212 



ROCKS AND SOILS 



noticed nature's way of maintaining soil fertility? On 

 the ground are rotting leaves, branches, and limbs of 

 trees. These decaying plant remains add humus to the 

 soil and restore minerals taken from it by the grow- 

 ing vegetation. Successful farmers imitate nature's 

 method by adding manure to the soil. If green plants 

 are plowed under, they are called green manure. Some- 

 times farmers grow cowpeas, rye, or clover after the 

 harvesting season and then plow them under before 

 the next planting season. The plants decay and add 



Bacteria, 

 change Proteins 

 toNitra.tes 



Nitrates 

 in Soil 



Plants use 

 Nitrates injbuil<ing 

 Proteins 



Proteins 



in 

 Plants 



Cinimals eat. plants 

 FIG. 342. NITROGEN CYCLE 



humus and nitrogen to the soil. Farmers also scatter 

 the manure from animals over their fields. This is one 

 of the oldest and most practicable methods of main- 

 taining soil fertility. Manure is more desirable than 

 commercial fertilizers because it adds humus to soil 

 at the same time that it enriches it with important 

 chemical elements. 



All plants and animals require nitrogen. In our 

 study of foods we learned that nitrogen occurs in sub- 

 stances called proteins. 



Plants build up proteins from water, carbon dioxide, 

 and the chemical elements they obtain from the soil. 

 Animals obtain their proteins from plants or animals 

 they eat. The waste products of plants and animals 

 and their dead bodies return to the soil. Here the pro- 

 teins they contain are broken down by soil bacteria 

 into simpler nitrogen compounds. These simpler sub- 



stances dissolve in soil water and are absorbed and 

 used again by other plants (study Fig. 342). Thus 

 nitrogen makes a cycle through plant and animal life. 

 Nitrogen is supplied to soil in another way. Al- 

 though four fifths of the atmosphere consists of the 

 element nitrogen, animals and most plants are un- 

 able to use this important element directly from the 

 air. Fortunately there is one family of plants called 

 legumes that assists in taking nitrogen directly from 

 the air. Plants of the legume family like clover, peas, 



r^^^Jl 



U. S. Bureau of Plant Industry 



FIG. 343. NODULES ON ROOTS OF A LEGUME 



and beans have little nodules growing on their roots 

 (see Fig. 343). In these nodules are certain bacteria 

 that combine free nitrogen of the air with oxygen to 

 form nitrates. A part of the nitrates is used by the 

 plant and a large part stays in the nodules. When the 

 plants die the nitrogen in the form of nitrates becomes 

 a part of the soil. Many legumes do not grow well 

 without these nitrate-forming bacteria in the soil. It 

 is now possible to buy these bacteria and to inoculate 1 

 beans and other legumes with them before the plant- 

 ing is done. 



Most farmers do not raise the same crop, year after 

 year, on a certain piece of land. Instead they alternate 

 their other crops with legumes (peas, beans, clover, 



'Inoculate, insert into (as buds, germs, or bacteria). 



