150 POMOLOGY 



is one of the oldest practices in agriculture and it would in- 

 deed be unfortunate to advocate the discontinuance of it 

 unless there is adequate data to warrant the position. This 

 the author beUeves is not the case, but rather that in the 

 past few years it has been demonstrated beyond question 

 that the practice is valuable both in the orchard and on the 

 farm. To test the soil for the presence of organic matter 

 (carbon) and to find an immaterial gain where abundant 

 organic matter has been returned to the soil for a long pe- 

 riod of time is hardly sufficient evidence on which to rest the 

 case. It is of more value to the orchardist to know that 

 where clean tillage has been followed in both citrus and de- 

 ciduous orchards, the soil failed properly to support the trees 

 within a relatively short time. When cover-crops were 

 again included in the orchard management, the trees be- 

 came vigorous and productive, even without the use of fer- 

 tilizers. As is pointed out elsewhere, the hotter the climate 

 and the longer the season, the quicker will the humus and also- 

 the nitrogen disappear from the soil through tillage methods. 

 True, there may not be sufficient material raised to main- 

 tain an increasing ratio of organic matter as the trees be- 

 come mature and shade a large portion of the ground. Then 

 outside material may be resorted to, to supplement the loss. 

 Hence the present teaching must be that abundant cover- 

 crops are the safest way of preventing depletion of the soil 

 where tillage methods are followed. 



132. Nitrification. — The student should not confuse the 

 two groups of bacteria that have to do with the nitrogen 

 transformations in the soil. Where legumes are grown, 

 the symbiotic organisms living in the nodules on the roots 

 fix nitrogen of the soil-air and leave it behind in combined 

 forms in their remains and in the tissues of the host. Among 

 the organisms concerned in the decay of legumes as well as 

 of other plants are those which break down the complex 



