3, i 



THE BIOLOGY OF THE SOIL. 



A plant derives its food partly from the air by means of its leaves 

 ana partly from tne soil by means of its roots ; and the whole process 

 depends on sunlight, which aftords the energy necessary ior the manu- 

 iacture of sugar Dy the green leaves, to which mineral salts from the 

 soil are carried up the stem from the roots. Here in the leaves new 

 chemical changes occur, raising the sugar a step higher in chemical 

 complexity by the addition to it of nitrogen, and the substance thus 

 formed travels away from the leaves to all parts of the plant in order 

 to undergo further chemical elaboration step by step, till new living 

 matter, protoplasm, has been built up. The plant is, indeed, a 

 creative agent it can build up living substance from wholly 

 inorganic or mineral matter ; in other words, from the dust and from 

 the air ,it can raise inorganic matter to organic matter. 



It is the business of the horticulturist and the agriculturist to 

 provide his plants with abundant and suitable food material. He is 

 not content with that quantity of mineral food that is already in 

 the soil, for that sooner or later would be exhausted under intensive 

 cultivation. 



It is at present impossible to alter the amount of food material 

 obtained from the air, but it has been found possible to improve and 

 increase the food material contained in the soil, effectively and eco- 

 nomically, and the knowledge that we have of this principle of 

 manuring has largely been brought about by experiments carried out 

 during many years by Lawes and Gilbert and their successors at the 

 experimental farm at Rothamsted, in Hertfordshire. Last year 

 Professor Easterfield dealt with the chemical aspect of this question. 

 To-night I propose to discuss the biological aspect, for biology has 

 played a considerable part in recent years in advancing our knowledge 

 of the chemical processes going on in the soil, whereby the nitro- 

 genous material of manure is rendered available to the plant. 



I say "available" for it is not every mineral salt that is useful. 

 The plant can select from the mixture we call the soil just what it 

 needs ; but it can only take it if it be in a special state of chemical 

 combination. 



The soil consists of irregular particles, varying in size and shape 

 and material in different kinds of soil fine in clays, coarser in sandy 

 soils and these particles consist of minerals of different sorts, includ- 

 ing the nutritive or food material of plants. This nutritive material 

 consists of the phosphates, sulphates, nitrates of such bases as 

 potash, lime, and magnesia. With these mineral materials there is 

 always more or less organic matter derived from the decay of plants 

 and animals that live in the soil. And in order that a soil may be 

 "rich" it must contain a proportion of this organic matter or 

 "humus," the material that is dark in colour, and supplies the 

 "open" character of soils. The soil particles are separated from one 

 another by smaller and larger spaces containing air, which is as 

 necessary for the health of the roots as for that part of the plant above 

 ground. These spaces also contain water, which forms from 15-30 

 per cent, of the weight of the soil. This water is mainly distributed 

 in very thin films over the solids, though of course after rain the 



