No. 6. DEPARTMENT OF AGRICULTURE. 375 



anced ration so far as nitrogen is concerned, then, is obtained from 

 the soil if pos^^ible without any application of fertilizer. If commer- 

 cial nitrogen is used, a study of the conditions and the needs of the 

 plant will cause one to decide what shall be the source of the nitro- 

 gen sui)i)lied. High grade, quick-acting nitrogen should be applied 

 in a relatively small amounts, for the soil does not possef<s the power 

 of fixing the nitrates that it does of fixing jdiosjihoric acid and jiotash. 



As the result of actual field ex})erimenls it is found that phos- 

 })]ioric acid is more often needed in balancing the ration for plants 

 than is any other element of plant food. This is without doubt due 

 in part, to the fact that phosphoric acid combines with many sub- 

 stances contained in soils as lime, iron, etc., and as a result of such 

 combination the phosphoric acid is very effectually fixed by the soil. 

 So firmly is it fixed in combination that it is often impossible for the 

 plants to ''unfix" or liberate it fast enough to supply their demands 

 and hence plants may suffer for lack of this important element of 

 their food, even though it be present in the soil in large quantities. 



As a principle, it should be remembered that the phosphoric acid 

 in a high grade fertilizer is never available in any larger per cent, 

 at any one time than when it is applied to the soil, but that the 

 tendency is constantly toward a reversion to forms which are less 

 available. 



There has in recent years been much discussion as to the relative 

 value of soluble and insoluble phosphoric acid as plant food. The 

 claim is made, and correctly made, that if soluble phosphoric acid is 

 applied it soon becomes reverted and then insoluble. The value of 

 an}' fertilizer depends, to a considerable extent, upon the even and 

 thorough distribution of the fertilizer through the soil. The soluble 

 phosphate is distributed in solution, and when it is finally ''fixed" 

 by the soil, it has been distributed evenly, and plant roots can come 

 into intimate contact with the fertilizer. 



It is not necessary in all cases to purchase high grade phosphate 

 fertilizers. Under certain conditions insoluble phosphoric acid as 

 contained in floats, crude ground rock, may prove beneficial. The 

 conditions, how^ever, under which such a fertilizer may be used to ad- 

 vantage should be well understood or failure of success Avill result. 

 Insoluble phosphoric acid of the soil is rendered slowly available for 

 plants use by means of the fermentation and decomposition of or- 

 ganic matter and by the action of plant roots in the soil. The pro- 

 cess goes on most rapidly in those soils which are well supplied with 

 tumus, moisture and high temperatures. In using insoluble phos- 

 phoric acid as a fertilizer we fertilize the soil rather than the crop 

 which is grow'ing upon the soil; immediate results should not be 

 expected, but there is some satisfaction in knowing that the total 

 amount of phosphoric acid in the soil has rather been increased 



