Chapter Seven 

 THE SOIL 



The soil, that complex substance forming the outer layer 

 of the earth's surface, is a great reservoir of water and 

 mineral salts essential for plant growth. Many different 

 kinds of soils will furnish the necessary materials for plants, 

 but a good soil has a proper balance of the following five 

 constituents: mineral particles, water, air, organic matter, 

 and living organisms. 



The basic material of most soils is the mineral particles 

 of varying size formed by the decomposition of rock. Sandy 

 soil has a high proportion of coarse particles; silt soil has a 

 base of exceedingly fine particles. In the eastern United 

 States most soils are mixtures of various sizes of particles. 

 The size of the soil particles is important because the surfaces 

 hold the water and mineral salts which the root hairs absorb. 

 A single cubic foot of fine garden soil may have more than 

 ten thousand square feet of soil particle surface, while a 

 sandy soil has only 10 per cent as much. Since this differ- 

 ence in soil surface results in a proportional difference in the 

 water-holding capacity of soil, sandy soils require more fre- 

 quent watering than fine soils for good plant growth. 



The kind of rock from which soil has been formed is very 

 important because some soil particles are continuously dis- 

 solving to furnish the mineral salts in solution for the plants. 

 A soil formed by a combination of rocks is more productive 

 than one from shale or slate alone. Some of the best soils 

 are largely from a limestone origin. However, the degree of 

 weathering usually is more important than the parent rock 

 from which it is formed. 



46 



