THE BIOLOGICAL CONDITIONS IN THE SOIL 239 



Rate of Renewal of Plant Nutrients. 



Whatever the effect of variations in concentration there is 

 no question that the rate of renewal of the supply of plant 

 nutrients to the roots is an important factor in soil fertility. 

 Some of the most fertile soils in England are not particularly 

 rich in plant nutrients, but they are able to keep the roots 

 well supplied. 



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, therefore, 

 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 temperature. As yet our knowledge of the 

 detailed composition of this medium is slight but it is steadily 

 improving (see Chap. IV.) ; and we shall get a very false 

 idea of the conditions of life in the soil unless we recognise 

 the main fact of its existence and fundamental significance. 

 Gola (108^) has discussed the influence of the colloidal com- 

 plex in determining plant habitats, and Sohngen (269) its 

 effect on the activities of micro-organisms. 



Sourness, or Reaction of the Soil. 



Many soils, including some of the most fertile, are neutral 

 or feebly alkaline to litmus paper and contain so much basic 

 material that interaction with dissolved salts leads only to an 

 interchange of bases without turning the solution acid. 



But other soils, including many infertile ones, are acid to 



