56 Gardening 



THE PHYSICAL CONDITION OF SOIL 



How soils are formed. Soils are formed from the 

 rock of the earth's crust mixed with the decayed remains 

 of plants and animals. Through the action of air, 

 water, and changes in temperature, the surface rock 

 in the course of time crumbles into particles which form 

 the rocky or mineral parts of the soil. Therefore soil 

 may be said to be composed largely of rocks that have 

 fallen into pieces. With a hammer one may pound 

 rock into powder, thus reducing it to small particles 

 like those which make up the soil. 



The organic substances in the soil are formed by decay 

 of plants and animals that die and fall to the ground. 

 Organic matter that is well decayed so that it is black 

 in color and in fine particles is called humus. The dif- 

 ferences in soils are due both to the different kinds of 

 rocks from which they were formed and to the amount 

 of organic matter in them. 



The breaking up of solid rock and of plant and animal 

 remains into fine particles meets the first need of living 

 plants, for it gives them a place to grow. It allows the 

 roots to find their way in among the soil particles, thus 

 making it possible for them to anchor the plant and to 

 secure a supply of water and raw food materials. 



Size of soil particles. The size of the rock particles 

 in the soil may range from large bowlders to minute frag- 

 ments of dust. The number of individual particles 

 in a spoonful of ordinary garden soil is so great as to be 

 almost inconceivable. It is estimated that in an ounce 

 of coarse sand there are about one hundred billion 



