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 1J 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 in 
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 by plants (Ref. No. 3, pp. 404, 405, 
412-418; or Xo. 10, pp. 166-174). — Soil materials must be dissolved 
in water before plants can absorb them. The plant-food elements of 
the soil 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 in 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 surrounding soil 
