68 CROP GROWING AND CROP FEEDING 



The writer has frequent inquiries from farmers who are anxious to know 

 whether the free sulphuric acid that they are told remains in the acid phos- 

 phate from the rock, will be injurious. The fact is there is seldom, if ever, 

 any such acid in a well-made superphosphate, and even if there was it would 

 at once seek some base in the soil and be changed to a neutral salt, either the 

 sulphate of lime, potash or magnesia, according as one or the other may be 

 present in the soil. 



It has been found that where superphosphates have long been used freely 

 the phosphoric acid will accumulate in the soil to such an extent that further 

 applications have no effect. This has been the case in a large section of 

 Eastern North Carolina, where the farmers say that they no longer get any 

 returns from the application of phosphates to their cotton crop. The fact 

 is that the soil holds on to phosphoric acid longer than anything else in the 

 way of plant food, and does not allow it to leach away as the nitrogen is apt 

 to do, but keeps it there till the crops call for it. Hence it is easy to see that 

 in making liberal applications of phosphates, whether merely pulverized bone 

 or rock or dissolved phosphate, we are in no danger of serious loss, but can 

 depend on any surplus staying there till wanted by the crops. Professor 

 Voorhees well says, "The real object of making it soluble is to enable its better 

 distribution. If it were possible to as cheaply prepare the dicalcic (or 

 reverted) form as the soluble, it would, perhaps, be quite as useful from the 

 standpoint of availability. After the soluble is distributed in the soil, it is 

 fixed there by combining with the lime and other minerals present." It is 

 thought that it at once assumes the reverted form, and that in the presence 

 of an abundance of lime may even become insoluble. The solubility of the 

 phosphoric acid lasts much longer in a light soil deficient in organic matter, 

 but even there it is fixed rapidly enough to prevent serious loss. Chemical 

 analysis of the drainage waters seldom shows any loss of phosphoric acid. 



THE VALUE OF INSOLUBLE PHOSPHATES. 



In the valuation of commercial fertilizers, as we have noted, some of the 

 Experiment Stations place no commercial value on the insoluble phosphoric 

 acid in a fertilizer. We have long been satisfied from our own experience 

 that this is an error, at least so far as the agricultural and crops-producing 

 value of the insoluble phosphate is concerned. Years ago, in farming on a 

 large scale, we found. that we did get the happiest results from the use 

 of the phosphatic guanos from the Carribean islands in which the phosphoric 

 acid was all insoluble. True, we did not get the same immediate results as 

 from the use of the dissolved acid phosphate, but the final result was as good 

 and more lasting, and when these insoluble forms of phosphoric acid were 



