LOSS OF NITRATES 157 



be turned under as late as possible in the life of the plants and not 

 in the hot summer when the plants are green. This agrees with the 

 findings of Causemann who recommends that they be turned under 

 late in September. It has been observed by Bredemann that the 

 addition of organic matter, such as hay and sugar, produces a harm- 

 ful effect the first year and a beneficial effect the two following years. 



2. Green manures change the number and kind of organisms 

 occurring in the soil. Hill found the total number of bacteria in 

 soils treated with green manures to be much greater than in those 

 receiving no green-manure treatment. Legumes gave in most cases 

 a greater increase than non-legumes. Miintz considers the value of 

 green manures proportional to the rapidity with which their nitrogen 

 is converted into nitric nitrogen. Heinze, on contrasting stall and 

 green manures, found that the latter carried but few organisms which 

 would break down the insoluble material. The decomposition of 

 green manure was found to be due to dust organisms and to organ- 

 isms found in soil. For this reason the number and kind of organisms 

 in a given soil determine in a great measure the influence of green 

 manures on succeeding crops. Decomposition is rapid in an open, 

 sandy soil rich in bacteria and relatively slow in a soil poor in bacteria. 

 In an open, sandy soil the nitrogen of the green manure may pass 

 over into nitrates and be washed out, whereas in a heavier soil the 

 nitrogen becomes available more slowly and is not washed out so 

 rapidly. For this reason in heavy soils green manures often give the 

 best results the second year. 



Koch thinks the good effects produced when green manure is 

 added to stable manure may be due to an increased nitrogen fixation 

 by Azotobocter, the organism using the cellulose of the manure as a 

 source of energy ; whereas Heinze considers that the results which he 

 obtained from studying the action of carbon bisulphid on soils may 

 help us to understand the peculiar effects produced at times by the 

 turning under of mustard, buckwheat, rye, and other non-leguminous 

 crops. It has been noted repeatedly that these crops when plowed 

 under in a green state lead to a better growth of the following cereal 

 or root crops on nitrogen-poor soils. As Heinze points out, there 

 may have been more or less justification for this belief, so far as the 

 indirect influence of mustard is concerned. It would seem at times 

 that the action of mustard is not unlike that of carbon bisulphid in 

 affecting the bacterial flora of the soil, and it really appears from 

 facts already known that the green mustard substance in the soil 

 retards the development of the acid-forming species and encourages 

 the growth of the nitrogen-fixing Azotobacter species. Heinze, 

 therefore, thinks that further study may enable us to make extensive 

 use of mustard as an indirect source of combined nitrogen, and tries 

 to find theoretical support for this belief in the fact that allyl 

 isothiocyanid mustard oil, C 3 H 5 N = C = S, which is a constit- 



