MANURES 105 



" catch crops " for this purpose is best done in the autumn, 

 since nitrification is then very rapid, and loss by the washing 

 out of the nitrates by the winter rains is to a great extent 

 prevented. Rye and mustard are favourite crops for the pur- 

 pose ; and obviously the ploughing in of the crop must take 

 place before the seed is formed, or otherwise the land would be 

 fouled for the next year. If leguminous crops are grown 

 and the crop be ploughed in, a still greater accession to the 

 nitrogenous store of the soil may be secured, for in the pre- 

 sence of the appropriate nodule-forming bacteria such crops 

 draw supplies of nitrogen from the air. 



In the case of most plants, the roots absorb from the soil 

 water, the phosphates, potash, nitrates, &c., present and the 

 plant is quite unable to obtain any sustenance from the free 

 nitrogen of the air. In the case of peas, beans, clover, lupines 

 and other leguminosae the roots often possess small nodular 

 swellings or tubercles, inhabited by micro-organisms (Bacillus 

 radicicola) which have the power of taking free nitrogen from 

 the air within the soil, building it up into complex organic 

 compounds, probably of an albuminoid nature, and handing it 

 on to the host plant. 



When a leguminous plant possesses the nodules and their 

 micro-organisms, it becomes independent of combined nitrogen 

 in the soil, and can thus flourish under circumstances which 

 would induce nitrogen starvation in other plants. Moreover, 

 after such a crop, the soil is often left richer in nitrogen than 

 before, owing to the root-debris remaining after the removal 

 of the crop. These facts were discovered by Hellriegel and 

 Wilfarth in 1886-88, and artificial cultures of appropriate 

 micro-organisms for several crops were put on the market 

 under the name of " Nitragin." 



In this connection attention may be called to the recent 

 improvements in the preparation of cultures of nodule-forming 

 bacteria for the various leguminous crops. Both in Germany 

 and in America modified preparations of "Nitragin" have 

 been prepared, which were said to be much more successful 



