ORGANIC CONSTITUENTS OF THE SOIL SOLUTION 87 



increased plant growth, but some of them, by themselves alone, 

 would retard plant growth. 



In a similar way green manures have been examined. If 

 fresh clover, alfalfa, or cowpeas, be macerated and an aqueous 

 extract thus prepared, it will in general be quite toxic to plants 

 such as wheat; and if this extract be allowed to stand and fer- 

 ment or sour the resulting solution will be totally unfit for the 

 growth of seedling plants. But if the clover, alfalfa, or cow- 

 pea vines be allowed to wilt thoroughly before being macerated 

 and extracted, or if they be macerated and incorporated with 

 soil and allowed to remain thus for ten days or a fortnight be- 

 fore being extracted; then, the resulting solution will be quite 

 stimulating to such plants as wheat, corn or the grasses, when 

 added either to water or soil cultures. It would seem, therefore, 

 that the mineral constituents of the legumes commonly employed 

 as green manures are less important than the organic, in affect- 

 ing the growth of crops subsequently planted, and the inhibitory 

 or toxic action of fresh green manure seems to be recognized in 

 the common practice of waiting some days after turning under 

 a green manure crop before seeding to a new crop. 



The wilting of a green manure involves a darkening and some 

 blackening of the mass, with apparently some absorption of 

 oxygen. This fact has suggested a trial of other organic sub- 

 stances which show a decided ability to absorb oxygen. Among 

 such substances, pyrogallol stands preeminent. It has been 

 shown that when pyrogallol, or better pyrogallol and lime, is 

 added to certain soils, naturally low in productive power, and 

 allowed to stand for a few days, these soils are readily brought 

 into good condition and support good crops of wheat, rye, or 

 grasses. Pyrogallol in water cultures' is rather toxic to wheat 

 plants, even in quite dilute solutions. But if the aqueous solu- 

 tion of pyrogallol be allowed to stand exposed to the air, and 

 better if the solution be made slightly alkaline as by the addi- 

 tion of lime, oxygen is absorbed, and a dark brown or blackened 

 solution is soon formed, which is stimulating to wheat seedlings. 

 Many experiments have indicated it to be a general rule that 



