EXTENSION COUESE IN SOILS. 47 



LESSON Vn. THE PHOSPHORUS AND POTASSIUM OF SOILS. 



The mineral elements of plant food in the soil which are most apt 

 to be so low as to limit crop production are phosphorus and potassium 

 (see p. 11). These elements, it will be remembered, come from the 

 disintegration of rock materials. The total phosphorus and potas- 

 sium content of a soil, therefore, depends primarily upon the kind of 

 rocks from which the soil was formed. On the other hand, the quan- 

 tity of phosphorus or potassium available to plants is not accurately 

 measured by the total quantity of these elements in the soil, but 

 depends much upon soil management. A soil may be rich in total 

 phosphorus and potassium and yet crops may not be able to secure 

 sufficient of these elements for large yields. The quantity of decaying 

 vegetable matter in the soil has much to do with the quantity of 

 mineral elements available to plants, but if a soil is low in phosphorus 

 or potassium, or if the system of farming is such as to draw heavily 

 upon these elements, materials rich in available phosphorus and 

 potassium compounds may be added. 



Any material which adds to the fertihty of the soil is a fertihzer. 

 This term, however, is more commonly applied to commercial 

 materials used for this purpose, especially when the product contains 

 two or more of the essential elements of plant growth. The phos- 

 phorus content of fertihzers is commonly expressed in textbooks and 

 fertilizer analyses as phosphoric acid and the potassium content as 

 potash. To think in terms of phosphorus, the compound phosphoric 

 acid may be reduced to phosphorus by multiplying by 0.4366; 

 potash may be reduced to the element potassium by multiplying 

 by 0.83. 



Phos'phorus in the soil (Ref. No. 5, pp. 183, 184). — The proportion 

 of this element in the most common soils of the United States is 

 very small. The total amount on the average is from 0.05 per cent 

 to 0.1 per cent. In many cases it is as low as 0.02 or 0.03 per cent. 

 SiQce the soil of the surface, 8 inches, in which most of the organic 

 matter occurs, weighs about 2,000,000 pounds on an acre, this means 

 that there are normally between 400 to 2,000 pounds of phosphorus- 

 per acre, which constitutes most of the supply which can be made 

 available to crops. Agricultural crops on the average take from 

 8 to 10 pounds of phosphorus per acre annually. The total supply 

 of phosphorus in the soil to the depth of 8 inches would be, on this 

 basis, sufficient to meet the needs of crops for from 50 to 250 years. 

 This period would be much shorter in case of low phosphorus content 

 or larger yields. Of course, it is probable that some of the phos- 

 phorus in the soil below a depth of 8 inches can be drawn on, but 

 even if we assume that a considerable amount comes from below 8 

 inches, it is still evident that if the phosphorus absorbed from the 



