Date of this Version
Young Delicious apple trees were grown in two soil types, clay loam and loess, in eastern Nebraska, under a variety of cultural conditions. The development of roots and tops was studied for the first three years after transplanting in the orchards. In all, 73 trees were excavated.
The first two years were about normal in precipitation at Lincoln (locale of the clay loam) while at Union (locale of the loess) there was a deficit of over 9 inches each year. The third year, 1934, was one of severe drought and heat at both stations. The total rainfall deficiency for the three years was 30.8 inches at Union and 11.3 inches at Lincoln. The development of root systems was extremely rapid, the roots reaching a Maximum depth of 8.8 and a lateral spread of 12 feet the first year, and 14.8 feet and 21.2 feet the second. During the third year the maximum lateral spread reached 29.4 feet and the maximum depth reached was 17 feet. This greatly exceeded the lateral spread of three-year-old tops, which was about 6 feet, and the height of the trees, which was 7 to 8 feet.
The root systems responded readily to changes in soil environment. Under clean culture a generalized root system was produced. The roots penetrated deeply and spread widely in such a manner that a very large volume of soil was thoroughly occupied. In competition with corn there was little lateral spread and most of the root growth was vertical. Under straw mulch the roots had a pronounced shallow, lateral development. Under sod mulch both tops and roots were dwarfed.
No change in the development of roots and tops could be attributed to the use of the fertilizers, ammonium sulphate and acid phosphate, except that the trees were injured somewhat by ammonium sulphate the first season.
Corn planted 7 feet from the trees had little effect upon tree-root development the first year, but when planted nearer to the tree row it resulted in dwarfing the root system. When planted 3.5 feet from the trees for two years, the average lateral spread of surface roots toward the corn was 8.4 feet, while below four feet in depth the roots were generally limited in spread to four feet. With corn planted 5 and 7 feet from the tree row, the average lateral spread of horizontal roots was 9.4 and 10.2 feet respectively. The average spread of roots of cultivated trees in loess soil was 15.5 feet after two years of growth.
The average spread of the roots of trees under cultivation in loess soil was 19.2 feet at the end of the third season. The most distinct difference between the root systems of trees under cultivation and those modified by competition with corn was the wide spread of roots of cultivated trees at great depths.
The trees in the mulched series at Lincoln all showed a marked lateral development of roots in contrast to those in loess soil at Union, but their vertical development was not so extensive. Cultivated trees after three years of growth had an average lateral root spread of 23.6 and 19.2 feet respectively at Lincoln and Union and an average root depth of 9.4 at Lincoln and 14.7 at Union.
There was a decided positive correlation between top growth and root growth for the trees in the mulched series in clay loam soil. Trees in loess soil did not show so distinct a correlation between top and root development.
Apple roots grew toward an adjacent optimum moisture supply. The response of the root systems to the various cultural treatments can be explained largely on the basis of the water content and its location in the soil. Straw mulch caused more water to be available in the upper two to three feet of soil. Here roots had a marked lateral development but vertical growth was less than under other treatments. Corn greatly reduced the soil moisture on each side of the tree row and tree roots turned downward as they approached this drier soil.
After three years of growth, apple trees on loess soil had absorbed about one-half the total available moisture present in the soil directly beneath the trees to a depth of 9 feet.