30 



data are erroneous. Nor is dispersion complete in the standard 

 dilution method for counting soil organisms, and the results of 

 any given plating are liable to the error of counting as a single 

 organism a group or colony on an undispersed soil aggregate. 



A further consequence of great interest to the expert has 

 also emerged : certain of the so-called " constants " of the text 

 books, such as the Hygroscopic Coefficient, the Wilting Co- 

 efficient, Moisture Equivalent, etc., are not " constants " at all 

 in the physical sense. Dr. Puri finds that the " Hygroscopic 

 Coefficient " (the percentage of soil moisture in equilibrium with 

 a saturated atmosphere) is so inherently difficult to determine that 

 marked discrepancies are almost inevitable. These so-called 

 11 single value " measurements which aim at characterising a 

 soil by a single determination are very liable to error. One, 

 however, is being studied : the moisture content of a soil when 

 the well-mixed mass is just becoming sticky. 



But all this fundamental work takes time, and meanwhile 

 there are important practical problems for which a working 

 solution can be found by empirical means. On the field side Mr. 

 Haines has obtained further readings of drawbar pull in plough- 

 ing and cultivating, as done under ordinary farm conditions, a 

 criterion which necessitates the dovetailing of the work into the 

 ordinary farm routine. This year detailed studies have been 

 made of the causes of the irregularities of drawbar pull in an 

 apparently uniform piece of ground. Careful measurements 

 showed that a level field uniform to the eye which would have 

 been selected by any Committee as suitable for ploughing or 

 tractor trials nevertheless had soil irregularities that caused 

 considerable differences in drawbar pull. The results for Sawyer's 

 Field have been set up in the form of a contour diagram (Fig. 1), 

 in which the peaks and ridges represent high, and the valleys 

 represent low, drawbar pulls. Had the field been used for a 

 test, the areas allotted to different implements might have been 

 very favourable to some and very unfavourable to others in spite 

 of the apparent uniformity. Methods are being devised whereby 

 a survey can be made beforehand that will show the distribution 

 of irregularities in the soil. 



The work has also shown how much reduction in drawbar 

 pull can be effected by applying lime, limestone or organic matter 

 to the soil, and how to obtain the best effects by these methods. 

 Further, it has been found that the friction of ploughing can be 

 reduced by an electric device simple in principle and only awaiting 

 exploitation to become important in practice. 



Much work is done in the Physics Department in studying 

 the moisture relationships of the soil. Measurements conducted 

 in large cylinders filled with Rothamsted soil show that 

 little water rises to the surface from the subsoil when 

 the ground water falls to 4 or more feet below the surface. A 

 definite relation was found between the potential evaporating 

 power at the surface and tin; change in ground water level. This 

 work will be greatly facilitated when the continuous recording 

 devices now being devised are installed. 



