98 AGRONOMY 



element is the characteristic element of all animal and plant 

 life, as silicon is of the mineral kingdom, but the carbon 

 fixed in any form of organic life is only one stage in a constant 

 cycle of changes. Upon the death of the organism it is again 

 united with oxygen by the processes of decay and liberated 

 as carbon dioxide, only to be selected by new plants and formed 

 into starch and plant tissues again. Though forming so small 

 a proportion of the air, it is nevertheless estimated that there 

 are 3,400,000,000,000 tons of it in the atmosphere more 

 than 25 tons for each acre of soil. In plants and animals carbon 

 is most frequently found united with hydrogen and oxygen to 

 form carbohydrates, a carbohydrate being defined as a substance 

 consisting of these three elements, with the hydrogen and 

 oxygen in the proportions in which they form water. Starch, 

 sugar, wood, and cellulose are all carbohydrates. 



Nitrogen. Four fifths of the air is nitrogen, but ordinary 

 plants cannot use it. Their supply is derived almost entirely 

 from the nitrogen in the humus of the soil. A few plants, to 

 be described later, are able to make use of atmospheric nitro- 

 gen, but the rest use nitrogen only in the form of nitrates ; 

 that is, nitrogen combined with other elements, such as calcium, 

 potassium, magnesium, sodium, and the like. One of the chief 

 uses of these latter elements to plants seems to lie in their 

 ability to combine with nitrogen in a form that the plants can 

 use. Nitrogen is one of the elements most frequently lacking 

 in soils, though there are not less than 35,000 tons in the air 

 over each acre worth about ten million dollars at present 

 prices if it were only available for plants. Nitrogen intensifies 

 the color of plants, increases the growth of leaves and stems, 

 and, when abundant, may hinder seed formation by favoring 

 growth processes. Some grain crops, when supplied with plenty 

 of nitrogen, grow so luxuriantly that the stems are unable to 

 support the weight of the plant. Nitrogen is also necessary for 

 the formation of protoplasm and all other proteins. 



