THE BIOLOGICAL CONDITIONS IN THE SOIL 233 



records of soil temperatures at East Lansing, Mich., have 

 been taken by Bouyoucos (49a), who has also discussed their 

 effect on the physical properties of the soil. 



Food Supply. 



In spite of numerous investigations our knowledge of the 

 plant food in the soil is very limited. On physiological 

 grounds it is supposed 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. 173), or whether the 

 carbon dioxide respired from the roots ^ effects the solution of 

 more material than is already dissolved, has not been ascer- 

 tained. The soil solution may safely be regarded as the mini- 

 mum food supply, which is reinforced to an unknown extent 

 by the soluble substances in the soil. 



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 compounds 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 im- 

 portance that it has been necessary to do something empirically, 

 and by common agreement the small fraction of the phosphorus 

 and potassium compounds soluble in dilute acids is called 

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



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

 to dissolve insoluble food materials in the soil. This idea, which was a sur- 

 vival 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 {Biochemie 

 der Pfianzen, Bd. 2, pp. 872 et seq.). So far as is known COj is the only acid ex- 

 creted. The evidence is of the negative kind and is therefore not entirely satis- 

 fying, so that the problem is periodically brought up again ; recently, for 

 instance, Pfeiffer and Blanck stated that other acids also are given off (224^) 

 Cf. (275), and footnote, p. 33. For a recent discussion, with references, see H. 

 Kappen, Untersuchungen an WurzelsUften (Landw. Versuchs-Stat., 1918,91, 1-40). 



