io8 SCIENCE AND SOIL 



proportion that is liberated from our common soils of the element 

 that limits the yield of the crop. 



In Table 21 are given the amounts of annually available plant 

 food in Illinois soils as roughly estimated by this method of com- 

 putation. 



Of course, these amounts would be smaller and smaller year by 

 year in proportion as the total supply is decreased, and accordingly 

 complete exhaustion is not only impracticable and unprofitable 

 because of the continual reduction in crop yields, but it is mathe- 

 matically impossible, just as it would be impossible to exhaust a 

 bank account if only one per cent of the remaining deposit could 

 be withdrawn each week. 



A peaty swamp soil containing 2930 pounds of total potassium 

 per acre in the first 6f inches would liberate during the season, 

 according to this estimate, about 7 pounds of potassium, which 

 would be equivalent to a crop of 10 bushels of corn, which represents 

 roughly about the average yield from such land when not treated 

 with potassium, as is shown in the following pages. The common 

 brown silt loam prairie soil, when well farmed, will average about 

 50 bushels of corn per acre, which would require n^ pounds of 

 phosphorus and 74 pounds of nitrogen, while 12 and 96 pounds 

 represent i per cent of the phosphorus and 2 per cent of the nitro- 

 gen, respectively, in the surface soil, where phosphorus is the first 

 limiting element and nitrogen the second. 



These illustrations are given not to prove that this rough esti- 

 mate is applicable, but rather to show the basis which suggests 

 such a computation. It has some value, chiefly, perhaps, in that 

 it helps one to understand why it is that with phosphorus enough 

 in the surface soil for 50 crops, we obtain only half a crop as an 

 average. 



. On this basis we should try to keep sufficient phosphorus in the 

 surface soil for 100 large crops, of which one per cent would then 

 be sufficient for one large crop. This would require about 2300 

 pounds of phosphorus per acre, or but little more than is actually 

 contained in the most productive Illinois corn-belt soil, as the 

 early Wisconsin black clay loam in such counties as McLean, 

 Champaign, Edgar, et al. 



While there are several agencies that tend to convert insoluble 



