BUREAU OF PLANT IISTDUSTEY. 281 



The beneficial effect of legumes upon the succeeding crop is only 

 partly due to the plant food that becomes available b}^ the decomposi- 

 tion of surface and root growths of the legumes. Special tests made 

 in the greenhouse showed clearly that the resulting crop increases are 

 frequently higher than they could be if the fertilizing effect alone 

 were taken into account. Leguminous and perhaps other crops exert 

 undoubtedly a favorable influence upon the soil bacteria, aiid a 

 thorough examination of these almost unknown but evidently im- 

 portant relations between cultivated plants and soil microorganisms 

 has been started. 



Foivnation of mineral deposits hy bacteria. — In cooperation with 

 the United States Geological Survey the formation of deposits of 

 calcium carbonate by bacteria has been investigated. Two groups, 

 ammonifying and denitrifying organisms, have been found to be 

 active in this respect. They precipitate the carbonate not only fi-om 

 organic but also from inorganic salts of calcium, e. g., calcium sul- 

 phate. 



Inocnlat'/on of legumes. — Experiments to determine the necessity 

 for inoculating leguminous crops have been conducted in the follow- 

 ing States: Pennsylvania. New Jersey, New York, Ohio, Michigan, 

 Illinois. South Dakota. North Dakota, Colorado. Oregon, Washing- 

 ton. California. Oklalioma. Georgia, North Carolina, and Virginia. 



PREVENTION OF ALKALI INJURY ON IRRIGATED LANDS. 



In manv districts the chief concern of the irrigation farmer is to 

 prevent the accumulation of alkali salts in harmful quantities in 

 good land rather than to reclaim salty land for use in crop produc- 

 tion. The causes that contribute to injury of land by alkali salts 

 are continuing causes and are largely subject to control by the irri- 

 gation fai-mer if the}- are correctly understood. Unless adequate 

 drainage is provided, the salts carried in solution by irrigation 

 water are left in the soil when the water evaporates or is taken up 

 by crop plants. Under these conditions the soil solution may be- 

 come excessiveh' concentrated as time goes on, which may result not 

 only in direct injuiy to growing crops but may also injure the 

 physical condition of the soil. This is particularly true where the 

 dissolved salts are chiefly compounds of sodium. The concentration 

 of the salts of sodium in the soil solution results in reactions with 

 other materials in the soil which increase the tendency of the soil 

 to become puddled and impermeable to the movement of water, and 

 when these conditions obtain it is difficult to remove the excessive 

 salts in the soil solution by leaching. 



The salts of calcium and magnesium as they occur in the soil 

 solution are not only less injurious to crop plants than the salts of 

 sodium but appear also to have a beneficial effect on the physical 

 condition of the soil. Wlien the irrigation water is relatively rich in 

 calcium and magnesium the reaction that takes place in the soil 

 appears to result in improving the physical condition and perme- 

 ability of the soil. 



These results indicate the importance of understanding, in any irri- 

 gation region where alkali troubles develop, both the character of the 

 soil solution and the irrigation water, so that the irrigation practices 

 may be designed to prevent the excessive accumulation of soluble 



