SOIL ANALYSIS AND ITS INTERPRETATION 145 



The " available " potash l shows no kind of regularity, but varies 

 between 5 and 50 per cent, of the quantity extracted by strong acids. 

 In deciding whether or not sufficient is present, attention must be paid 

 to the soil, the crop and the rainfall. Thin chalky soils, sandy soils 

 and soils rich in organic matter are peculiarly responsive to potassic 

 manures, whilst clay soils generally are not. Carbohydrate-making 

 crops, like sugar beets, mangolds and potatoes also invariably want 

 more potash than they find in the soil or in dung. Potassic manures 

 also tend to prolong the life of the plant, and, therefore, to increase the 

 yield in dry districts where the conditions all tend to early stoppage of 

 growth. Illustrations are afforded in Table LIX. where soils in dry- 

 districts, known to respond profitably to potassic manures, are com- 

 pared with soils in places of much higher rainfall where potassic manures 

 do not prove profitable. 



TABLE LIX. "AVAILABLE" POTASH IN SOILS OF KNOWN BEHAVIOUR TOWARDS 



POTASSIC MANURES. 



All are arable soils. The chalk pastures on the South Downs 

 usually contain less than -01 per cent, of available potash (e.g. the 

 Patching soil), and they respond to potassic manures. It will be ob- 

 served that -015 per cent, is insufficient in East Kent where the rainfall 

 is 23 inches, whilst 'Oio per cent, suffices in West Sussex under 10 

 inches higher rainfall and generally better water-supply in the soil. 



Phosphoric Acid. Generally speaking, the largest amount of phos- 

 phoric acid is found in chalk soils, '2 to -25 per cent, being present; 

 about -15 to 2 per cent, is found in good loams, sandy loams contain 

 about -I per cent., while poor clay pastures and poor sands contain still 

 less. Little if any direct connection can be traced between the phos- 

 phoric acid and the productiveness ; in general i.t tends to increase as 



1 i.e. t extracted by i per cent, citric acid. 



