4G Report of the Department ok Agronomy of the 



Calcium and magnesium for the purpose of plant food are abun- 

 dantly supplied if these elements are present in the form of carbonates 

 (limestone) in sufficient quantities for neutralizing acidity and for 

 good physical condition of the soil. For many crops enough calcium 

 and magnesium as plant food may be furnished with little or no 

 carbonate present; but some, especially legumes, seem to require 

 the easily available carbonate to furnish all these elements they 

 need. At any rate the question of supplying calcium and magnesium 

 as plant food is entirely taken care of if limestone is applied as needed 

 "for neutralizing acidity, etc. 



The problem is now narrowed clown to the familiar one of nitrogen, 

 phosphorus and potassium. With soils such as we are considering 

 and under ordinary conditions of management the addition of small 

 quantities of either nitrogen, phosphorus or potassium in easily 

 soluble compounds usually produces an increase in general farm 

 crops. But whether or not by the most practical efforts enough of 

 these elements already in the soil can each year be made available 

 or whether their addition as fertilizers is profitable are questions 

 which can be decided only by carefully conducted field experiments. 

 However, a few general considerations first deserve attention. 



General considerations. — Potassium is contained in the soil in 

 25 to 30 times the amount of phosphorus, and as much as 10 times 

 the amount of nitrogen. If potassium could be made available 

 as fast as needed the supply would last indefinitely. Considering 

 also that a good part of the potassium of most crops is again returned 

 to the soil in crop residues or farm manure, it would seem entirely 

 practicable by good management to do away with the purchasing 

 of potassium for a fertilizer. The potassium problem then is appar- 

 ently one of liberation rather than supply. 



In proportion to the amounts used by crops, nitrogen is contained 

 in the soil in smaller quantities than phosphorus; also there is much 

 loss of nitrogen by leaching. The total supply in the soil would 

 not last more than 20 or 25 years if it could be drawn upon as fast 

 as needed. Of course it is impossible to draw upon it in this way 

 and so it is evident that even at the outset some special provision 

 must be made for supplementing the amount of nitrogen in the soil. 

 The addition of decaying organic matter in the form of crop residues, 

 green-manure crops and all available farm manure not only supplies 

 nitrogen but is also necessary as means of liberating mineral plant food 

 and maintaining good physical condition of the soil. We can 

 determine by field experiments only, to what extent it pays to sub- 

 stitute for these materials or supplement them by the use of high- 

 priced commercial nitrogen. 



Phosphorus is contained in the soil in relatively small quantities, 

 and clearly its supply must be renewed at an early date if large 

 crop yields are to be maintained. To what extent it is now profitable 

 to supplement this supply must be determined by experiments. 



