20 CHAPTER III 



the water is further distributed and on freezing again the soil 

 is broken up to a further extent. Drying and freezing have a 

 tendency to coagulate the soil colloids and by so doing promote 

 a granular condition. 



(c) Plants and Animals. — The root residues of plants 

 upon decay assist granulation. Earthworms and other burrow- 

 ing animals have also a beneficial effect on the tilth of the 

 soil. 



(d) Addition of Lime. — Lime tends to granulate clay, 

 thus making it more friable. 



(e) Tillage. — At the correct time, i.e., under proper mois- 

 ture conditions, tillage tends to promote granulation. 



(f) Drainage assists granulation by the removal of surplus 

 water. 



(g) Addition of Organic Matter. — Decaying organic 

 matter assists granulation directly, but also to a very large 

 extent indirectly, due to its great changes in volume upon 

 wetting and drying, thus making processes (a) and (b) more 

 effective. 



The chief agencies which destroy good tilth are : — 



(a) Excessive Moisture, which causes deflocculation or 

 breaking down of the granules. The poor physical condition 

 of over irrigated (water-logged) soils bears testimony to this. 



(b) Tillage at the wrong time, i.e., when certain soils 

 are too wet or too dry. 



(c) Depletion of Humus. — The tendency of soil to settle 

 and bake into large clods after years of cultivation is often due 

 to humus depletion. 



(d) The Presence of Sodium Carbonate. — This substance 

 has a strong deflocculating effect. It is a characteristic con- 

 stituent of black " brak " in soils. 



When to Till. — The correct stage at which the various 

 classes of soil should be tilled is indicated when a mass of 

 moist soil pressed in the hand retains its form but does not 

 show free water. A clay soil will become puddled if worked 

 when too wet and if ploughed when too dry will produce a 

 cloddy condition. Sandy soil, however, can be manipulated 

 without detrimental results even when the moisture content 

 is very high or very low. 



A brief review of some of the fundamental facts relating 

 to plasticity and cohesion of soils will make the matter clear. 

 The following is a short summary of the remarks of Lyon, 

 Fippin and Buckman in this connection : — ' 



