1 66 SOIL CONDITIONS AND PLANT GROWTH 



of sol to gel through the presence of electrolytes in the 

 ground water. 1 Some of the humus gel becomes oxidised, 

 some of both humus and ferric gels change their colloidal 

 properties in other ways. When wet weather comes on again 

 it is no longer possible for the whole of the deposited gel to 

 change back to a sol : some will no doubt change, and there 

 will be a certain washing down of the gel deposited on the 

 bleached sand. But where the bulk had accumulated deposi- 

 tion has begun, and the place where this happened serves as 

 a seat of further action. 



This view seems more in accordance with the facts than the 

 older one and it does not involve any unproved assumptions 

 such as reduction of ferric to ferrous iron and presence of ferrous 

 iron in the pan. 



There is another type of pan formation which must not be 

 confused with this. On clay soils the continued ploughing to 

 a uniform depth with heavy ploughs often leads to the consoli- 

 dation of the underlying soil and the formation of a compact 

 layer of clay or ploughsole which behaves like rock. This effect 

 is purely physical, it can be overcome by periodically sending 

 a subsoiler behind the plough to break through the mass. 



The Retention of Water by Soil and the Rate of 

 Evaporation. 



The older chemists and physicists divided the soil moisture 

 into two kinds : free water, which was supposed to be suspended 

 on the particles just as sea water is suspended on grains of sand 

 or as oil on leaden bullets ; and hygroscopic water, which was 

 retained in a closer sort of way so that it could not be taken up 

 by plants or micro-organisms. Methods for discriminating be- 

 tween these were devised, and pot experiments were made to 

 ascertain directly the amount of free water available for the 

 plant. 



The first experiments made on the rate of evaporation of 

 water from soil gave broken curves which could be explained 



1 Ramann lays stress on this ; Morison and Sothers do not, because their sols 

 were very stable in presence of electrolytes. 



