ii2 SOIL CONDITIONS AND PLANT GROWTH 



and it further accounts for the remarkable benefits produced by even 

 small additions of nitrate of soda to the soil at this period. 



In dry regions the accumulation of plant food and other soluble 

 decomposition products in the soil may be too great to admit of plant 

 growth, and bare patches or regions arise known as " alkali soils " from 

 the circumstance that sodium and potassium carbonates are often present. 

 In wetter climates the soluble substances tend to be washed out more 

 completely, but notable quantities often persist in heavy clay soils, 

 especially where the drainage is bad, and may produce injurious effects 

 on vegetation. 



In all discussions of the plant food in soils it is assumed that the 

 only significant plant nutrients are nitrates, phosphates, sulphates and 

 salts of potassium, calcium, magnesium and iron, commonly called the 

 " essential " plant foods. Perfect plants can be raised in water cultures 

 containing only these substances. Reference to Fig. 5 (p. 39), however, 

 shows that productiveness is not always maintained indefinitely simply 

 by supplying these salts, and not nearly so well as when the complex 

 farmyard manure excreta, litter, etc. is used. It is known also that 

 certain other compounds besides the " essential " foods may increase 

 plant growth (p. 46). Further, Russell and Petherbridge have shown 

 that on heating soil to 1 00 something is formed that stimulates root 

 production to a remarkable degree. Are these effects the results of 

 nutrition or of stimulation ? Are the manganese, lithium, etc., com- 

 pounds and the beneficial soil constituents indispensable nutrients of 

 which only traces are required, or are they, as Armstrong expresses it, 

 " condimental " foods ? The evidence is not very decisive, and may be 

 made to serve either hypothesis, but in many cases Armstrong's view 

 is the simpler. At any rate there is evidence that our view of the plant 

 nutrition in the soil has hitherto been too narrow and ought to be 

 widened. 



The Nature of the Medium on which the Soil Life Goes On. 



It is a mistake to suppose and this point cannot be too strongly 

 emphasised that the medium on which the soil organisms live and 

 which is in contact with the plant roots, is the inert mineral matter 

 that forms the bulk of the soil. Instead the medium is the colloidal 

 complex of organic and inorganic compounds, usually more or less 

 saturated with water, that envelops the mineral particles ; it is, there- 

 fore, analogous to the plate of nutrient jelly used by bacteriologists, 

 while the mineral particles serve mainly to support the medium and 

 control the supply of air and water and, to some extent, the tempera- 



