EFFECTS OF SOIL TEMPERATURE AND LIGHT INTENSITY ON ROOT GROWTH OF LOBLOLLY PINE SEEDLINGS

Abstract

made under controlled conditions. Reed noted a marked reduction in root growth during the time when soil temperatures were lowest and also at times when the soil was driest. Turner's data also indicate this same general trend. It was, therefore, decided to measure root growth of loblolly pine under controlled conditions to find whether or not a definite and perhaps quantitative relationship to soil temperature and other factors could be shown. Materials and methods Two series of experiments were performed, the first during the spring and summer of 1941, the second during the winter of 1946. The seedlings need in 1941 were grown from seed collected in North Carolina in 1940, and the seedlings used in 1946 were grown from seed collected in Louisiana in 1945. The seedlings were grown in glass tubes about two inches in diameter and 18 inches long. These tubes were closed at the lower end with rubber stoppers and a layer of cinders placed over the stoppers. The tubes were then filled with a loamy greenhouse soil, which was kept wetted to field capacity. Although the soil was heated in an electric sterilizer before use, it still contained two species of small animals, a nematode and a collembolan. Neither seemed to be harmful; apparently they fed only on dead tissue sloughing off the roots. The collembolans may have aided in aeration by opening small passageways in the soil. The soil atmosphere in the tubes was analyzed with a Haldane apparatus and found to have approximately the same concentration of oxygen as the outside air. The concentration of carbon dioxide in the soil was somewhat higher than outside. The average amount was 1.0% and the maximum 2.88%. Cannon and Free (?) reported that for several species there was no detrimental effect on root growth of 20% or more carbon dioxide if oxygen were not deficient. Since the soil atmosphere normally contains more carbon dioxide than ordinary atmosphere and excesses from 0.2 to 5% are not unusual (28), it is believed that growth was not abnormally influenced by deficient oxygen or by an excess of carbon dioxide. 146