Soil Aggregation Improved by Rotating Crops

Much of the benefit from growing sod crops is due to the influence they have upon aggregation, or the binding together, of soil particles. A high degree of aggregation permits rapid movement of water and air within the soil and consequently prevents excessive runoff.

Marshall silt loam plots on which alfalfa and bluegrass had been grown were found to have 50 and 58 percent greater soil aggregation, respectively, than plots on which corn had been grown annually. The improvement in aggregation on the plots formerly in alfalfa and bluegrass was still evident when these plots had been cropped to corn for 3 years. The losses of water and soil from the plots for the 3 years they were in corn, 1943-45, were markedly less than those from plots where a single crop of corn followed one of clover. In other words, land should be kept in a hay crop two or more years at a time in order to improve aggregation and stabilize the soil.

To find what effects different cropping systems had had on the stability of aggregates in soil at Bethany, Mo., we made counts of the drops of water, falling 30 centimeters, required to disperse an aggregate about the size of a BB shot and wash it through a 20-mesh screen. Tests were made on 180 aggregates taken from the surface inch of soil oil each plot. Aggregates of soil on which corn had been grown and of soil that had lain fallow were collected in early spring when oats had been seeded and had grown to a height of about an inch. The results :

Average number of drops required to disperse aggregate Cropping system.

Corn annually 6. 2 Alfalfa 13 years 40. 2 First-year meadow 37. 7 Bluegrass 13 years 31. 2 Corn after meadow 10. 1 Clean fallow 13 years 7. 5 Second-year meadow 41.2 All over the country one can observe the superior physical condition of soils under sods. Where sod has been turned under for corn, more rapid infiltration and less runoff and erosion occur. Also, microbial activity and aeration are greater than where corn has been grown annually.

Rotations Increase Soil Organic Matter and Yields

On nine experimental plots having an 8-percent slope at Bethany, the organic-matter content of the soil at plow depth ( the top 7 inches ) was 3.25 percent in 1930. For the next 13 years the plots were variously cropped or permitted to lie fallow, and plots 6 and 7 were treated with lime and phosphate. In 1943 a second test was made to determine soil organic-matter content. The result was:

The depletion of nitrogen in soils that had been farmed poorly for 50 to 75 years was disclosed in tests by the Ohio Agricultural Experiment Station of seven such soils and comparable virgin soils of adjacent areas. The nitrogen content of the top 7 inches of these cropped Ohio soils had been reduced by from 17 to 48 percent.

After 60 years of cropping to corn and oats the soil organic matter at Plow depth of a Shelby loam area in northern Harrison County, Mo., averaged 1.83 percent. On an adjacent area that had remained in grass during the period, the corresponding percentage was 5.91. Similar Harrison County land cropped to a good rotation for nearly 60 years still retained 4.11 percent organic matter in the plow depth. Land under rotation cropping during this 60-year period lost about 30 percent of its original supply of organic matter; without rotation, the loss averaged 69 percent, or 2.3 times as much.

Crop rotations with varying proportions of corn were tested on Wooster silt loam by the Ohio station in 1921-35. Rotations of 4 and 5 years with only one crop of corn each produced larger amounts of digestible proteins and digestible nutrients annually than shorter rotations or continual cropping to corn. They produced five times as great an amount of digestible protein and twice as great an amount of digestible nutrients as continuous cropping to corn. Where a 5-year rotation had been practiced, the nitrogen in the plow depth of soil was 2,487 pounds an acre, compared with 1,425 pounds where corn had been grown annually.

Corn yields on Marshall silt loam at Clarinda, Iowa, were found to be affected markedly by previous cropping. For the 3 years 1943-45, plots cropped to a 3-year rotation of corn-oats-meadow yielded, on an average, 88.6 bushels an acre, or 3.87 times as much as nearby plots that had been cropped to corn annually since 1932. The lower yield on the annual-corn plots was attributed to damage by the northern rootworm, nitrogen deficiency, and the poor physical condition of the soil. Great damage was done by the northern rootworm where two or more crops of corn were grown in succession. Such damage was rather common in southwestern Iowa.

Similar insect injury to crops is associated with continuous cropping in many sections. High yields of corn are common after crops like alfalfa and bluegrass, which have a fine network of roots that permeate the soil. Each year, many of the roots die and supply organic material to the soil. The action of roots in binding soil particles together, plus alternation of wetting and drying and of freezing and thawing, brings about a granular condition characteristic of soil under sod. When land in grass or meadow is turned for corn, the rotting of the roots and stubble makes much plant food available for the corn.

On the Fry farm, near Wooster, Ohio, injury from drought to corn that was grown in a rotation was much less where corn occupied the land only a small part of the time than where two or more crops of corn were grown in sequence. Where a 5-year rotation included 3 years of alfalfa and 1 year each of corn and wheat, corn yielded 40 bushels an acre in the drought year 1944, in contrast with only 15 bushels where the rotation included 3 years of corn. Where the 5-year rotation was supported by applying manure or leaving crop residues, the yield in 1944 was 45 bushels an acre. The corn yield for the 3-year rotation of corn–wheat-sweetclover was markedly lower in both 1943 and 1944 than that for the longer rotations.