TRANSACTIONS OP SUB-SECTION B. 585 



Joint Meeting with Section C. 



The following Papers were read : — 



1. Soil Surveys for Agricultural Purposes. 

 Bxj A. D. Hall, F.R.S., and E. J. Russell, D.Sc. 



The object of a soil survey is to give an account of the soils of an area in their 

 relation to the local agriculture. The methods adopted must be such that it is 

 possible (1) to classify together soils of the same formation which have similar 

 agricultural properties, and differentiate between others with dissimilar pro- 

 perties ; (2) to bring out clearly and unmistakably any connection that may 

 exist between type of soil and special crops or special agricultural methods ; (3) to 

 afford guidance as to crops that may succeed, or are not likely to do so ; (4) to 

 throw light on the manorial requirements of the soils. 



The geological formation affords the best basis on which to carry out a soil 

 survey. Where fairly constant physical and other conditions have obtained a 

 fairly constant type of soil may be expected. Thus, in their survey of Kent, 

 Sussex, and Surrey, the authors found that soils arising from the Bagshot and 

 Folkestone beds, although both light sands, possessed definite characteristics 

 whereby it was generally possible to distinguish them. In other cases gradual 

 changes can be traced in passing along the formation : thus, the Hythe beds in 

 the east contain a distinctly large proportion of the finest particles of soil, which 

 becomes less and less in moving westwards. 



Considerable difficulty comes in, however, where the formation is obscured by 

 drift. 



Of the various determinations made in the course of the analysis, the most 

 important is the mechanical analysis whereby the particles are graded according 

 to their sizes. For agricultural purposes the size of the soil particles is more 

 significant than their actual composition. The finer particles of soil regulate the 

 water-supply available for the plant, and profoundly influence the tillage opera- 

 tions ; unless a sufficient proportion is present the soil cannot be cultivated, but 

 is left as waste, agriculturally speaking, though it may be valuable for building 

 purposes. On the other hand, too great a proportion is detrimental, because it 

 increases the difficulty of tillage. The coarser particles determine the openness 

 and ' lightness ' of the soil. A number of crops have very special soil require- 

 ments, and are only found in any quantity on a particular type of soil. It is 

 possible to define these types over a given area by means of mechanical analysis. 



Account must always be taken of the other factors determining the water- 

 supply, such as rainfall, topographical position, nature of the subsoil, &c. Tem- 

 perature is equally important, and the altitude and exposure has also to be 

 considered. 



Of the chemical determinations the total carbonate is, perhaps, the most im- 

 portant. Calcium carbonate is the commonest of these compounds, and it acts 

 in two ways : it modifies the properties of the finer particles, making them more 

 like coarser particles, and it prevents the soil from becoming acid. It is impos- 

 sible to lay down any limits w-ithin which the carbonates must fall, an amount 

 sufficient on a light sandy soil being wholly insufficient on a clay. The amount 

 should increase pari passu with the amount of fine particles. 



The amount and nature of the organic matter is important, and it is necessary 

 to know whether free acid is present or not. Like calcium carbonate, humus 

 modifies the properties of the finest particles. 



The inter-relationships between size of particles, water-supply, and amounts of 

 calcium carbonate and organic matter cannot at present be reduced to any mathe- 

 matical expression, and, owing to their complexity, certain field observations are 

 necessary. But it is usually possible to interpret the results with sufficient pre- 

 cision for ordinary purposes. 



Of the bases, the amounts of alumina and of potash vary with the clay, but iron 

 oxides do not necessarily. The proportion of ferrous to ferric iron is important 

 as an indication of the amount of aeration in the soil. 



