LAND AND OTHER AGENTS OF PRODUCTION 159 



6f inches deep (corresponding to 2, 000,000 pounds of soil) would con- 

 tain 50,000 pounds of potassium. In Illinois, the normal soils actually 

 do contain from 25,000 to 45,000 pounds per acre of this plant-food 

 element in the first 6f inches, while less than 4 pounds of potassium 

 would be added in an application of 200 pounds of the most common 

 commercial fertilizer. The Illinois system of permanent fertility 

 does not provide for. the purchase of potassium for normal soils, but 

 it does provide for the liberation of an abundance of that element 

 from the practically inexhaustible supply in the soil. This liberation 

 is accomplished by the action of decaying organic matter plowed under 

 in the form of farm manure or crop residues, including clover or other 

 . legumes. 



Only where the soil is positively deficient in potassium susceptible 

 of liberation, as is the case with some sand soils and with most peaty 

 swamp lands, need potassium be purchased in permanent systems of 

 either grain farming or live-stock farming; but in market gardening, 

 or in raising timothy hay for the market, commercial potassium may 

 be required, and on some worn soils especially deficient in decaying 

 organic matter the temporary use of kainit is often advisable. 



Magnesium and calcium. As a general average, the normal soils 

 of Illinois contain more than four times as much potassium as mag- 

 nesium, while the loss by leaching and cropping in rational systems 

 of grain or live-stock farming may be actually greater for magnesium 

 than for potassium, so that magnesium is more apt to become deficient 

 in soils than is potassium. 



The calcium supply in normal soils is also only one-fourth that of 

 potassium, while the average loss by cropping and leaching is four 

 times as great, so that 16 to i expresses the relative importance of 

 calcium and potassium in the problem of permanent fertility on nor- 

 mal Illinois soils. 



All limestones contain calcium; and the common dolomitic lime- 

 stone in the almost measureless deposits of northern Illinois contains 

 both calcium and magnesium in very suitable form both for plant 

 food and for correcting or preventing soil acidity. In the Illinois 

 system of permanent fertility, ground natural limestone is applied, 

 where needed, at the rate of about 2 tons per acre every four years. 



Phosphorus. Attention was called to the fact that 2,000,000 

 pounds of the average crust of the earth contains 50,000 pounds of 

 potassium; but comparefl with this we find only 2,000 pounds of 

 phosphorus. Likewise, the plowed soil of an acre of average Illinois 

 land contains about 35,000 pounds of potassium but less than 1,200 



