THE SOIIi 61 



The superiority of soil B in " available " phosphates is thus 

 evident ; it contains more than four times as much as A. 

 0*01 per cent. *' available " phosphates and 0-005 per cent, 

 " available " potash are suggested as the lower limits foi 

 fertility for most crops. If less be found the soil requires 

 manure. It is obvious that the limits will be diflferent for 

 different crops, since both their requirements and power of 

 assimilating food materials differ greatly. The amounts of 

 potash and phosphoric acid extracted by 1 per cent, citric 

 acid solution, though not claimed to be an accurate measure of 

 that which plants are able to obtain from the soil, probably 

 furnish one of the best chemical means of estimating their 

 fertility so far as their requirements are concerned. Another 

 point to consider in this connection is the rate at which the 

 " unavailable " plant food becomes " available." In some cases 

 it has been found that a soil deprived of all "available " plant 

 food by prolonged treatment with a one per cent, solution 

 of citric acid soon acquires, when kept moist, additional 

 quantities of the former. There seems little doubt that in 

 warm climates the renewal of the available plant food in soil 

 occurs more rapidly than in colder ones. In tropical soils, 

 therefore, the occurrence of smaller quantities of " available " 

 plant food may suffice for the requirements of crops, owing to 

 the greater rapidity with which it is renewed. This is one of 

 the reasons why such soils, which appear poor on analysis when 

 compared with English soils, often prove very fertile. 



For details of analytical processes which cannot be given 

 here, a manual of agricultural analysis should be consulted. 



