248 Journal of Agricultural Research voi.v, no.6 



place the fact that sulphates have very little effect as compared with 

 soluble phosphates on the soil flora. This difference in action will remove 

 the sulphates from the category of effective fertilizers for all crops. 

 Nevertheless, for certain plants and types of soil they will be beneficial 

 if their only action is as a source of sulphur. 



The plants most affected were the members of the Leguminosae and 

 Cruciferae. It is probable that we should expect these classes of plants 

 to be more responsive to the higher concentration of sulphates in the 

 soil water than, for example, the Gramineae, owing to the higher protein 

 content of the first group and the special sulphur-bearing bodies abun- 

 dantly formed in the second group. In this soil, however, there was 

 noticeable stimulation to seed production in both barley and oats, 

 although there was little or no effect on the development of the quantities 

 of straw. 



In the case of clover the increase in air-dried matter due to calcium 

 sulphate alone was about 23 per cent. With rape the greatest increase 

 occurred where the calcium sulphate was superimposed upon a complete 

 fertilizer, giving an increase of 17 per cent over the complete fertilizer. A 

 similar order of increase was likewise observed with the radish crop, 

 where the increase above a complete fertilization, due to the calcium 

 sulphate addition, averaged 9 per cent. 



In general, the calcium sulphate was more effective than the more 

 soluble sodium sulphate. The special influence of sulphates on root 

 development is pointed out. They were particularly effective with red 

 clover and rape. In the case of red clover, which was more especially 

 studied, the roots were much elongated where sulphates entered into the 

 ration. This must result in a more extended feeding area for the plant 

 and, in addition, increase its ability to withstand periods of drought. 



The somewhat common observation of the benefit of land plaster to 

 this plant can probably be closely correlated with this special effect of 

 sulphates on root development, as well as its high protein character, 

 which would make special demands for sulphur. 



Whether recorded failures in the use of land plaster are to be correlated 

 with wet seasons, a high sulphur content normal to the soil under obser- 

 vation, or the variety of plants used is a matter for future observation. 



In these greenhouse experiments elemental sulphur was generally 

 harmful. These harmful results occurred even in the presence of a 

 generous supply of calcium carbonate. These results indicate that ele- 

 mental sulphur may be toxic through its incomplete oxidation to sul- 

 phites; toxicity may also arise in the absence of sufficient basic material 

 through the development of acidity from sulphuric acid. 



Application of these results to field practice is reserved until more data 

 on field plots are available. 



