WHAT THE SOIL SUPPLIES 311 



is of the first importance. In removing crops the nitrogen 

 compounds obtained by the plants from the soil are removed 

 also, and if there were no way of restoring these compounds, 

 the soil would sooner or later become so impoverished in 

 them that it could not produce a good crop ; in other words, 

 it would result in a nitrogen famine. 



Since the nitrogen-fixing bacteria are continually adding 

 nitrogen compounds to the soil, the loss is represented by the 

 difference between what the crops remove and what the 

 bacteria add. This difference varies with the crop and with 

 the soil, but in most cases there is a real loss, so that con- 

 tinuous cropping reduces the amount of nitrogen compounds 

 to the danger point. If a field has become reduced to the 

 point of a nitrogen famine, it may be restored to usefulness 

 by " resting " for a time until the bacteria have restored the 

 nitrogen compounds. This resting of a field, that is, not 

 working it for a crop, is called letting a field lie fallow. 



The restoration of nitrogen compounds in this way, how- 

 ever, is usually too slow a process for our purpose. A much 

 more rapid way is by means of a " rotation of crops." It is 

 found that soil is kept in much better condition as to nitrogen 

 if the same crop is not grown continuously upon it. Crops 

 vary as to their wastefulness of nitrogen, so that if a crop that 

 removes a minimum amount of the nitrogen compounds 

 alternates with a crop that removes a maximum amount of 

 these compounds, a nitrogen balance may be maintained. 

 By far the most effective rotation is secured by using legu- 

 minous plants as the alternating crop, notably the clovers and 

 alfalfa, for these plants add nitrogen compounds to the soil. 

 This peculiar property of leguminous plants is due to the fact 

 that their roots become intimately associated with nitrogen- 

 fixing bacteria which inhabit tubercles (little tubers) that 

 form on the roots, and in these tubercles there is an accumu- 

 lation of nitrogen compounds not obtained from the nitrogen 

 compounds of the soil, but produced by the bacteria in using 



