PROCEEDINGS OF SECTION K. 655 



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. 



Effect of Fertilizers on Physical Properties of Soil. 



Soluble salts in small quantities exert an influence upon the 

 physical properties of soils. Aikman {Manures and Manuring, 

 p. 273) points out that the quantities of fertilizing matter in farm- 

 yard 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. Davis (Bull. 82, Bureau of Soils, 

 U.S.A.), has studied this influence more particularly in the case 

 of tlfe ajDparent specific volume of the soil, rate of capillary action, 

 and change in vapour pressure. He finds that most fertilizers ac- 

 celerate 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. 



Effect of Fertilizers on Soil Moisture. 

 The action of soluble salts in affecting the moisture conditions 

 of the soil is of great importance. Cameron and Gallagher 

 (Bulletin 50, Bureau of Soils, U.S.A.) have shown that the 

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

 consider that for every soil there is an optimum moisture content 

 at which its physical condition is most favorable for plant growth. 



Of the various problems presented by a study of the physical 

 nature of the soil, the one which is of the greatest importance is 

 the questio'n of the behaviour of water in the soil. This applies 

 with special force to us in Australia where the problem of conserv- 

 ing the soil-moisture is of even greater importance than that of 

 manuring. The action of fertilizers, especially of potash salts, in 

 keeping the surface soil moist, is well known. The application of 

 fertilizers has been found to have a very considerable effect upon 

 the transpiration ratio of plants, and to enable them to make a 

 better use of the available moisture. 



In fact, J. W. Leather {Memoirs Dept. Agric, India, 

 Cheml. Series, Vol. 1, No. 8, p. 170), in the course of an investi- 

 gation into the water requirements of crops in India finds that the 

 transpiration ratio (that is, the relation between the weight of 

 water transpired by the crop and the weight of the dry crop) is 

 always lower when suitable manures are employed, and concludes 

 that ' ' speaking generally the effect of suitable manure in enabling 

 the plant to economize water is the most important factor which 

 has been noticed in relation to transpiration." 



