548 NATURE AND PROPERTIES OF SOILS 



The combination of rye and vetch, both of course in a snccn- 

 lent condition, seems especially efficacious. Sod as a green- 

 manure always appears more or less at a disadvantage. 



315. The use of green-manures. — The indiscriminate 

 use of green-manures is of course never to be advised, as the 

 soil may be injured thereby and the normal rotation much 

 interfered with. When soils are poor in nitrogen and organic 

 matter, they are very often in poor tilth. This is true whether 

 the texture of the soil be fine or coarse. The turning-under 

 of green-crops must be judicious, however, in order that the 

 soil may not be clogged with undecayed matter. Once or twice 

 in a rotation is usually enough for such treatments. Proper 

 drainage must always be provided. In regions where the rain- 

 fall is scanty, great caution must be observed in the handling 

 of green-manures. The available moisture that should go to 

 the succeeding crop may be used in the process of decay, and 

 the soil left light and open, due to an excess of undecomposed 

 plant tissue. In western United States, it is still a question 

 whether green-manures have any advantage over summer 

 fallowing. 



It is generally best to turn under green-crops when their 

 succulence is near the maximum and yet at a time when 

 abundant tops have been produced. This occurs at about the 

 half mature stage. A large quantity of water is carried into 

 the soil when the crop is at this stage, and the draft on the 

 original soil-moisture is less. Again, the succulence encour- 

 ages a rapid and more or less complete decay, with the maxi- 

 mum production of humus and other products. The plowing 

 should be done, if possible, at a season when a plentiful supply 

 of rain occurs. The effectiveness of the manuring is thereby 

 much enhanced. At Cornell University various green-manures 

 were seeded in the summer and plowed under that fall or the 

 next spring. The experiment was continuous for three years, 

 the nitrates being determined in the soil each year in April 

 and in June. The results are as given on the next page. 



