90 THE TEXTURE OF THE SOIL [chap. 



soil being left to stand for a time with a cover over the 

 basin in order to let the water diffuse thoroughly. If the 

 moisture is increased beyond the optimum point the 

 soil becomes obviously wet and gets sticky and puddled 

 as it is worked. The experimenter must use his 

 judgment as to when the right point has been reached, 

 whereupon a sample of the soil is taken, weighed and 

 dried in order to ascertain its water content. With 

 a little practice successive determinations will agree 

 within one or two per cent. The mean of several such 

 determinations may be taken as the optimum water 

 content. 



Variations in Surface Tension. 



The surface tension of water is very high, but it is 

 easily raised or lowered by the presence of small 

 amounts of material in solution. The effect of altering 

 the surface tension of a film at any point is to cause 

 motion, as is seen in the well-known experiment of 

 covering a plate with a thin film of coloured water and 

 dropping a little alcohol into the middle of the film. 

 The alcohol immediately weakens the surface tension of 

 the film in the middle to such an extent that all the 

 liquid runs to the outer edge and leaves the plate bare 

 in the middle. Most of the salts which are used as 

 artificial manures and are soluble in water increase the 

 surface tension of the soil water, hence an application of 

 salt or nitrate of soda may, by increasing the tension of 

 the surface film, lift more water from the subsoil and 

 maintain the top layer of soil in a moister condition. 

 Per contra, solutions of organic matter, particularly of the 

 many organic substances used as manure which have a 

 little oil in them, extracts of dung, etc., have a surface 

 tension below that of water. To this fact may be 

 attributed the "burning" of soils which is sometimes 



