THE SOIL 47 



Plants absorb their minerals from the capillary water 

 solutions. A rich soil has a greater supply of soluble material 

 than a poor soil. Thus many soils can be improved by in- 

 creasing their acidity so that the rock particles will be dis- 

 solved more rapidly, and by checking excessive drainage 

 which loses fertility to the underground water. 



The amount of soluble matter may vary from time to 

 time even though in general the insoluble portions continue 

 to become soluble. It may be reduced by greater absorption 

 at times of rapid plant growth, and during rainy periods 

 when some of the soluble material may be carried from the 

 soil in the underground drainage water. A soil with a greater 

 water-holding capacity should be better able to supply 

 plants with mineral salts. The soluble and insoluble compo- 

 sition of fertilizer, as shown by analysis, should be studied in 

 any effort to improve soil. It should be kept in mind that 

 while the insoluble is unavailable to the plants, it slowly 

 becomes soluble and available. Bonemeal and other organic 

 materials are among our best and safest fertilizers because 

 they become slowly available. 



The growth and development of all organisms is a 

 response to environment, which for plants is air and soil. 

 Man has been able by plant breeding (discussed in Chapters 

 18 and 19) to make plants that are better adapted to certain 

 environments, and although he has little power to regulate 

 the temperature, the sunlight, and the rain, he can use the 

 basic mineral soil available to him and can alter the proper- 

 ties of the soil complex most markedly in many ways. Soil 

 science has made it possible to make a soil suitable for practi- 

 cally any plant. The first steps of improvement might in- 

 clude the increase or decrease of the organic matter, the 

 number and size of air spaces, the numbers of living organ- 

 isms, the relative acidity, and of the temperature and water- 

 holding capacity of the soil. Soil improvement requires a 

 careful study of the soil and the response of the plants to the 



