92 AVAILABLE PLANT FOOD 



seven days at the ordinary temperature of the laboratory. The 

 liquid is then filtered, best through a Buchner's " nutschen " or por- 

 celain funnel with parallel sides and perforated base, by the aid of 

 a filter pump. 500 cc. of the filtrate are taken for the determination 

 of the available potash and a like quantity for the available phosphoric 

 acid. 



These two portions are evaporated to dryness and gently ignited to 

 destroy organic matters. The potash and phosphoric acid are then 

 determined exactly as already described. 



That this method of determining the available potash and phos- 

 phoric acid affords valuable help in judging of probable fertility is 

 shown by its application to the two soils whose analyses have been 

 given. 



The results were as follows : 



It is thus evident that, so far as potash is concerned, the two soils 

 are almost alike, but that soil A, though containing more total phos- 

 phoric acid than B, contains less than one-fourth as much in an avail- 

 able form. The beneficial effect of basic slag on soil A is thus easily 

 understood, soil B containing an abundance of available phosphates. 



Dyer points out that less than *01 per cent of available phosphoric 

 acid in a soil indicates that it stands in immediate need of phosphatic 

 manures. With reference to the limit for the available potash, Dyer 

 thinks, though he expresses more doubt in this case than with the 

 phosphoric acid, that soils containing much less than -005 per cent 

 available potash require application of potash manures. 1 



" Available " plant food in soils as determined by Dyer's method 

 is undoubtedly a valuable method of gauging the relative fertility of 

 soils which exist under similar climatic conditions, so far as their power 

 of supplying potash and phosphoric acid is concerned, but the writer's 

 experience has led him to conclude that there is another factor which 

 greatly influences fertility, and that is the rate at which the " avail- 

 able " matter is renewed by the processes of weathering which occur 

 in soils. 



This occurs with considerable rapidity even in England, for it was 

 found that a soil, which had been deprived of the potash and phosphoric 

 acid soluble in 1 per cent solution of citric acid, exactly as in Dyer's 

 method, after a few months, again contained quite a large proportion 



*In a subsequent paper (Proc. Roy. Soc., 1901, 11) Dyer states that for cereals 

 the limit denoting deficiency in phosphoric acid is between O'Ol and 0-03 per cent 

 soluble in 1 per cent citric acid solution. In root crops, especially turnips, the 

 limit would probably be higher. He also states that soils containing 0-01 per cent 

 of potassium soluble in 1 per cent citric acid probably require no further application 

 of potash manures. 



