14 SCIENCE BULLETIN, No. 9. 



and in acidity, although ammonia and amines are formed. Heating the 

 soil produces simultaneously both beneficial and harmful organic compounds. 

 Amongst the beneficial are xanthine and hypoxanthine, guanine, cytosine 

 and arginine, and among the harmful hydroxystearic acid. These substances, 

 if already present in the soil, are increased by heat, and if not originally 

 present are produced by the action of heat. The heated soil possesses at 

 first a decreased fertility, owing to the production or increase of dihydroxy- 

 stearic acid, but when this ingredient disappears, either through oxidation, 

 cropping, addition of lime or nitrate, the fertility of the soil is increased. 

 This explanation, it will be seen, opposes the conclusions of Russell and 

 Hutchinson as far as the effect of heating is concerned, and attributes this 

 effect to the alteration of the proteid matter of the humus, rather than to the 

 action of micro-organisms. 



There are thus several theories in the field to account for the action of heat 

 and of antiseptics upon the soil. On the one hand, it is attributed in both 

 cases to a partial sterilisation of the soil, as a result of which certain 

 organisms are destroyed which are hostile to the ammonia-producing bacteria ; 

 on the other hand, the action of antiseptics may, it is suggested, be due to 

 the removal of an impervious wax -like material surrounding the soil grains, 

 the presence of which hinders their being attacked by soil-solvents ; and in 

 the case of heating a third suggestion is that at first both harmful and 

 beneficial organic substances are produced, the harmful ones being readily 

 oxidised. 



Effects of Fertilisers on Physical Properties of Soil. 



Soluble salts in small quantities exert an influence upon the physical pro- 

 perties of soils. Aikman* points out that the quantities of fertilising 

 matter in farmyard manure are insufficient and in an unsuitable form for 

 the growth of crops, and that the chief influence of such manure is on the 

 structure of the soil. R. O. E. Davisf has studied this influence more 

 particularly in the case of the apparent specific volume of the soil, rate 

 of capillary action, and change in vapour pressure. 



He finds that most fertilisers accelerate capillary movement, sulphate of 

 potash and a mixture of sulphate of potash and phosphoric acid retard it. 

 Soluble salts, whether acting as plant-food or not, may produce in the soil 

 changes in structure which in turn influence plant growth. Their effect is 

 most pronounced in soils containing a large amount of fine particles. 



Influence of Fertilisers on Soil-moisture. 



The action of soluble salts in affecting the moisture conditions of the soil 

 is of great importance. Cameron and Gallagher:}: have shown that the 

 physical nature of the soil changes with its moisture-content, and consider 

 that for every soil there is an optimum moisture-content at which its 

 physical condition is most favourable for plant growth. 



* " Manures and Manuring," p. 273. 

 t Bull. 82, Bureau of Soils, U.S.A. 

 J Bull. 50, Bureau of Soils, U.S.A. 



