22 USE OF FELDSPATHIC EOCKS AS FERTILIZERS. 



always been held by the majority of chemists and agriculturists that 

 the potash combined as it is in the mineral silicates becomes soluble 

 with extreme slowness, if at all, it is customary to distinguish between 

 available and total potash. According to the official methods in 

 vogue in this country the available potash is determined by boiling 

 1.0 grams of the sample with 300 grams of water thirty minutes and 

 analyzing the water extract. In different places different methods 

 are used ; in some cases acid solutions are used to extract with, so that 

 it is difficult to give an exact definition of available potash. How- 

 ever, it mav be defined as all the potash that is present, in Avhatsoever 

 form, which the crop can make use of in one crop season. The text- 

 book definitions, as well as the State fertilizer laws, are in need of 

 revision in this particular respect. This is especially true in view of 

 the fact that the value of a mixed fertilizer is figured, as far as 

 potash is concerned, on the available or Avater-soluble potash alone. 

 Even the potash contained in such organic fertilizers as cotton-seed 

 meal and tobacco stems, which undoubtedly becomes available in one 

 season, is not estimated as being available by the present methods of 

 analysis, and therefore in many cases great injustice is done to manu- 

 facturers of mixed fertilizers, who should certainly be entitled to all 

 the plant food contained in their mixtures which can be made use of 

 by a crop in a single growing season. 



Tobacco is particularly dependent upon abundant supplies of pot- 

 ash. In some of the tobacco-growing districts barnyard and stable 

 manures are used, often enriched with an addition of commercial fer- 

 tilizers. Since sulphates and muriates of potash are found to be harm- 

 ful in certain ways to high grades of tobacco, the use of the strongly 

 alkaline potassium carbonate has been largely resorted to on the 

 tobacco crop. Potassium carbonate, which contains about 66 per cent 

 of potash (KoO), is an expensive compound, costing $90 to $95 per 

 ton, f. o. b. port of entry, or 6 to 7 cents per pound of unit potash. 

 In addition to the cost of this salt, its strong alkalinity is an objec- 

 tionable feature, and it is a grave question whether the annual addi- 

 tion to the land of a superabundance of such an alkaline salt will not 

 be followed by an actually harmful effect. In view of the successful 

 results which have attended the efforts of the Bureau of Plant 

 Industry" to improve the quality and yield of native tobacco by 

 proper selection and breeding, it was decided to make a systematic 

 investigation of the possible use of ground feldspar as a potash 

 fertilizer. 



Preliminary to the field and crop experiments which were to follow 

 later, greenhouse experiments were begun early in the winter of 



oBul. 9G, Bureau of Plant Industry, U. S. Dept. of Agriculture; Yearbook 

 Dept. Agr., 1905, p. 219. 

 104 



