entirely derived from the carbon dioxide of the air. Although this com- 

 pound forms but 3 or 4 parts in 10,000 parts of the atmosphere, the quan- 

 tity is sufficient, owing- to the wind continually bringing- fresh supplies 

 to the leaves. Thus there is an abundance of air around the leaves of 

 the plant, but, if the soil is not open and porous, there may not be enough 

 in contact with the roots, for it is worthy of note that air in the soil in 

 which crops are growing is as essential to the life of plants as air in the 

 stable is to the animal. This ventilation of the soil is necessary to supply 

 oxygen required in germination of seed, to permit the roots to live, for 

 they, too, must breathe, and to supply this life-giving element to the 

 millions of little organisms in the soil which are busy preparing soluble 

 food for the plant. The ventilation of the soil is also required to supply 

 free nitrogen for the use of nitrogen-fixing germs, and to remove the 

 excess of carbon dioxide which is being continually set free in the soil. 

 From the soil the plant derives nitrogen, chiefly in the form of nit- 

 rates, the ash substances, and water. Fortunately, although ten ele- 

 ments are essential for the growth of the plant, there are only four that 

 particularly interest the farmer, as the other six are usually found in 

 abundance. These four are, nitrogen, potassium, phosphorus, and cal- 

 cium. A continuous supply of all the essential elements of plant growth 

 is absolutely necessary; for, if one constituent is absent, or present in an 

 insufficient quantity, no matter what amount of the other nutrients may 

 be available, the plant cannot be fully developed. Consequently, just as 

 a chain is only as strong as its weakest link, so the crop-producing power 

 of a soil is limited by the essential nutrient present in relatively the small- 

 est quantity. 



Function of Plant Food Constituents. 



In the absence of nitrogen the plant makes no appreciable growth. 

 With only a limited supply, the plant commences to grow in a normal 

 way, but as soon as the available nitrogen is used up, the lower and 

 smaller leaves begin gradually to die down from the tips and all the plant's 

 energy is centred in one or two leaves. Nitrogen is one of the main con- 

 stituents of protein, which is possibly the most valuable part of a plant, 

 It is also a constituent of chlorophyll, the green coloring matter of plants; 

 hence with a limited supply of nitrogen, the leaves will have a sickly 

 yellow color. Plants with large, well-developed leaves are not suffering 

 for nitrogen. An abundance of this substance will produce a luxuriant 

 growth of leaf and stem, but it will retard maturity, and, with cereals, 

 will frequently cause the crop to "lodge." Therefore, when crops, such 

 as cereals, tomatoes, potatoes, etc., are to be matured, an over supply 

 of nitrogen is injurious; but with crops, such as lettuce, cabbage, etc., 

 which are harvested in the green condition, an abundance of nitrogen 

 will, other fertilizing constituents being present, tend to produce a strong 

 vigorous growth, and give crispness or quality to these crops. 



Potassium, or potash, as it is commonly called, is one of the most 

 important and least variable of all the elements of the ash of plants. It 



