PLANT FOOD AND SOIL FERTILITY 35 



use the cheaper form, if its application secures favorable 

 results compared with those obtained by the use of acid 

 phosphate. As has been stated, the quick action due to 

 solubility must not be expected of phosphate rock. There 

 is a question whether the use of rock phosphate fertilizer is 

 ever warranted. There is good authority for believing that 

 its use under certain circumstances does give results equal, 

 or nearly equal, to those obtained from acid phosphate and 

 at less cost. It is 

 supposed that rock 

 phosphate becomes 

 slowly available for 

 plant use through the 

 action of bacteria in 

 the presence of an 

 abundance of organic 



matter. Diagram showing yield of potatoes. Ma- 



in rtrnrtiVp SQ ms nure and rock P hos P hate produced a yield 

 47 per cent greater than that of manure alone 

 nure accumulates, on equal areas. (Wisconsin Agr. Exp. Sta- 



finely ground rock tion -> 



phosphate is mixed with it; later, when the mixture is applied 

 to the land and plowed under, the rock phosphate becomes 

 thoroughly incorporated with the soil along with the organic 

 matter of the manure. If the soil is not already rich in lime, 

 lime or limestone should also be liberally applied. The 

 insoluble phosphate seems to be changed into a soluble form, 

 though so slowly that the effect of the fertilizer does not 

 begin to show much until the second season; but from this 

 time on for a few years, it seems to have the same effect as 

 acid phosphate. The slowness of action may not be a serious 

 matter, for in good farm practice permanent fertility of the 

 soil is of more importance than the results of one season. 



