158 Soil Analysis [264 



mineral acids, without reference to the fact that only a very 

 small proportion of this total may be available for plant use. 7 



We know that plants can only take up food from the soil in 

 a state of solution, the chief solvents being water saturated 

 with C0 2 or the acid excretions of the plants' own roots. 



Dr. Dyer has estimated the acid contents of the root sap of 

 over a hundred varieties of plants, and finds that, on the average, 

 a 1 per cent, solution of citric acid is very similar in its action 

 to this acid secretion. 



A long series of experiments on Rothamsted soils, of which 

 the history was known, confirmed his opinion that a citric acid 

 solution of this strength would give an accurate idea of the 

 available potash and phosphoric acid in the soils. 



As this method occupies some time, it is described first.' 

 The student should start this analysis, and whilst it is standing 

 he should proceed with the fuller analyses described in para- 

 graph 270. 



264. Solubility in Citric Acid Solution. — Take a Win- 

 chester quart bottle which has been used for the storage of strong 

 acids, and which, therefore, will be unlikely to yield up potash, 

 &c, to the citric acid solution, and rinse it thoroughly. Weigh 

 out 200 grams of air-dried soil and place it in the Winchester. 

 Dissolve 20 grams of citric acid in 2 litres of distilled water 

 and pour it over the soil. Stopper the bottle and shake up 

 thoroughly. This shaking must be repeated several times each 

 day for seven days. At the end of seven days the solution is 

 decanted through a large filter, two portions, each of 500 c.c, 

 are collected, and treated as described in the next two para- 

 graphs. 



At the same time weigh out 50 grams of the same air-dried 

 soil in a tared evaporating basin. Place this in a steam oven 

 and leave for about five hours ; after this weigh every twenty 

 minutes until the weight is constant. The loss represents 



