POTASH AND ITS FUNCTION IN AGRICULTURE. By H. W. WILEY, Chief of the Division of Chemistry, U. S. Department of Agriculture. ORIGIN OF POTASH. The potash naturally present in a soil, in common with its other mineral constituents, is the residue of the decomposition of the min- erals which composed the original rocks. The common salts of pot- ash, viz, phosphate, chloride, sulphate, etc., are soluble in water, and where disintegrated rocks are subjected to leaching these salts are in a great measure removed. In the original rocks the potash is chiefly held in the structure of silicates, more or less complex, and wholly insoluble in water. In the débris of these rocks, as found in the soil, it is evident that the potash must still be held in the insoluble state, but, nevertheless, so thoroughly decomposed as to be yielded grad- ually to the demands of the growing plant. LOSS OF POTASH DURING W^EATHERING. During the progress of decomposition a portion of the potash passes into the soluble state and is removed by the leaching produced by heavy rains. This fact is conclusively shown by the chemical analysis of fresh and decomposed rocks of the same structure and occurring in the same locality. For instance, in the analysis of a fresh and decomposed rock (diabase) near Medford, Mass., it was found by Mr. G. P. Merrill that the undecomposed sample contained 2.16 per cent of potash, and the disintegrated portion 1.75 per cent. From these data it is seen that by leaching during and after weathering the diabase lost 19 per cent of its potash. In comparing the soil, partially disintegrated rock and undecom- posed granite, from which the fresh and decomposed rocks were de- rived, near Rock Creek, in the District of Columbia, almost the same relative loss was found, the percentages of potash in the rock, decom- posed rock, and soil being, respectively, as follows: 2.71, 2.11, and 2.10, showing a loss in passing from the fresh rock to the complete soil of 23.5 per cent. From these data it is safe to conclude that in virgin soils, formed in situ and free from erosion, from 70 to 80 per cent of the potash pres- ent in the original rocks will still be found. It would evidently be useless to seek for any constant relation between the potash in sedi- mentary soils and that in the rocks from which they were originally 107 108 YEARBOOK OF THE U. S. DEPARTMENT OF AGRICULTURE. formed. In these soils is found a mixture of sediments of very dif- ferent origins and of different degrees of fineness, the finest particles being evidently carried to the greatest distance, or being last depos- ited. In the finer particles of such a soil there is naturally a great disturbance of relations, and the potash itself is distributed among such particles in proportion to its solubility and the specific gravity of the intact rock fragments containing it. PERCENTAGE OF POTASH IN FELDSPARS. The mineral constituents of rocks which afford the largest quan- tities of potash to the soil are the potash feldspars. The rocks containing this mineral are widely distributed. Feldspar itself is essentially a silicate of alumina associated with the silicates of potas- sium, sodium, or calcium. Magnesia and iron are either absent or occur in very small quantities. The predominating alkali is either potash or soda, although where potash predominates there is nearly always some soda, and where soda predominates a small quantity of potassium is found. The amount of potash in feldspars varies widely; it is in general from 5 to 15 per cent. In a variety of feld- spar found at French Creek mines, Warwick, Pa., 15.99 per cent of potash has been found; at Magnet Cove, Hot Springs, Ark., 15.60 per cent; at Leverett, Mass., 12.20 per cent. Feldspars of this kind have as much potash as the fertilizing material known as kainite, described further on. It is evident that if such feldspars were readily decomposable, yielding the potash in a form soluble in water or easily dissolved by the vital activity of the rootlets, they would be quite as valuable for fertilizing purposes as the kainite of commerce. In point of fact, however, these feldspars, as a rule, disintegrate slowly, and it is probable that there would be no immediate effect produced by applying them, even in a finely ground state, to the soil. Sooner or later, however, the process of disintegration would be sufficiently advanced to render the potash assimilable, little by little, by the grow- ing crops. Feldspars may be altered or disintegrated by infiltrating waters carrying more or less carbon dioxide in solution, and also by the action of waters rendered acid by the decomposition of sulphides, or a min- eral containing the protoxide of iron is often the first occasion of the change. When the infiltrated waters contain carbon dioxide, the feldspar first loses its lime, by a combination of the lime with this acid, and next a portion of its potash is carried off as carbonates. The residue, being chiefly silicate of alumina, becomes a kind of kaoline. The carbonate of soda or potash or the silicate of those bases may be used in the formation of other minerals, while the alkali goes to supply the saline ingredients of fresh and marine waters. The decomposition of feldspathic rocks near the surface of the earth evidently proceeds in such a way as to distribute extremely fine POTASH AND ITS FUNCTION IN AGRICULTURE. 109 particles of the partially undecomposed minerals throughout the soil. These mineral particles contain the residual potash of the original mineral in a form which may be slowly used for agricultural purposes. In localities subjected to heavy rains the water-soluble form of the potash resulting from weathering is not found except in very limited quantities, but, as indicated in the analyses cited, the quantity of the potash lost by this solvent action does not in many instances exceed 20 or 25 per cent of the total quantity present in the original rock. DISTRIBUTION OF THE POTASH IN THE SOIL. When a soil is separated by water into groups of particles of approximately the same size, the potash is unequally distributed among them. In one instance it has been shown that a soil contain- ing 0.63 per cent of potash afforded sediments in which the potash was distributed as follows: In the clay (21.64 per cent of the whole) 1.47 per cent; in silt less than a quarter of a millimeter in diameter (23.56 per cent of the whole) 0.53 per cent; in silt half a millimeter in diam- eter (13.67 per cent of the whole) 0.12 per cent. The total potash in all the sediments amounted to 0.49 per cent of the weight of the soil, showing a loss of 0.15 per cent, which is ascribed to the solvent action of the water used in effecting the separation. RELATIVE SOLUBILITY OF PARTICLES OF DIFFERENT DEGREES OF FINENESS. The solubility of soil particles of different degrees of fineness in an acid, such as hot hydrochloric, is, as a rule, inversely proportional to their size. The clay, therefore, possesses the highest degree of solu- bility, and the coarser particles in the order of their size are less sol- uble. Thus, while a soil may lose a portion of its potash in passing from a state of larger to one of smaller aggregates, the residual potash becomes more readily soluble, and therefore more easily assimilated by the plant. In the coarser particles of the soil are retained those mineral constituents of plant food which, in the course of years, become gradually available. Nature thus conserves the potash, as well as other mineral foods, in a most careful manner, giving up only limited portions each year. RELATION OF POTASH TO OTHER MINERAL INGREDIENTS IN SILTS OF DIFFERENT MAGNITUDE. As a result of the studies of the composition of the silts, the follow- ing conclusions may be drawn: 1. The iron and alumina exist in almost identical relative propor- tions in each sediment, making it probable that they are in some way definitely correlated. 2. Potash and magnesia also exist in almost the same quantities. lio YEARBOOK OF THE U. S. DEPARTMENT OF AGRICULTURE. and their ratio to each other in all the sediments being almost constant seems to indicate that they occur combined, perhaps in some zeolitic silicate which may be a source of supply to plants. 3. Manganese exists only in the clay, a mere trace being found in the next sediment. 4. The lime appears to have disappeared in the clay, having prob- ably been largely dissolved in the form of carbonate by the large quantity of water used in elutriation. Its increase in the coarser portions may be owing to its existence in a crystallized form not so readily soluble. 5. In a summary of the ingredients, it is seen that there is a loss in potash, magnesia, and lime in the sediments as compared with the original soil, and this loss is doubtless partly due to the solution of these bodies in the water of elutriation. A noteworthy fact is the rapid decrease of acid-soluble matter in the coarser sediments. DISTRIBUTION OF POTASH IN THE SOIL AND SUBSOIL.. A study of the distribution of the potash in the soil and subsoil may be undertaken from two points of view. In the first place, the total quantity present in each may be determined.