AVAILABILITY OF MINERAL PHOSPHATES FOR PLANT 



NUTRITION 1 



By W. L. BuRLisoN, 

 Associate Professor, Crop Production, Agricultural College, and Associate Chief, Crop 

 Production, Illinois Agricultural Experiment Station^ 



INTRODUCTION 



Phosphorus is the key to permanent systems of agriculture for a large 

 portion of the common soils of the com belt. These soils contain, as an 

 average, 5,000 pounds of nitrogen, 1,200 pounds of phosphorus, and 

 35,000 pounds of potassium for the surface soil to the depth of 6^ inches. 

 If the land were producing com at the rate of 100 bushels per acre, the 

 nitrogen would be sufficient for 50 crops, the phosphorus for 70 crops, and 

 the potassium for about 1,842 crops. The nitrogen supply can be main- 

 tained by the growth and judicious management of leguminous crops. 

 Potassium is present in quantities adequate for many years. With 

 phosphorus the problem is different. This element can not be gathered 

 from the soil air by legumes; nor is it one of unlimited supply. When 

 once removed, phosphorus must be returned to the land in crop residues, 

 in farm manures, or in commercial fertilizers which contain phosphorus. 



Since the introduction of commercial fertilizers, more or less discussion 

 has been carried on concerning the value of insoluble mineral phosphates 

 as a source of phosphorus for the nutrition of plants. In Europe (28, 

 p. 329)^ the highest authorities on agricultural problems have discouraged 

 the use of insoluble phosphates, while in America scientists and practical 

 men have disagreed. Investigations which have been conducted on the 

 use of insoluble minerals are by no means conclusive. Therefore it is 

 the purpose of the work reported in the following pages to throw more light 

 on this question, which is of so great economic importance and scientific 

 significance. The subject matter will be presented according to the fol- 

 lowing divisions: 



I. Review of literature regarding the availability of phosphate minerals. 



II. The aviailability of phosphoms in Tennessee brown rock phosphate 

 for wheat {TrUicum vulgare), oats (Avena saliva), rye (Secale cereale), 

 barley {Hordeum sativum hexasiichon) , cowpeas (Vigna caijang), soybeans 

 {Glycine hispida), timothy (Phleum pratense), red clover {Trifolium 

 praiense), and alfalfa (Medicago saliva). 



* This paper is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy 

 in Agronomy in the Graduate School of the University of Illinois, 1915. 



* The author wishes to express his appreciation for the suggestions and encouragement tendered by 

 Dr. C. G. Hopkins and Dr. A. L. Whiting, of the Illinois Experiment Station. 



* Reference is made by number to "Literature cited," p. 513-514. 



Journal of Agricultural Research, Vol. VI, No. 13 



Dept. of Agriculture, Washington, D. C. June 26, 1916 



ef , III.-3 



37770°— 16 2 (485) 



