SOIL ACIDITY AS INFLUENCED BY GREEN MANURES 



By J. W. White 



Associate Professor of Experimental Agronomy, The Pennsylvania State College 

 Agricultural Experiment Station 



INTRODUCTION 



The study of the general fertilizer series of plots at the Pennsylvania 

 Experiment Station, which have been continuously treated according to 

 different systems of fertilizing ever since 1881, has shown some instances 

 of large lime requirements, particularly in the case of the soils of plots 

 30, 31, and 32, treated, respectively, wath a complete fertilizer containing 

 different quantities of nitrogen supplied in the form of ammonium sul- 

 phate. Experiments reported in recent years have shown clearly by the 

 ordinary tests, that these soils are distinctly acid, that the common 

 rotation crops fall off in yields in proportion to the degree of acidity of 

 these soils, and that the use of either lime or finely ground limestone in 

 quantity sufficient to make the lands neutral is all that is necessary to 

 bring them back to normal bearing. 



One of the questions raised by the facts just stated concerns the cause 

 or causes producing the sour or lime-deficient conditions here observed. 

 Among these causes commonly regarded as operative to set up an acid 

 condition, or one of pronounced lime requirement in a soil, is the accumu- 

 lation in it of a large amount of organic matter decaying under conditions 

 or by the action of agents that form acid materials from it. On all 

 cultivated soils, sods and crop residues are customarily plowed under and 

 might serve as the raw material for the production of acid substances 

 and, indeed, may have at the outset a direct acid effect, because they 

 themselves contain, especially in the immature condition, considerable 

 quantities of acid-acting constituents. 



Doubtless, however, the heavy cover crops and intercrops, turned 

 under as green manures, should most strikingly show the kinds of effects 

 here in question. Practical experience has sometimes shown temporary 

 ill effects to follow the turning under of especially rank growths of green 

 manures, and their stage of maturity and their kind are regarded as 

 having something to do with the appearance or nonappearance of these 

 ill effects. This particular phase of the green manure subject has not 

 to our knowledge, been elsewhere studied. 



Journal of Agricultural Research, Vol. XIII, No. 3 



Washington, D. C. Apr. 15, 1918 



nix Key No. Pa. — 7 



(171) 



