COMPOSITION OP SOILS || 



CHAPTER XVI.— Composition of Soils 

 (Continued) 



83. Plant Food in Soils. — In the last chapter we con- 

 sidered four substances — water, organic matter, sand, 

 and clay, which make up so large a part of all soils. But 

 while these substances make up a large part of all soils, 

 they furnish the plant with but a small part of its 

 mineral matter. Water and organic matter in both 

 indispensable as plant foods, but sand is of almost do 

 value, and pure clay is little better than sand. Sand 

 and clay are not of much use as plant foods, but they 



us a sort of storehouse for the plant supply of food 

 and water, and they also make up the soil in which the 

 plant roots grow and develop. The clay particles in the 

 soil hold water from which the plant may draw its sup- 

 ply. They also hold certain valuable plant foods which 

 they give up to growing plants. Sand prevents the soil 

 from becoming too wet and sticky, and also absorbs 

 much heat from the sun. Mixed in with the sand, clay, 

 and organic matter are small quantities of various min- 

 eral compounds that make up the ashes of plants. Four 

 mineral elements are absolutely necessary to the growth 

 of plants; they are phosphorus, potassium, calcium, and 

 iron. Many others are found in the ash of plant b, 

 they do not seem to be as necessary as the four men- 

 tioned. 



84. Phosphorus in Soils. — This element in some form 



