EFFECT OF OXIDATION OF SULPHUR IN SOILS ON 

 THE SOLUBILITY OF ROCK PHOSPHATE AND ON 

 NITRIFICATION » 



By O. M. ShEdd^ 



Chemist, Kentucky Agricultural Experiment Station 



INTRODUCTION 



The widespread use of different forms of phosphate on a large majority 

 of our soils has been justified by the increased returns. In fact, out- 

 side of certain areas of our State, the need of phosphate has been so 

 conclusively proved that seldom is the question ever raised as to whether 

 it is necessary but rather what is the most economical kind to use — the 

 soluble form, such as acid phosphate, or the less soluble form, such as 

 ground rock phosphate. 



A large amount of work has been done in the greenhouse and the 

 field in comparative tests of the various forms of soluble and insoluble 

 phosphate fertilizers. The writer does not care to discuss in this con- 

 nection what the results show regarding the relative merits of the two 

 kinds further than to state that on soils very deficient in this element the 

 use of a soluble form has been generally found to be more profitable for 

 immediate returns. 



HISTORICAL REVIEW 



In the course of some work by the writer during the early part of 

 1914 (12-14),^ on the sulphur content of Kentucky soils and the effect 

 of this element and its compounds on plant growth, it was found that 

 comparatively large amounts of added sulphur are easily oxidized to sul- 

 phate in the soil. The results also indicated that the sulphuric acid 

 formed would act on rock phosphate when present and convert it into 

 a water-soluble form. The time of contact of the mixture of soil, rock 

 phosphate, and sulphur was about 10 weeks. Since the amounts of 

 material used were small and only one soil was tested, the work was not 

 conclusive. It was the writer's intention to continue it later in order to 

 arrive at more definite conclusions, and for this reason it was not men- 

 tioned in the earlier publications. 



Mar^s (11), in 1869, mentioned the fact that sulphur is converted to 

 sulphate in soils. It was suggested in 1877 by Charles F. Panknin (11) 

 of Charleston, S. C, that sulphur, if mixed with ground bone or ground 



> Published by permission of the Director of the Kentucky Agricultural Experiment Station. 



* The writer desires to express his appreciation to Dr. A. M. Peter for helpful suggestions and to the Agron- 

 omy Department, especially to Prof. P. E. Karrakcr. for assistance in starting the earlier experiments. 



* Reference is made by number (italic) to " Literature cited," pp. 344-345- 



Journal of Agricultural Research, Vol. XVIII, No. 6 



Washington. D. C. Dec. is. 1919 



tc Key No. Ky.-g 



(329) 



