SOIL TEXTURE AND COMPOSITION 87 



Plant Requirements. — All agricultural plants require ten 

 elements, and the growth of any farm crop is dependent upon a 

 supply of these. If one is lacking there is no plant growth. The 

 extent of plant growth is governed by the one present in least 

 quantity in proportion to the plant's needs. Of these ten ele- 

 ments, all plants secure two elements from the air, one from the 

 water, and seven from the soil. 



Water, which constitutes from 70 to 90 per cent of the plant, 

 comes wholly from the soil. Carbon and oxygen which compose 

 90 per cent of the dry matter of the plant are drawn from the 

 air. The air is one-fifth oxygen; hence, the supply is inexhaus- 

 tible. There is only one pound of carbon in 10,000 pounds of 

 country air, but this is being continually renewed through the 

 burning of coal, wood, and other material. Moreover, all ani- 

 mals return carbon to the air in large quantities as carbon dioxid. 

 Therefore, we need not concern ourselves with the carbon supply 

 of plants as it is automatically governed. 



If we include hydrogen with the carbon and oxygen weac- 

 count for 95 per cent of the dry matter of the plant. This is 

 supplied the plant in the water. Therefore, we need concern 

 ourselves with only seven elements which constitute only 5 per 

 cent of the dry matter of the plant. 



Nitrogen — The element taken from the soil by plants in larg- 

 est quantities is nitrogen. It enters into the composition of the 

 protoplasm of the plant. The lack of nitrogen in the soil results 

 in a reduction of leaf surface, stunted abnormal growth, yellow- 

 ing of the leaf, especially in cold weather. Excessive nitrogen 

 given to cereals causes excessive production of straw with a great 

 tendency to lodge. Beets produce excessive tops with small roots. 

 Nitrogen-starved plants and also plants receiving excessive quan- 

 tities of nitrogen are especially susceptible to attacks of insects 

 and fungus pests. 



Nitrogen differs widely from the other essential elements in 

 the following. It does not constitute a part of the original 

 earth's crust. It belongs exclusively to the atmosphere to which 

 it constantly tends to return. In its atmospheric conditions it 



