CULTURAL METHODS IN ORCHARDS 



161 



In another investigation ^ it was found that the "clean 

 cultivation with cover-crop and the straw mulch occupy 

 the extreme positions in soil temperature behavior." The 

 bare soil will respond very quickly to a change in the air 

 temperature, rising rapidly during warm weather and con- 

 versely showing the lowest temperature in the winter. By 

 examining Fig. 27 it will be seen how closely the soil temper- 

 ature follows that of the air. (Maximum air temperature.) 



Fig. 28. — Curves showing the relative formation of nitrates under sod, 

 tillage, and tillage cover-crop systems of orcharding. Parts per 

 million, dry soil. 



137. Nitrates. — The nitrogen problem is of paramount 

 importance in discussing the fertility of an orchard. This 

 is because in many cases nitrogen is the limiting factor, and 

 also because it is a very expensive element to purchase in 

 the form of artificial fertilizer. Hence, the bacterial activ- 

 ities in the soil, particularly nitrification, become of great im- 

 1 Ind. Agr. Exp. Sta. Bull. 205. 1917. 



