THE USES OF LIME IN AGRICULTURE AND HORTICULTURE. 2^9 
the latter contains a preponderance of larger particles, the finer above- 
mentioned gelatinous substances being almost absent. 
When lime is added to a clay soil it collects together these gela- 
tinous or " colloid " substances, as they are technically called, forming 
them into loosely-bound particles ; that is to say, it coagulates the 
finer particles into coarser grains and automatically frees the larger 
particles from the adhesive action of the colloids. This action is 
technically known as flocculation. A simple illustration of it is 
easily afforded by the addition of a little lime to a glass vessel con- 
taining muddy water from a clay soil. It will be seen that the lime 
immediately flocculates the fine particles held in solution and pre- 
cipitates them to the bottom of the vessel, leaving a perfectly clear 
fluid. The practical result of this flocculation of the fine particles 
of clay is to give the effect of a coarser-grained soil. Under these 
circumstances not only is the soil more friable when dry but it is 
also less retentive of moisture when wet ; it therefore becomes per- 
manently drier and warmer and permits of earlier cultivation, a matter 
of the utmost importance on heavy soils. 
The disintegrating effect of lime upon a heavy soil is also due to 
the chemical change of the oxide CaO to the carbonate CaC0 3 . On 
light soils it is generally claimed that lime acts as a binding agent 
in the same way as it does in mortar, and enables them to obtain 
moisture from the subsoil by capillary attraction, at the same time 
increasing their retentive powers — in short, making a sandy soil more 
like a sponge and less like a sieve. A good example of the cementing 
power of lime on light soils is afforded by the lime pan, frequently 
found in such soils rich in lime ; and also in the practice, frequently 
adopted, of liming sandy roads to make them bind. It should be 
added, however, that it is doubtful whether this action of lime on light 
soils is as beneficial as is sometimes claimed ; in any case it is not so 
definite and valuable as is the case with heavy soils. 
Chemical Action of Lime on Soils.- — The value of lime lies in the fact 
that it is an " active base " ; it is what is known to chemists as a 
powerful alkali, and as such combines with and neutralizes any acid 
with which it may come in contact. This action is the first and most 
important that we have to consider under this heading. Acids are 
continually being formed in the soil by the decay of organic matter 
in the form of stems, roots, and other plant residues of previous 
crops which are periodically ploughed or dug in. These acids of decay 
(known as " humic " acids) are also produced when dung or other 
organic manures are applied to the soil. Other and stronger acids 
are introduced into the soil by such manures as superphosphate and 
sulphate of ammonia. It is one of the most important functions of 
lime to neutralize these acids and render them harmless to plant life. 
The experiments at Rothamsted and Woburn afford good examples 
of the results of the lack of lime, especially when ammonium salts 
have been used year after year. The Rothamsted soils only become 
