108 SOIL CONDITIONS AND PLANT GROWTH 



Food Supply. 



In spite of numerous investigations our knowledge of the plant 

 food in the soil is very limited. On physiological grounds it is sup- 

 posed that the whole of the nutrient material coming from the soil 

 enters the plant in the dissolved state, but whether the actual soil 

 solution is all the plant gets, as Whitney and Cameron suppose (see 

 p. 76), or whether the carbon dioxide respired from the roots ^ effects 

 the solution of more material than is already dissolved, has not been 

 ascertained. The soil solution may safely be regarded as the minimum 

 food supply, which is reinforced to an unknown extent by the soluble 

 substances in the soil. All attempts to get any further have broken 

 down, partly because the soil solution cannot be separated and subjected 

 to examination (p. 63), and partly because the soil compounds can- 

 not be sharply divided into two groups, one soluble and the other 

 insoluble. 



Nitrogen nutrition presents a tolerably simple case because plants 

 growing on cultivated soils probably absorb all their nitrogen as nitrates, 

 which are readily and completely dissolved by water, whilst plants in 

 undisturbed soil — grass land, etc. — probably utilise ammonium com- 

 pounds as well. Potassium and phosphorus nutrition present greater 

 difficulties because very little is known about the compounds of 

 these elements in the soil. This particular problem is of such 

 technical importance that it has been necessary to do something 

 empirically, and by common agreement the small fraction of the phos- 

 phorus and potassium compounds soluble in dilute acids is called 

 " available " food material, while the rest is said to be " unavailable ".^ 

 Here, however, the agreement ends, for no two dilute acids give the 

 same results and no two Associations of Agricultural Chemists recom- 

 mend the same dilute acid. Dyer's i per cent, citric acid (91, 92) is 

 adopted in Great Britain, and its use has been justified by Wood's inves- 

 tigations (320 and 321). N/200 hydrochloric acid has been recom- 

 mended in the United States, 2 per cent, hydrochloric acid by 

 Nilson in Sweden, aspartic acid in Hawaii, and so on. The results 

 obtained by different acids are shown in Table XLVI. 



^ At one time it was supposed that special acids were excreted by plant roots to dis- 

 solve insoluble food materials in the soil. This idea, which was a survival of the mediaeval 

 view that plants are wholly analogous to animals, persisted into our own times, but has 

 been shown to be untenable by Czapek. 



2 Daubeny (78) originated this distinction, using the terms " active " and "dormant ". 



