EXTE]SrSION COURSE IN SOILS. 13 



Phosphorus also seems to bear an intimate relation to the development 

 of plant cells. Potassium and calcium are closely allied with stem 

 and root structure. A liberal available supply of these elements 

 favors stiff, strong stems in grain and other crops. Potassium is also 

 essential in starch formation. A good supply of available potassium 

 in soils is needed, therefore, for root crops. Sulphur has an important 

 function in cell structure. Iron is necessary in the forming of 

 chlorophyll grains which give the green coloring to leaves and which, 

 in the presence of sunshine, aid in the manufacture of starch m the 

 leaves, largely from carbon dioxid and water. Carbon, together with 

 water, composes a large percentage of plant structure and is the 

 basis of all organic substance. Oxygen not in combination with 

 other elements enters the plant and causes the breaking down, or 

 oxidation, of other materials in the plant. 



Soil materials removed hy crops (Rei. No. 3, pp. 418-420). — In nature, 

 as plants mature and decay, the soil materials used in plant growth 

 are largely returned to the soil. The loss to the soil of inorganic or 

 mineral substances by leaching and erosion is usually counter- 

 balanced by the natural agencies of disintegration, while the organic 

 or vegetable decomposition enriches the soil in nitrogen and returns 

 the mineral substances again to the soil. Mineral compounds from 

 vegetable decay, it should also be noted, become more readily avail- 

 able in the soil than do the minerals from rocks. Under ordinary 

 farm practice, on the other hand, soil materials are removed in 

 crops, waste occurs in connection with the management of manures, 

 straw, and plant residues, and the soil often leaches and erodes very 

 readily. All of these things deplete the fertility of the soil. The 

 plant-food elements removed by crops vary with the yield, the crop 

 grown, and the available materials in the soil. A reliable table 

 showing the average quantity of nitrogen, phosphorus, and potassium 

 removed from the soil by crops is found in reference No. 5, page 154. 



Plant-food materials contained in soils (Rei. No. 5, pp. 58-60). — 

 The amounts of the essential plant-food elements in soils are ex- 

 tremely variable. Since the nitrogen in soils comes almost entirely 

 from vegetable decay, the supply of this important element depends 

 upon the plant materials returned to the soil and the activity of the 

 agencies of decomposition. The total supply of the mineral ele- 

 ments present in the soil, as stated in Lesson I, depends largely upon 

 the original rocks from which the soil was formed. The quantity 

 of materials available for plant growth, it must be understood, 

 depends upon good soil management as weU as upon the type of 

 soil formation. Hopkins says: 



We can assume for a rougli estimation that the equivalent of 2 per cent of the 

 nitrogen, 1 per cent of the phosphorus, and one-fourth of 1 per cent of the total po- 

 tassimaa contained in the surface soil can be made available during one season by 



