70 REPORTS ON THE STATE OF SCIENCE.—1918. 
REPORT ON THE PART PLAYED BY COLLOIDS IN 
AGRICULTURAL PHENOMENA. 
By E. J. Russet, D.Sc., F.R.S., Director of the 
Rothamsted Experimental Station. 
Agricultural chemists only slowly recognised the part played 
by colloids in the soil. For many years the soil was regarded as 
a mass of crystalloid mineral matter, and in discussions of its 
properties it was treated as if it were ‘sand mingled with certain 
soluble salts and organic matter. Sand cultures were commonly 
adopted in pot work; sand particles were used in experiments on 
the He ae properties of the soil; and in the early attempts at 
mathematical analysis the particles were supposed to be spherical 
and impenetrable, though this assumption was recognised as an 
approximation only. As experimental results and deductions 
accumulated, it became obvious that there was a wide discrepancy 
between the properties expected and those actually found in 
natural soils. It therefore became necessary to re-examine the 
fundamental propositions. 
The first demonstrations of the unsoundness of the old views 
came from the Dutch investigator van Bemellen. It had long 
been known that soil possessed the remarkable property of absorb- 
ing certain soluble substances from their solutions: ammonia was 
taken from ammonium sulphate solution, potash from potassium 
sulphate, and so on. It was this property that justified the use 
of soluble salts as artificial fertilisers. The first explanation was 
offered by Way, who supposed that the process was a simple 
chemical reaction of the double decomposition type, and he 
assumed the existence in the soil of a series of reactive silicates in 
order to account for the observed phenomena. Subsequent 
writers, adopting the simple expedient of keeping away from the 
soil, elaborated the properties of these double silicates; and when, 
at a later date, mineralogists directed attention to the zeolites, 
some of the agricultural chemists assumed that these substances 
existed in quantity in the soil and were the reactive constituents 
in question. 
Shortly after Way had offered his chemical hypothesis Liebig 
advanced a physical explanation. He supposed that soil had 
some power of attracting dissolved salts similar to the power 
possessed by charcoal for condensing gases. Only the substances 
physically held in the soil were considered of immediate value to 
the plant, although the chemically combined substances might be 
a reservoir in maintaining supplies. 
Further investigations showed that neither explanation was 
quite sufficient : Knop therefore combined the two hypotheses and 
explained the absorption of acids as a chemical combination with 
iron or aluminium oxides, and the removal of bases partly as a 
physical attraction and partly as a chemical combination with 
silica or double aluminium silicates. But the compromise was 
not very satisfying and aroused little enthusiasm; moreover, it 
did not help to account for the ever increasing number of 
