1 10 AGRICULTURE 



in the fact that they are nitrogen-gatherers. Nitrogen is a costly and 

 scarce element of plant food, costing, in commercial form, about 

 sixteen cents a pound. An average crop of peas or clover adds 

 about one hundred and fifty pounds of nitrogen to each acre of soil. 



Free Nitrogen. Where do the plants get it ? From the 

 air. Over every acre of soil there are more than three thousand 

 tons of free nitrogen. This is of no service to crops, such as 

 the cereals, which have no power to use nitrogen until it is changed 

 into certain compounds. With vast quantities of nitrogen around 

 it, a plant- may starve, just as a sailor may perish of thirst with 

 the great ocean surrounding him. Nitrogen is there, water is 

 there, but not in forms available to the plant and the man. 



Tubercles. Legumes, however, feed indirectly on this free 

 nitrogen by means of certain forms of bacteria, which live in 

 knots, called tubercles, on their roots. The decay of the tuber- 

 cles leaves nitrogen in the soil. On different legumes and under 

 different conditions these tubercles vary from the size of a tiny 

 pin head to that of an egg. They are thought to be rootlets, 

 changed in form by thousands and millions of bacteria. 



Bacteria in Soils. In soils where legumes have been culti- 

 vated, these bacteria are abundant. Where legumes have not 

 been grown, they are apt to be lacking. In that case the legumes 

 will produce a smaller crop and the soil will be less enriched. 

 The bacteria increase rapidly, and often legumes thrive the second 

 year on land where they failed the first. Bacteria may be sup- 

 plied by sprinkling land with soil from a field on which has been 

 grown the legume desired; soil from an alfalfa field must be 

 used for alfalfa, from pea land for peas, and so on. Sprinkling 

 the soil, or in oc'u lat ing it, as it is called, with soil from another 

 field is troublesome and often inconvenient ; it often introduces 

 weeds and insect pests and fungous diseases. 



