272 MATERIALS OF INDIRECT VALUE [chap. 



it was inevitably regarded as a necessary constituent of 

 the food of such plants, and was naturally enough 

 supposed to contribute to the stiffness of the straw. 

 In his manures Liebig supplied the alkalies combined 

 with silica, and when Way discovered that certain 

 strata of the Upper Greensand, near Farnham, con- 

 tained considerable quantities of silicates readily dis- 

 solved by acids, the rock was for a time extracted 

 and ground up as a manure for cereals. But Sachs 

 showed that these plants, however rich in silica 

 their ash was when they had grown on ordinary soil, 

 could yet be grown with complete success in a water 

 culture devoid of any silica, and Jodin succeeded even 

 in raising four generations of maize in water cultures 

 with no more silica than was contained in the original 

 seed. It was also shown that the stiffness of the straw 

 depended upon such physiological factors as light and 

 exposure, rapidity of growth, etc., and was independent 

 of the amount of silica present, so that the use of 

 silicates for manurial purposes ceased, except at the 

 instance of one or two unscrupulous firms puffing 

 worthless materials. However, it must not be supposed 

 that so large a constituent of a plant's ash is entirely 

 without physiological function, and from the Rothamsted 

 barley experiments (which include plots receiving 

 sodium silicate) it may be seen that soluble silica does 

 play some part, at present not properly understood, 

 which enables the plant to make better use of the 

 dormant phosphoric acid in the soil. The silicates, 

 however, possess no practical use as fertilisers, the 

 increase thus produced would not repay the expense 

 of applying the silicate of soda. 



Green manuring. — Green manuring consists in the 

 ploughing under of some rapidly growing crop — mustard 

 or tares in this country, lupins on sandy soils on the 



