336 SOILS 



must get old soil, or an artificial culture, for each 

 kind of leguminous crop grown. It is probable 

 that the several kinds of bacteria will be round to 

 be more or less interchangeable, but the safest way 

 is to get the kind that go with the crop to be grown. 



It is often found that the first year a leguminous 

 crop is grown the stand is poor and the growth 

 unsatisfactory; or that there are few nodules on 

 the roots, snowing that little nitrogen is being 

 secured. But the second year the crop will be 

 better and the roots have more nodules, because 

 the bacteria have increased. In growing legumi- 

 nous crops, therefore, it is often best to re-seed on 

 the same land until the soil becomes well filled with 

 bacteria. Often if a poor stand of clover or alfalfa 

 is plowed and the land at once re-seeded a much 

 better stand is secured. 



Poor Soils Benefited Most. If the soil is already 

 quite well supplied with available nitrogen legu- 

 minous plants growing in it get very littlenitrogen 

 from the air; they will draw upon the nitrogen in 

 the soil. Leguminous plants live on nitrogen of the 

 air only when they have to ; when there is very little 

 nitrogen in the soil. Cowpea plants on poor soil 

 usually have many more nodules on their roots 

 than cowpea plants on a rich soil; showing that 

 the former are living mainly on the nitrogen of the 

 air, while the latter are living mainly on the nitro- 

 gen in the soil. Even on a soil already rich in 

 nitrogen, leguminous'crops do return more nitrogen 

 to the soil than they draw from it; but the poorer 

 the soil the more nitrogen there is added to it. The 

 same crop of cowpeas may add 100 Ibs. of nitrogen 

 to the soil or 25, according to the extent to which 

 the plants have been obliged to get nitrogen from 

 the air. This calls attention again to the peculiar 



