342 SOIL CONDITIONS AND PLANT GROWTH 



they tend to promote root development, an obvious advantage 

 in a dry soil where the plant will fail unless the roots strike 

 into the deeper, moister layers ; they also stimulate the vital 

 processes going on at the end of the season and are thus 

 valuable in wet, cold districts. But rainfall and water-supply 

 are important factors in determining the choice of phosphates 

 on arable land ; basic slag proves less useful than super- 

 phosphates on dry soils, but it is sometimes nearly as useful 

 on moister soils or under higher rainfall. The amount of chalk 

 in the soil is not the determining factor, but the moistness ; 

 if, as often happens, a chalky soil is dry, superphosphates will 

 prove the more useful ; where the soil is moister, basic slag is 

 as good, and of course cheaper. Evidence is accumulating 

 that mineral phosphates are often of value. On grass land 

 basic slag is often more effective than superphosphate. 



The Relative Advantages of Mechanical and Chemical 



Analysis. 



The fundamental distinction between mechanical and 

 chemical analysis is that the former deals with the whole of 

 the soil, which it sorts out into fractions of varying sizes, 

 while the latter deals only with the part that is readily dis- 

 solved by acids. Mechanical analysis, therefore, gives a 

 picture of the whole (albeit very incomplete), while chemical 

 analysis does not ; it is in consequence eminently suited for 

 the purpose of a soil survey, the chief object of which is to 

 classify and describe the soils. Further, it enables the in- 

 vestigator to explain to some degree the observed water re- 

 lationships of the soil when sufficient is known about the 

 water-supply, and also to account for many of the peculiarities 

 observed in cultivation. It enables him to say, as far as can 

 be said on our present knowledge, whether any observed de- 

 fects are due to defects in the soil or its situation, or to the 

 system of management that has been adopted. As it cannot 

 be interpreted fully without a knowledge of the amounts of 

 organic matter and calcium carbonate present, these two quan- 

 tities must be determined in every sample. 



