IMPORTANCE OP SOIL 



209 



The rock materials must also contain humus (decayed 

 vegetable matter). Soils may be made fertile in the 

 following manner. Lichens and some other low forms 

 of plant life are the first to establish themselves on in- 

 fertile rocks and rock particles, as they need almost no 

 soil to live. They die and leave small amounts of min- 

 erals and humus from which mosses can gain enough 

 nourishment to grow. The soil is further enriched as 

 the mosses die, and later ferns and grasses appear. 

 This process goes on for centuries, and as the fertility 

 of the soil increases, higher and higher forms of plant 

 life become established. Leaves, stems, and roots from 

 shrubs and trees decay and add still more humus to 

 the soil. Thus we see that our natural fertile soils are 

 formed from particles of rock that supply the mineral 

 matter and from decayed plants that supply humus. 



Photo by C. K. Sibley 



FIG. 336. LICHENS GROWING ON A ROCK 



What are the different kinds of soil? If you exam- 

 ined samples of soil, as suggested in one of the experi- 

 ments, you perhaps noticed that they consisted of par- 

 ticles of different sizes. Soils are classified according 

 to their sizes into four principal types : gravel, sand, 

 silt, and clay. The following table gives a classification 

 of soils based on the size of particles. 



KIND OF SOIL 



Gravel 

 Sand 

 Silt 

 Clay 



SIZE OF PARTICLKS 



Over 1/50 inch in diameter 

 1/50 to 1/500 inch in diameter 

 1/500 to 1/5000 inch in diameter 

 Smaller than 1/5000 inch in di- 

 ameter 



Soils also contain varying amounts of humus. The 

 larger the amount of humus in soils, the less the soils 

 weigh. For this reason humus soils are spoken of as 

 light soils. 



Vnderu'Ood 



FIG. 337. VEGETATION IN AN EXCESSIVELY MOIST REGION 



Plants do not grow well on soils made up of par- 

 ticles of only one size. A clay bank or a sandbar could 

 not be profitably cultivated. Soils most useful for farm 

 and garden crops consist of mixtures of sand, clay, and 

 humus. A nearly equal mixture of clay and sand is 

 called loam. Soils containing more clay than sand are 

 called clay loams while those in which sand predomi- 

 nates are called sandy loams. 



Upon what factors does crop raising depend? Crop 

 raising depends on a number of important factors such 

 as temperature, rainfall, sunshine, and soil fertility. 

 Man cannot control the weather, but he can do much 

 toward providing the proper soil conditions necessary 

 for good plant growth. These include proper condi- 

 tions of moisture, air, temperature, and soil fertility. 

 Let us consider now some of the problems involved 

 in maintaining a proper supply of moisture. 



Soil water. All soils contain water, but the amount 

 in different soils varies greatly. Plants also vary 

 greatly in the amount of water they need to flourish. 

 The cactus can thrive where there is little moisture, 

 whereas the willow is commonly found where there is 

 a large supply. As a rule, our common garden plants 

 do not grow well in extremely wet or extremely dry 

 soils. Every one-has observed crops dying in a swampy 

 place and also in a time of drought. 



How does water rise in soils? Water moves through 

 soil by means of capillary action. Soil particles form ir- 

 regular tubes much as in the case of a sugar lump or 

 blotting paper. The fine tubes are called capillary tubes. 

 They provide passageways for the deep soil water to 

 reach the plant roots and the surface of the ground. 

 The water goes not only upward in these tubes but in 

 any direction in which they may lead. The amount of 

 water and the distance that water rises in soils by 

 capillary action depends upon the size of the spaces 

 and the fineness of the soil particles. The finer the 

 soil, the smaller the tubes. Thus the distance traveled 



