160 Plant Physiology 



86. Availability of the nutrients. Plant nutrients 

 exist in the soil in conditions most diverse with respect to 

 availability, and chemical analysis does not satisfactorily 

 distinguish between availability and nonavailability. 

 Potassium, for example, may be present in conjunction 

 with aluminium silicate, or it may be present in far more 

 soluble form ; but there are at present very few data con- 

 cerning the nature of these compounds. If any element 

 is present in markedly unavailable form, that element 

 will be needed as a fertilizer, especially to hasten the early 

 stages of growth. 



Fertilizers are generally applied, not only to keep up 

 fertility, but to increase availability. In the latter case, 

 therefore, from the immediate standpoint of the plant, 

 fertilizers are supplied either (1) as directly available 

 nutrients; (2) as substances, effecting readjustments in 

 the soil, so that needed elements become more available 

 to the plant; or (3) in order to counteract the effects due 

 to some unbalanced condition of the nutrients (later dis- 

 cussed at length), injurious acidity, alkalinity, and the 

 like. 



87. The solvent action of roots. It is well known 

 that roots and root-hairs are able to render available a 

 certain amount of nutrient materials. There is a solvent 

 action of the roots. The case almost universally cited is 

 the corrosion of marble (limestone) by roots. The nature 

 of this solvent action has been much studied and discussed. 

 It is certain that the excretion of aqueous CO 2 is sufficient 

 to account for much, and probably for nearly all of this 

 action. 



Kunze and others seem to have convincingly demon- 



