Rotations in Conservation

by R. E. UHLAND

GOOD CROP rotations provide for systematic cropping of land in a way that will maintain or improve soil fertility, yields, and the nutrient value of the crops. The type of rotation that should be used on any given piece of farm land depends on the characteristics of the land. If the land is very steep, only sod crops can protect it against serious erosion. If it is gently rolling, rotations that provide a sod cover 1 or 2 years out of every 3 to 5 will keep it from deteriorating. Some level lands can safely be tilled almost every year. Any cropping system, to be fully effective, should be supplemented with conservation practices such as contouring, terracing, application of needed plant foods, soil amendments, return of crop residues and manures, and strip cropping, according to the needs of the particular field or farm.

A glance back at some recent experiences will show the importance of rotations in our future farming.

Although our land has suffered a great deal because of erosion and deterioration, it still possesses enormous unexhausted soil productivity, as demonstrated by the 30-percent increase in wartime food production. That production was something to be proud of, but it involved a speedup in exploitation of our soil resources, which was not counterbalanced by soil-conserving practices. Many farmers, in fact, dropped legumes from their rotations during the emergency, with the result that much Soil organic matter and nitrogen were lost.

The tremendous part that unexploited soil productivity played in. Supplying food during the war years 1942 and 1943 is illustrated by the relative production during those years of the lands that were more recently brought into use for crop production. In those 2 years the North Central Crop Reporting Division, which includes Ohio, Indiana, Illinois, Minnesota, Nebraska, Kansas, North Dakota, and South Dakota, produced these large percentages of the Nation's total of principal food and feed crops: Corn, 76.7; soybeans harvested for grain, 92.8; wheat, 59.5; oats, 82.3; tame hay, 49.9; alfalfa, 37.8; cattle and calves, 54.1; sheep and lambs, 35.9; and hogs, 74.7.

In 1935-39, intertilled crops were grown on 46 percent of all the land used for crops in the five Corn Belt States-Ohio, Indiana, Illinois, Iowa, and Missouri. In 1943-44, the corresponding percentage was 52.1. In Iowa, the percentages of all cropland on which intertilled crops were grown in the two periods were 45.7 and 52.5; in Illinois, they were 50 and 56.9; in Ohio, 38.2 and 45.5. With such proportions of all cropland under tillage in 1943-44, the best possible sequence of crops would not have prevented excessive exploitation of soil resources.

A productivity balance-that is, the net result of the processes contributing to soil productivity and those tending to exhaust it-has been computed for the soils of Ohio farm lands for each of the years from 1929 to 1945 according to a system devised at the Ohio Agricultural Experiment Station. The results indicate that deterioration of the soils slackened during- the first decade of that period; the productivity balance changed from a 0.65-percent loss in 1929 to a 0.5-percent loss in 1939. Under the pressure of war, the trend was reversed; the loss increased from 0.61 percent in 1942 to 0.78 in 1945.

If the same system of measuring the soil-productivity balance were applied for all Iowa and Illinois, the calculated wartime losses would be greater than those for Ohio; in those States, much larger percentages of cropland were used to grow intertilled crops and the practice of growing such crops year after year on the same land was much more common. These figures point to an inescapable conclusion of the highest importance to every farmer and every other citizen: During the next few years agriculture in the Corn Belt and in the entire United States must be directed toward attaining a positive balance in soil productivity through appropriate crop rotations and supporting practices.

Crop Rotations Control Runoff and Erosion

Experiments conducted by a number of State agricultural experiment stations and the Soil Conservation Service have shown that type of crop and type of cultivation have a marked influence upon runoff and erosion. I give a few examples.

At Bethany, Mo., a Shelby loam area cropped annually to corn for 10 years lost 27.2 percent of the rainfall as runoff and, on. an average, lost annually 50.9 tons of soil per acre, equivalent to more than one-third inch. Adjacent land that was cropped to a 3-year rotation of corn-wheat-hay Lost only 16.5 percent of the rainfall as runoff and lost only 7.51 tons of soil per acre annually. Continuous covers of alfalfa and bluegrass permitted only 6.7 and 8.1 percent runoff, respectively, with only a trace (0. 15 ton an acre) of soil loss.

At Temple, Tex., a plot of Houston clay soil with a 2-percent slope cropped annually to corn for 11 years lost 8.8 percent of the precipitation and 22.72 tons of soil an acre a year. Adjacent land in Bermuda grass lost only a trace of water and soil. When oats followed the corn, 13.4 percent of the precipitation ran off and annual soil loss per acre averaged 2.06 tons.

The runoff and erosion from a 7.7-percent slope of virgin Stephensonville fine sandy loam cropped to a rotation of cotton-wheat-sweetclover at Guthrie, Okla., were compared with those from adjacent plots where cotton was grown annually. The water losses were 9.29 percent and 11.35 percent, respectively. The annual soil losses from the land in cotton averaged 4.4 times as great as those from the land in rotation. Percentages of water lost from areas in cotton, wheat, and sweetclover averaged 10.07, 11.55, and 6.25; annual soil losses averaged 9.04, 1.69, and 0.52 tons an acre. Soil loss from cotton in rotation was only a little more than half that from annual cotton. Here again the results clearly show the value of wheat and legumes in a rotation in retarding loss of soil.

Near Zanesville, Ohio, in 1934-42, the runoff from land in a 4-year rotation was only half that from land cropped continuously to corn, and the soil loss was less than one-seventh as great.

Similar results were obtained on Marshall silt loam near Clarinda, Iowa. In the 10 years 1933-42, land cropped annually to corn lost 2.3 times as much of the precipitation and 5.32 times as much soil as land in a 3-year rotation of corn-oats-clover. Low runoff and only a trace of soil loss were recorded for plots in alfalfa and bluegrass. A marked cumulative influence of these two crops in reducing erosion became evident when the land was plowed and planted to corn.

Investigations on many farms in different sections of the Corn Belt have yielded evidence as to how type of cropping and farm cultural practices affect depth of topsoil and, consequently, yield of corn. Loss of topsoil through erosion from fields under different cropping systems and from different locations within a field was clearly reflected in crop yields.

You will note that where practically all the topsoil was gone the yield of corn to the acre averaged much less than half of what it was where 7 to 8 inches of topsoil remained and less than a third of what it was where the topsoil was 11 to 12 inches deep. Similar results were obtained in other studies near Fowler, at Coshocton, Ohio, at Bethany, Mo., and at Shenandoah and Greenfield, Iowa.