INFLUENCE OF GYPSUM UPON THE SOLUBILITY OF 
POTASH IN SOILS 1 
By Paul R. McMillbr, 
Assistant Soil Chemist, Agricultural Experiment Station of the University of Minnesota 
INTRODUCTION 
The use of gypsum as a fertilizer was probably familiar to the Romans. 
Its beneficial effect has been noticed particularly with such field crops as 
clover and alfalfa, which are especially dependent upon a generous sup¬ 
ply of potash, and its action is commonly assumed to be due to an ability 
to replace potassium in the soil minerals and, hence, to increase the 
water-soluble portion of this constituent. More recently its favorable 
effects upon such crops has been attributed by some investigators to its 
sulphur content. Some of the recently reported laboratory experiments 
show that applications of gypsum have a very marked effect upon the 
solubility of potash, while some others indicate that it either has no 
effect whatsoever or actually decreases the solubility of the potash. 
Bradley (3) 2 found that gypsum added both to soils from western 
Oregon and to the mineral pegmatite markedly increased the content of 
water-soluble potash. 
Dumont (5), studying the effect of gypsum upon both granitic soils 
and the separates from these obtained by mechanical analyses, found 
that when mixed with about one-third its weight of gypsum, moistened, 
and allowed to stand, the soil gave increasing amounts of water-soluble 
potash with lengthening periods of contact between soil and gypsum. 
In the case of the soil separates the fine sand showed an increase of 
0.016 part per 1,000 of soil, while the coarser sands and the day showed 
no increase even after 34 days’ contact. 
Morse and Curry (7, p. 49-30) found that when powdered feldspar was 
treated with gypsum the solubility of the potash in water was increased. 
Likewise Andr6 (2) observed a greatly increased solubility of the 
potash of microdine when this was treated with gypsum. 
On the other hand, Fraps (6), from an extended laboratory and green¬ 
house study of the effects of additions of gypsum upon the availability 
of soil potash, condudes that gypsum is often injurious. He states (p . 30 ): 
Additions of sulphate of lime . . . have no such effect upon rendering potash 
available to plants as has been claimed. . . . 
Most recently of all, Briggs and Breazeale (4, p. 28) found that— 
gypsum solutions depressed the solubility of the potassium in orthoclase, the quan¬ 
tity of potash in solution decreasing progressively as the concentration of the calcium 
sulphate increased. 
1 Published, with the approval of the Director, as Paper 115 of the Journal Series of the Minnesota Agri¬ 
cultural Experiment Station. 
* Reference is made by number (italic) to “ Literature cited,” p. 65-66. 
Journal of Agricultural Research, 
Washington, D. C. 
oc 
Voi. XIV, No. 1 
July 1, 1918 
Key No. Minn.-29 
m 
(61) 
