IMPORTANCE OF SOIL 



211 



the soil moisture so that there will be enough water 

 for a crop the following year. This is called dry fann- 

 ing. Dry farming does not mean that crops are made to 

 grow without water, but only that whatever water 

 falls is carefully conserved. 



Soil temperature is another factor in plant growth. 

 Seeds develop faster and plants grow more rapidly 

 when the temperature is between 65 and 75 degrees 

 Fahrenheit. We have seen that cultivation retards the 

 evaporation of soil water, and this in turn reduces the 



Keystone-Underwood 



FIG. 340. DIGGING DITCHES TO RECLAIM SWAMPY LANDS 

 AND TO DESTROY MOSQUITOES 



amount of cooling which always accompanies evap- 

 oration. Thus in the one process of cultivation three 

 essential factors in soil conditioning may be provided 

 for : the conservation of soil moisture, the conserva- 

 tion of soil temperature, and the proper circulation of 

 air around the roots. 



How are waste lands sometimes reclaimed by irri- 

 gation and drainage? Desert waste lands in regions of 

 the United States have been transformed into fertile 

 farms and gardens by supplying water artificially to 

 the soil. This process is called irrigation. Soils that are 

 irrigated successfully possess naturally the proper 

 minerals and humus for crop raising, but they do not 

 receive enough rainfall for plants to grow. In some 

 regions to make up for deficient rainfall great reser- 

 voirs have been built by damming a valley with 

 enormous cement walls. These reservoirs hold the 



water of the rainy season for use during the dry sea- 

 son. As the farmers need water it is delivered to them 

 through irrigation canals. Already large areas of arid 

 and semi-arid lands in Arizona, California, Colorado, 

 Texas, and other states have been reclaimed for farms, 

 gardens, and orchards. 



In other regions of the United States there are fer- 

 tile lands that receive an excess of water. This condi- 

 tion is corrected by constructing an artificial drainage 

 system. Care must be taken, when artificial drainage is 

 employed, to see that sufficient water is left in the soil 

 for the proper growing of plants. 



How is soil fertility maintained? As plants grow 

 they take certain elements from the soil. Scientists 

 have determined that the following ten chemical ele- 

 ments are the essential elements of plant food and 



From A Textbook of General Botany by Smith and Overton. By permission 

 of the Macmillati Company, publishers. 



FIG. 341. PLANTS OF THE SAME SPECIES, SHOWING EFFECTS ON 

 GROWTH OF THE OMISSION OF DIFFERENT ELEMENTS 



that with the exception of some carbon and oxygen 

 they are obtained directly from the soil : 



carbon 



hydrogen 



oxygen 



nitrogen 



calcium 



potassium 



phosphorus 



sulphur 



magnesium 



iron 



From the standpoint of soil fertility, phosphorus, po- 

 tassium, nitrogen, and calcium are the most important 

 elements to be considered. The other elements are 

 present in sufficient quantity in most soils. Plants, 

 however, vary in the amounts of phosphorus, po- 

 tassium, nitrogen, and calcium they need for healthy 

 growth. Corn, for example, removes all of these ele- 

 ments, whereas wheat uses more of the phosphorus. 

 The important thing to remember is that crop raising 

 reduces soil fertility and that any soil will eventually 

 become infertile unless the elements removed from it 

 by growing plants are again restored. It is for this 

 reason that successful farmers artificially fertilize 

 their soil. 



In walking through a wood or a forest, have you 



