48 CLOVERS 



to this view, the stems and leaves of the plants were 

 thus equally benefited and, consequently, when these 

 were plowed under where they had grown these also 

 added plant food to the cultivated portion of the soil, 

 in addition to what it possessed when the clover seed 

 which produced the plants was sown upon it. In 

 brief, this theory claimed that fertility was added by 

 the clover plants gathering fertility in the subsoil 

 and depositing it so near the surface that it became 

 easily accessible to the roots of other plants sown 

 after the clover and which had not the same power of 

 feeding so deeply. This theory was true in part. 

 The three important elements of plant food, nitro- 

 gen, phosphoric acid and potash, were and are thus 

 increased in the soil, but this does not account for 

 the source from which the greater portion of the 

 nitrogen thus deposited in the soil was drawn, as 

 will be shown below. 



It was also noticed that when the seed of any vari- 

 ety of clover was sown on certain soils, the plants 

 would grow with more or less vigor for a time and 

 then they would fail to make progress, and in some 

 instances would perish. It was further noticed that if 

 farmyard manure was applied freely to such land, 

 the growth made was more vigorous. Yet, again, 

 it was noticed that by sowing clover at short inter- 

 vals on such soils, the improvement in the growth 

 of the plants was constant. But it was not under- 

 stood why clover plants behaved thus under the con- 

 ditions named. It is now known that ill success at 

 the first was owing to the lack of certain micro- 

 -organisms, fno're commonly termed bacteria, in the 



