66 ELEMENTARY BIOLOGY 



94. Exhaustion of soiL One of the dangers of the past was 

 the fact that after many crops of plants had been taken from 

 a soil, there was a lack of the mineral salts required for plant 

 growth. In addition to nitrogen, plants need phosphorus, pot- 

 ash, calcium, and iron. There is no danger of ever exhausting 

 the iron in a soil, for this element is used in such small 

 quantities that plants will have stopped growing for lack of 

 some of the other elements (for example, phosphorus or nitro- 

 gen) long before the iron supply is considerably reduced. In 

 many kinds of soil the same thing is true of calcium. But the 

 other elements are used in such large quantities (in propor- 

 tion to the amounts present in most soils) that they practically 

 limit the use of soil for crop raising. It is chiefly for this 

 reason that so many farms in the eastern parts of this country 

 have been abandoned ; the farmers found that they could not 

 raise crops on the old soil. 



95. Fertilizers. To make up for the withdrawal of materials 

 by crops, it has for ages been customary to put various sub- 

 stances on or into the soil. These substances are called ferti- 

 lizers and include limestone or gypsum, barnyard manure and 

 guano, crushed bones and ground phosphate rock, and many 

 other substances. In this country the farmers spend about 

 $125,000,000 annually for commercial fertilizers, besides what 

 they use from their own dungheaps. 



The first thought in the use of fertilizers is to replace in the soil 

 materials that are lacking for plaut growth. Some fertilizers, how- 

 ever, are sometimes added not so much to supply material that the 

 ])lants may use as to produce chemical changes in the soil, to make 

 the latter more suitable for the growth of plants. For example, 

 gypsum is sometimes used to supply calcium, but it may also be 

 used in some cases to make the phosphorus in the soil more easily 

 available for the plants. 



96. Biology of soil. The soil is more than a mixture of 

 substances having physical and chemical properties. It con- 

 tains many different kinds of very small plants and animals 



