EXTENSION COURSE IN SOILS. 11 



quantity with different crops and with the stage of development of 

 the plants. In general, the mineral elements and the nitrogen make 

 up only about IJ per cent of the dry weight of plants, while the 

 carbon, hydrogen, and oxygen comprise about 98| per cent of the 

 total dry weight. While silicon, sodium, and chlorin are present ia 

 growing crops, these elements do not appear to be indispensable to 

 the successful growth of plants. Attempts to grow plants without 

 any of the elements carbon, hydrogen, oxygen, nitrogen, potassium, 

 phosphorus, calcium, magnesium, iron, or sulphur have resulted only 

 in failure. These elements have been called, therefore, the 10 essential 

 elements of plant food. Whenever all conditions favorable to the 

 best growth have been furnished to plants, with the exception that 

 some one essential element was supplied only to a limited extent, the 

 plants have never developed beyond the point made possible by the 

 element which was limited in supply. When this principle is applied 

 to crop production, it means that no matter how favorable the water 

 supply, the tilth, and other essentials for growth may be, the harvest 

 will never exceed what is made possible by the element which rela- 

 tively is least supplied to the crop from the soil. The element of 

 plant food thus limiting growth is called the limiting factor in crop 

 production. The elements commonly considered as limiting crop 

 production are nitrogen, phosphorus, and potassium. The manage- 

 ment of soils so as to build up the supply of these elements of plant 

 food is specially treated in Lessons VI and VII. 



How soil materials are utilized hy plants (Ref. No. 3, pp. 404, 405, 

 412-418; or No. 10, pp. 166-174), — Soil materials must be dissolved 

 in water before plants can absorb them. The plant-food elements of 

 the soU. go into solution in the form of compounds called salts. A 

 salt results from a chemical reaction between an acid and a base. 

 An acid is a substance which will turn blue litmus paper red, while 

 a base is one which will neutralize an acid and will turn red litmus 

 paper blue. Vinegar contains an acid, while slaked lime is a base. 

 When muriatic acid is added to slaked lime they react and form calcium 

 chlorid, which is a salt. Calcium phosphate, potassium sulphate, and 

 sodium nitrate are examples of salts which serve as sources of plant 

 food. While these and all other salts must be dissolved before they 

 can be utilized by plants, it is not necessary or even desirable that 

 large quantities of plant food be m solution in the soil at any one time. 

 Plant-food substances in solution or in condition to become so from 

 the action of natural agencies are called available; those not in con- 

 dition to become soluble for plant use are said to be unavailable. 



Plants during growth absorb the soil solution through many small 

 projections called root hairs. These root hairs constantly develop 

 anew near the ends of protruding rootlets and keep in close contact 

 with soil grains and immersed in. the water film surroimding soil 



