THE CONSTITUTION OF THE SOIL 63 



soils contain less coarse sand than one-half the quantity of clay present. 

 When the coarse sand exceeds the clay in amount the soil becomes 

 light, unless of course the clay is above 20 per cent, when the soil 

 must always remain heavy. Not being a colloid, it possesses no power 

 of absorbing water or soluble salts. Soils containing 40 per cent, or 

 more, of coarse sand and less than 5 per cent of clay are only culti- 

 vated where large quantities of dung are available, or where the water 

 supply is exceptionally good. As the amount of coarse sand increases, 

 the soils become less and less suited to cultivation, till finally the sand 

 dune condition is reached. 



Fine gravel is not usually present to any great extent, and is of 

 importance only when the coarse sand is already dangerously high. 

 Stones cannot be determined quantitatively by any method of sampling 

 in use, and their effect must be judged by a visit to the field. If they 

 are uniformly scattered through a stiff soil, as in the Clay-with-Flints, 

 they are on the whole beneficial, because they facilitate tillage. Where 

 they form a bed underlying the soil they may do harm by causing 

 over-drainage. Some typical examples are discussed in chapter VII. 



Calcium Carbonate. 



Calcium carbonate is often present in small amounts only, but it 

 plays a controlling part in soil fertility. It produces both chemical 

 and physical effects. It prevents the formation of certain conditions 

 that otherwise tend to arise, conditions that are not yet investigated, but 

 are unfavourable to many agricultural plants and soil micro-organisms. 

 It gives rise to the soluble bicarbonate that flocculates clay, and thus 

 physically improves the soil texture. There is a certain critical stage 

 where comparatively small changes in the amount of calcium carbonate 

 may very materially alter the native flora, the predominant weeds, the 

 soil micro-organisms, the liability of the plants to disease and the 

 tractability of the land. Soils sufficiently supplied with calcium car- 

 bonate stand out in sharp contrast with those containing too little, 

 although they may be of similar composition in all other respects. So 

 great is the effect that the practical man has long since adopted the 

 special term " sour " to describe soil deficient in calcium carbonate, a 

 term we shall find it convenient to retain. Table XXVIII. shows pairs 

 of soils similar in constitution and general external conditions, tempera- 

 ture, water supply, etc., but very different in agricultural value because 

 of their different content of calcium carbonate, one being readily culti- 

 vated while the other is wet and sticky, and only suitable for pasture 

 land : 



