EXTENSION COUESE IN SOILS. 47 
LESSON VII. 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 fertility of the soil is a fertilizer. 
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 fertilizers 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. 
Phosphorus 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. 
Since 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 
