SOIL FERTILITY 391 



of lessened production. From the table on page 390, which gives 

 the amount of plant food used by some common crops, it will be 

 seen that a fifty-bushel crop of wheat requires ninety-six pounds of 

 nitrogen, sixteen of phosphorus, fifty-eight of potassium, eight of 

 magnesium, and eleven of calcium, a total of only one hundred and 

 seventy-nine pounds. This is only two and one-fourth per cent of 

 the weight of grain and straw produced. The percentage of plant 

 food taken from the soil is the same for corn, while for oats it is 

 slightly more than two and one-half per cent. For the crop to 

 obtain even this small amount, it is necessary that much larger 

 amounts be present in the soil, since only a small proportion is 

 available each season. The yields given in the preceding table are 

 high, but no larger than rich soils will produce under favorable 

 conditions. 



The legumes take most of their nitrogen from the air, but the 

 other elements given in the table are taken from the soil. Other 

 crops take all of their supply of these elements from the soil. Be- 

 sides the elements given in the preceding table, iron is taken from 

 the soil. However, there is such an abundance of iron in the soil 

 and plants require so little that soils probably will never become 

 deficient. In the case of sulphur, the amount needed is small and 

 the soil receives some from the air during rains. 



Supply of Plant Food in Soils. The supply of plant food de- 

 pends upon several factors. Probably the most important is the 

 rock from which the soil was derived. A soil derived from a sand- 

 stone may contain very little plant food of any kind. A granitic 

 soil will probably contain large amounts of calcium, potassium, 

 some magnesium, and phosphorus. A limestone soil would contain 

 considerable amounts of each element. Soils formed by mixtures 

 of various rocks usually contain the largest supply. 



Nitrogen is nearly always a later acquisition. Very few rocks, 

 as that term is commonly used, contain nitrogen. 



Leaching removes large amounts of plant food, and for this 

 reason the soils of humid regions contain less than those of arid 

 ones. Some exceptions occur in swamps, where the mineral plant 

 food has been carried in by washing and leaching from the higher 

 areas. Conditions are favorable for the accumulation of nitrogen 

 through the more luxuriant growth and less rapid oxidation of vege- 

 tation. The physical composition of the soil plays a very important 

 part in leaching, since the smaller the soil particles the less the 

 leaching. 



