INVESTIGATIONS ON THE PRODUCTION OF PLANT FOOD. 175 



the mineral plant foods, including phosphates, salts of potassium, 

 calcium, &c , have been studied in the main from geological and 

 physico-chemical standpoints, while those resulting from vegetation 

 have been worked at almost entirely from the biochemical standpoint. 

 But the division line is only in the laboratory and does not exist in 

 Nature ; we shall see that all of the plant nutrients fall into each 

 group. 



The beginning of the soil goes back to remote ages when the 

 particles of sand, grit, or clay got split off from the original rocks and 

 began their wanderings by stream, wind, or glacier that have finally 

 brought them to their present place. Many of their properties were 

 determined during these wanderings and cannot now be altered ; thus 

 some of the most striking differences between the red soils of South 

 Devon and the grey soils of Dorset arose out of the differences in 

 conditions between Triassic and Lias times ; these differences have 

 persisted all through the ages, and we cannot now go back and undo 

 the work of the past. 



But the mineral particles do not constitute soil. The final stage 

 in soil formation is not complete until vegetation has sprung up and 

 died, and its remains have mingled with the mineral fragments 

 and begun to decay. During its lifetime vegetation takes certain 

 substances from the mineral matter and the atmosphere and builds 

 them up into complex organic matter. Like other constructive work, 

 this process requires energy, which in this case is derived from 

 the sunlight and is stored up in the complex substances of the cell 

 tissues. 



When the plant dies and its remains mingle with the mineral 

 fragments it begins to decay. The whole process then reverses : 

 instead of a building up tfyere is a breaking down ; the fabric of complex 

 material slowly elaborated during life is disintegrated and resolved 

 into the simple substances out of which it was formed, and all the 

 stored-up energy is dissipated. The old life is cleared away and the 

 ground is left clear for new life ; the old plant tissues are converted 

 into food for another generation of plants. So prodigal is Nature 

 with life that even this process of dissolution and decay affords the 

 means whereby more life may manifest itself. A whole population 

 of the most varied description springs up in the soil, feeding on these 

 plant tissues, deriving its energy from the energy stored up during 

 the lifetime of the plant, and reversing completely the changes effected 

 by the plant. 



Scheme showing how Soil is formed. 



Mineral fragments 

 [No energy.] 



+ Plant Residues = Soil 



[Organic matter comprising [Comprising 

 Carbohydrates (soft cellulose, gums, &c.) Mineral fragments 



Cellulose (resistant fibres, &c.) Organic matter 



Proteins Sources of energy 



Mineral substances Micro-organisms.] 

 Waxes.] 



