier years.

The crop or cropping system can have a controlling influence upon the amount of runoff and hence moisture storage occurring on sloping land.

For example, G. M. Horner observed that the cropping system employed on the land had a profound effect on runoff losses at Pullman, Wash. During a 30-day period beginning January 9, 1953, rain amounted to 9.23 inches. Wheatland, after alfalfa and grass, yielded 0.9 inch of runoff. Wheat after fallow in a wheat-fallow system yielded about 3.1 inches of runoff. Subsurface-tilled land following sweetclover green manure yielded about 2.6 inches; when moldboard plowing was done, the runoff was 5.6 inches.

ADEQUATE FERTILITY is essential for the efficient use of soil moisture. Symptoms of deficiency of plant nutrients especially those associated with insufficient nitrogen have been wrongly diagnosed as drought injury.

Proper distribution of adequate amounts of all essential nutrients in the soil permits plants to make more effective use of limited moisture supplies in surface soil and also to extend their roots into subsoils for stored moisture.

In experiments at the Missouri Agricultural Experiment Station in 1952-1953, Dwight D. Smith found root development under fertilized crops was greater and penetrated deeper than without fertilization. Corn receiving a complete fertilizer yielded 79 bushels an acre and required 16 inches of water. Corn grown without fertilizer produced only 18 bushels and required about 14 inches of water. The fertilized corn not only extracted more moisture from the soil; it used the extracted water more efficiently. Each inch of water used by the fertilized plants produced about 5 bushels of grain, but each inch of water extracted by plants that did not receive fertilizer yielded only 1.3 bushels of grain.

PROPERLY FERTILIZED crops help conserve moisture in another way. Of the small amount of rainfall received at McCredie, Mo., in 1953, nearly 1 inch was lost as runoff from the unfertilized corn plots. The loss was only one-fourth this amount on fertilized corn land. In the previous 6 years, when precipitation in the crop season averaged nearly 20 inches, 3 inches of runoff occurred under corn without fertilizer. In contrast, fertilized corn in a 2- and 4-year rotation yielded slightly less than 1 inch and one-half inch of runoff.

Crop production in the Great Plains is curtailed oftenest by limited moisture but periodically by inadequate amounts of nitrogen. Wide fluctuations in moisture resulting from climatic variations create difficult and complex problems in adjusting nitrogen to the moisture available at any one time. An overabundance of available nitrogen in relation to moisture can cause marked reduction in yields. Not only is the total supply of nitrogen important; the timing of its availability in relation to moisture availability and stage of plant development is critical.

That the balance between supplies of moisture and nitrogen is important in the efficiency of moisture use by winter wheat was demonstrated in 1954-1955 by R. E. Ramig at North Platte, Nebr. In each season, 5.75 inches of available moisture in the soil at seeding time was best from the standpoint of efficiency of water use. The efficiency, however, varied widely in the two years and with the rate of nitrogen fertilization. In 1954, a year when seeding-to-harvest precipitation measured only 8.9 inches, the highest efficiency of 1.85 bushels per inch of water used was realized when 40 pounds of nitrogen fertilizer was applied to the acre. In 1955, when precipitation during the crop season was 17.1 inches, the top efficiency was 2.3 bushels per inch of water used, but that occurred when 80 pounds of nitrogen an acre was applied. In both years, as the applied nitrogen rate was reduced from those giving the highest efficiencies, the grain produced per inch of water used also was less. Without nitrogen fertilizer, only 0.9 bushel in 1954 and 1.1 bushels in 1955 were produced per inch of water. Each year, the efficiency of water use was approximately doubled by adjusting the nitrogen supply to a level that made possible the best use of stored and seasonal moisture.

Lack of other nutrients also lowers the efficiency of water use by plants. C. O. Stanberry and his coworkers in a study at Yuma, Ariz., in 1949-1952 showed that the combined 4-year yield of alfalfa varied from 25.8 to 50.0 tons an acre, depending on the quantities of irrigation water and phosphorus fertilizer applied. Efficiency of water use varied nearly 100 percent among different phosphorus treatments. It was 7.8 acre-inches per ton of hay when 800 pounds of P₂O₅ per acre were applied during the 3-year period and 14.2 acre-inches per ton when only 100 pounds of P₂O₅ were applied. The results substantiate the importance of adequate soil fertility in achieving the goal of maximum conservation of soil moisture.