154 SOIL CONDITIONS AND PLANT GROWTH 



if the drift map is interpreted in the light of the Memoirs of the Geo- 

 logical Survey. But as agriculture is influenced by altitude and rainfall, 

 the investigator must also use an ordinary contour map and a rainfall 

 map which, unfortunately, he must construct for himself from data in 

 British Rainfall. Within each vegetation area a number of soil sam- 

 ples must then be taken from points representing the area as closely 

 as possible: situated, for example, on level regions or long gentle 

 slopes, and not on made ground, steep slopes, or places of local 

 disturbance, etc. 



In seeking for typical places the investigator is likely to meet with 

 a good deal of discouragement ; farmers will tell him that half a dozen 

 or more different kinds of soil occur on their particular farms, while the 

 vegetation of a wild area may show considerable changes. But these 

 differences often arise from differences in the compound particles and 

 not in the ultimate particles ; small variations in the amount of calcium 

 carbonate or of organic matter, or differences in the water supply, or 

 management, may considerably affect the ease of cultivation and the 

 vegetation relationships and give the impression of a wholly different 

 type of soil. So little does cropping, cultivation, etc., affect the ulti- 

 mate particles that it is quite immaterial for the purpose of a soil 

 survey whether the sample is taken from pasture land or arable 

 land, but it is well to take a number of samples from both. With a 

 little practice abnormal places are easily avoided. It is necessary to 

 take a large number of samples not only to get at the type, but also to 

 trace out the causes of the phenomena noted by the farmer, and to 

 discover the main factors determining the cultivation and vegetation 

 relationships of the soil. Very full inquiries must be made on the 

 spot as to the agricultural value of the land, the crops and manures 

 most suitable, its behaviour during drought and wet weather, and any 

 special points to be observed during cultivation. Information is also 

 wanted about the most troublesome weeds, the native vegetation, 

 hedgerow and other timber, etc., and note must be taken of the 

 position of the soil in regard to water supply, the nature of the strata 

 down to the permanent water table, etc. The most reliable informa- 

 tion is obtained only by properly conducted manurial trials. 



It is usual to take the sample to a depth of 9 inches and a lower 

 sample to a depth of 1 8 inches, but if any marked change occurs in 

 the soil the sample should only be taken to the point where it sets in. 

 The subsoil sample does not characterise the formation any better than 

 the surface sample, but it affords a useful check and helps in detecting 

 abnormalities. 



