Residual Effect of Varying Applications of Potash on the Replaceable Potassium in Several Mississippi Soils

P must be applied along with nitrogen arid phosphate to a number of soils in the South in order to obtain the most profitable yields of cotton. A review of the literature reveals that there have been several studies conducted on the residual effect of applied nitrogen and phosphate in fertilizers. However, only a few such studies have been made on the residual effect of applied potash. This may be explained by the smaller tonnage of potash used as compared to either nitrogen or phosphate. When potash is applied to soils, it may be leached out into the drainage water, fixed in nonreplaceable form, removed in the harvested portion of crops, or held by the clay in the replaceable form. Truog and Jones (14)* in a summary review of the available data relative to the leaching of potash concluded that not more than 10 to 15 pounds of K2O per year may be expected to be leached from the plow layer of the average soil and that excessive losses by leaching can only be expected to occur when heavy applications of potash are made on a soil of light texture under conditions of humid climate. It has been well established by Volk (16) and Hoagland and Martin (7) that a considerable amount of applied potash becomes fixed by the soil colloids in nonreplaceable form and such amount is, therefore, not subject to leaching. DeTurk, et al. (4) have recently concluded from an extensive study of Illinois soils that "potassium fixation is well-nigh universal in the soils of the corn belt when soluble potassium salts are added." Worsham and Sturgis (19) also found from a study of 14 soil types of the lower Mississippi delta that potash fixation varied from none in Lintonia silty clay loam and Portland silt loam to 209 p.p.m. in Sharkey clay loam, and that potassium fixation increased as base exchange capacity increased. While some potash is removed each year in harvested crops, the amount removed in the case of cotton or corn is quite small. For example, an average crop of seed cotton yielding 1,000 pounds, according to Beeson (i), will remove only 12 pounds K2O per acre in the seed and lint, while the grain from a 30bushel per acre corn crop will remove only about 8 pounds K2O per acre. When legumes are grown for hay, medium to large amounts of potash are removed each year with the harvested portion of the crop. Dunkle, et al. (5) have shown that in 67 out of 72 Pennsylvania orchard soils, the surface soil was higher in replaceable potassium than the subsoil, which supports the idea that in Pennsylvania, potash is not readily leached from the surface soil. Reuther (u) found that heavy applications of straw over a 4-year period nearly tripled the replaceable potassium content of the soil in both the oto 3-inch and the 3to 6-inch depths and nearly doubled it for the 6to 12-inch depth. Gourley and Wander (6) found that there was some downward movement of applied potash, but that the lateral movement was equally as great. The Rhode Island Agricultural Experiment Station (12) found that phosphate and potash applied over a period of 20 to 25 years continued to affect yield of Japanese millet 6 years after their use was discontinued. In some contrast to results reported above, Sturgis and Moore (13) found that on Lintonia silt loam and Teller fine sandy loam there was little fixation of potassium applied in the form of muriate of potash and that 4 months after its application nearly half of the potash applied was soluble in water. From this study they concluded that the low absorptive capacities of these two soils for potassium make the maintenance of available potassium difficult in soils that leach easily. Volk (15) reported that the amount of potash leached below the 8-inch depth is directly proportional to the amount applied and found winter legumes effective in reducing the percentage of applied potash which is leached below the 8-inch depth. Since some of the results relative to the leaching and fixation of potash in southern soils are at some variance with 'those reported elsewhere, and since there seems to be few studies of residual effects of applied potash on soils of the South, it is the purpose of this work to present data obtained from a number of field tests where varying amounts of potash have been applied for a period of 5 years or more.