ROOT SYSTEMS OF BRIGHT BELT TOBACCO

PRACTICALLY NONE of the recent research on root systems of cultivated plants has dealt with tobacco. Since a knowledge of tobacco roots may lead to a more effective use of soil and plant resources, these investigations were planned to determine root development and distribution in different soils and soil profiles and to obtain sufficient data for a statistical study of tobacco plants. The study was extended over two years, 1937 and 1938. The experimental plots were located in Harnett County, North Carolina, on the upper coastal plain. The soils used were podzolic of the Norfolk and Ruston series (Jurney, et al., 1917). The Norfolk soils included sandy loam and sandy loam deep phase while those of the Ruston series were sandy loam and gravelly sandy loam. The records of the weather bureau station located in Raleigh, 25 miles to the north, show a mean annual temperature of 60.1 OF. (740 average for May, June and July) and mean annual precipitation of 54.91 inches. Root studies in general have been on the "typical" root system in relation to arbitrary depths (Weaver, 1919, 1926, etc.) or to soil profile (Lutz, et al., 1937), or on the ratio existing between roots and shoots (Schulze, 1906; Crist and Stout, 1929; Weaver and Himmel, 1929). Details of methods involved in root study are discussed by Weaver (1919, 1926) and by Pavlychenko (1937). Most work on tobacco roots in America has been incidental to study of nematode injury, rootrot, etc. The one study which was made definitely to determine development and distribution of tobacco roots was by Bruner (1932) in Oklahoma. In his experiments, the soil was rot disturbed deeper than one inch in cultivation, which resulted in root systems being "somewhat more extensive than they would be in plants suitable for harvesting." The taproot was altered somewhat by transplanting and Bruner referred to the resulting system as "an extensively branched but not very symmetrical system." Adventitious roots grew at all angles from the stem base. He found the mature root system extended to a depth of five feet and to about nine feet in diameter, although "competition among the horizontal branches caused them to develop less strongly at the extremities as the season progressed." While this is the extent of the work in America, much more has been done abroad, particularly in Germany and the Balkans. Most of these reports are not available in America. Piescu's investigation (1938) is the most recent and it carries a good summary of other foreign work. His earlier history is taken directly from Behrens (1892). Up until this time, several investigators had 1 Received for publication June 19, 1940. The writer is especially grateful to Dr. H. J. Oosting, Dr. F. A. Wolf, Dr. T. S. Coile, and Prof. F. X. Schumacher for their help in this study and to Mr. Alton Johnson for providing and cultivating the experimental plots. variously described the root system of tobacco as being either a taproot system of no great extent with strong little branched laterals or as being a shallow system made up of an unimportant taproot and 6 or 8 laterals that were covered with long fibrous branches. However, Behrens is credited, by Piescu, with being the first intensively to study the taproot, which is lost in transplantation, and the resultant irregularly netted system. Behrens, himself, does not indicate that it was a special study, but, on the contrary, said, "Was ich daruber gelegentlich gefunden habe." He began with germinating Sumatra tobacco seed and followed root development through the time of transplanting. Ten days after setting in th-e field, he noticed numerous adventitious roots forming on the stem base. A plant which was not transplanted had a well developed taproot system, while one that was twice transplanted had no root system except adventitious roots. Popovici (1896) found some varietal differences. Jaca had a poorly developed root system while Samson and Persician were well developed with large deeply penetrating taproot and laterals with many rootlets. Girard and Rousseaux (1904) excavated a large number of roots of several varieties of tobacco by digging a trench and then washing the soil away with a stream of water under force. They estimated that they missed about 10 per cent of the roots. Their main observations were on the rate of plant growth and they concluded that fertilizer requirements of tobacco are greater than those of other crops. They found most of the roots in the upper 30 cm. of soil and decided that subsoil plays a lesser part in the "nutrition" of tobacco than in other crops. There was a constant ratio of 4:1 existing between weights of the shoots and of the roots. They gave, as the total weight of roots per hectare, 1,032 kilograms (418 kgms. per acre), with very little difference between varieties. From southern Rhodesia, we have the observation of Odlum (1905) that the fibrous root system of tobacco obtains its "food materials" near the surface and he adds that the subsoil should not be neglected because of this, because moisture content of the surface soil is dependent upon the condition of the subsoil. A more recent paper (Prjanischnikow, 1930) points out that necessity for deep cultivation is decreased because of the shallow nature of the root system. Tollenaar (1930) made a comparative study of the root systems of two strains of tobacco in Sumatra, one of which was grown in both irrigated and non-irrigated sandy volcanic and clay soils. He found a varietal difference similar to that found by Popovici (1896). Also, following cultivation, there was more profuse branching of the cut ends of roots in moist soil. The number of roots, depth of penetration, length and branching, appeared to be less in dry soil. In hard dry clay, the roots were vertical,