EXTENSION 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 tK.e presence of sunshine, aid in the manufacture of starch hi 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 by crops (Ref. 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 (Ref. 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 well as upon the type of 
soil formation. Hopkins says: 
We can assume for a rough 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- 
tassium contained in the surface soil can be made available during one season by 
