146 SOIL CONDITIONS AND PLANT GROWTH 



the clay, fine silt, and silt increase the poor clay pastures form a 

 readily explained exception but it does not appear to be closely 

 associated with any one fraction like the potash. The amounts of 

 " available " phosphoric acid vary enormously ; Kentish hop gardens 

 commonly contain from '05 to *i8 per cent. ; well farmed arable soils 

 contain some -015 per cent, while in poor worn-out pastures the quan- 

 tity may sink as low as 'OO2 per cent. In most cases these quantities 

 are insufficient for some of the crops grown, especially where high 

 quality or feeding value is aimed at ; hop growers regularly, and, they 

 maintain, profitably, apply phosphates to gardens already containing 

 05 per cent, of available phosphoric acid, whilst arable farmers use 

 them for swedes when -015 per cent, or sometimes even more, is 

 present. The exceptions to this rule are the light soils sufficiently 

 provided with moisture and a forward climate ; on these the need for 

 phosphates appears to be less. But in all cases where much purchased 

 food is fed on the land phosphates appear to be of advantage to the 

 succeeding crop. 



Rainfall does not appear to have so marked an effect in controlling 

 the need for phosphates as it has for potassic manures. The explana- 

 tion is to be found in the fact that phosphates are useful both in dry 

 and in moist situations : 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 ; basic slag proves less useful 

 than superphosphates on dry soils, and at least 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, superphosphate will prove the more useful ; where the soil 

 is moister, basic slag is as good, and of course cheaper. 



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 only deals with 

 the part that is readily dissolved by acids. Mechanical analysis there- 

 fore gives a complete picture 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 



