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 andof 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 
