13 



FULL MINERALS, AND, IN ADDITION:— 



when compared with the unmanured plot. 



The mineral manures have caused some reduction in power 

 requirement, and a still further reduction has been caused by 

 addition of sulphate of ammonia, but nitrate of soda has acted the 

 other way and increased the power requirement. 



There are, however, other ways of altering- the resistance of 

 soil to the plough, and an interesting electrical method is being 

 studied. 



The depth of ploughing influences the power consumption 

 more than might have been expected. An increase of only one 

 inch in depth, i.e., going from 5" to 6" deep, increased the power 

 consumption no less than 32%, a portion of which is due to the 

 resistance offered by the "plough-sole" produced below 5" depth. 

 Against this, maladjustments of the hitch were not particularly 

 wasteful of power, although they caused bad ploughing. Perhaps 

 the most surprising result was that the drawbar pull was practi- 

 cally the same whatever the speed of ploughing within the 

 ordinary limits of the tractor ; hence the power consumption per 

 acre depends mainly on the speed and is smallest at the highest 

 speeds. Another way of stating this fact is that the paraffin con- 

 sumption per hour for the same tractor is approximately the same 

 whether it is taking 1| hours or 3 hours to plough an acre of 

 ground. 



The factors determining the resistance and the power con- 

 sumption are intimately bound up with the physical properties of 

 the soil which are systematically studied in the Physical Depart- 

 ment. These physical properties determine also the water 

 relationships — evaporation of water, percolation, etc. — which are 

 being carefully investigated. This work has important applica- 

 tions in tropical and sub-tropical countries where irrigation is 

 practised, and the Indian Government regularly sends experts to 

 study for a year or two in the Physics Department. 



Dr. Keen is also co-operating with Professor Sven Oden, of 

 Stockholm, in elaborating the original Oden apparatus for 

 estimating the amount of fine material of different sizes in soils. 



SOIL ACIDITY. 



The electrometric method used in the Physics Department by 

 Mr. E. M. Crowther is giving good results and is sharply distin- 

 guishing soils of varying degrees of acidity. The values are 



