On Light and Heavy Soils 259 



well as its subsequent transformation in the soil. The 

 physical character of the soil determines not only the 

 depth at which nodules may be formed on the roots of 

 legumes, but also the usefulness of these nodules. It 

 also determines the rate at which the vegetable matter 

 in the soil shall decay. 



In general, it should be remembered that in lighter 

 soils a green-manure may be covered up with a deeper 

 soil layer than in heavier soils, since in the former the 

 processes of decay are active at greater depths. On the 

 other hand, the deep plowing of green-manuring crops 

 on heavy soils 'may not only retard the decomposition 

 of the organic materials, but also may lead to the accu- 

 mulation of acid substances by encouraging the growth 

 of certain acid-forming bacteria. This danger exists 

 scarcely at all in open soils. In their case, the danger 

 lies rather in too great a degree of aeration, with the con- 

 sequent drying out of the upper soil layer, the destruc- 

 tion of the decay bacteria and the preservation of the 

 green-manure in an unchanged condition for a long 

 time. In such cases, a heavy roller may be employed 

 to advantage in overcoming this difficulty. 



Leguminous green-manures and the succeeding crops. 

 Leguminous cover-crops affect the quality as well as 

 the quantity of the succeeding crop. It has already 

 been noted that they may pump out of the soil so great 

 an amount of moisture as to affect injuriously the fol- 

 lowing harvest. In countries where the rainfall does 

 not exceed, on the average, thirty inches per annum, 

 this is a matter of great moment. It has been found by 

 Schultz, of Lupitz, and likewise by many others, that 



