PHYSICAL PROPERTIES OF SOILS. 41 



more sand or gravel the heavier the soil; the more 

 vegetable matter, the lighter it will be. It is said 

 some peat soils weigh not more than thirty pounds per 

 cubic foot. Few soils ordinarily cultivated will weigh 

 less than seventy-five or eighty pounds per cubic foot. 

 The soil on an acre, 43,560 square feet, to the depth 

 of one foot, will weigh rarely less than 3,000,000 

 pounds and often more than 4,000,000 pounds, or 

 1,500 to 2,000 wagon-loads of one ton each. It will be 

 seen that even though only a small percentage of the 

 soil is ever in condition to be available as plant food, 

 there may still be an enormous quantity of such food 

 present in the first few inches of soil — enough to pro- 

 duce the largest crops. A sandy soil may have a 

 greater supply of plant food than a clay soil, because 

 of the greater weight, even though it have, as is usu- 

 ally the case, a smaller percentage of such food. 



Soils are often spoken of as "light" or "heavy" not 

 with reference to their actual weight, but to their te- 

 nacity. In this sense a sandy soil is "light" and easily 

 worked, and a clay soil "heavy," although lighter in 

 weight than the sandy soil. A soil with much vegeta- 

 ble matter will be light in both senses of the word. 



Texture. — xill soils are porous, that is, have small 

 open spaces between the solid particles. Into these 

 the air and water pass, gases are condensed, and the 

 roots of plants penetrate in search for food. A solid 

 mass of pure plant food would give little nourishment 

 to a plant. A finely divided soil in which there is a 

 very small percentage of plant food may produce fair 

 crops. A large number of small pores is desirable. 

 Coarse sand has large openings between its grains. 

 A good loam or a well pulverized clay soil has a molti- 



