286 SOIL TYPES [chap. 



any deep-rooting crop can be properly grown. Its 

 origin is perhaps not entirely explained as yet; the 

 respective shares of the iron bacteria of Winogradsky, 

 or the purely chemical actions of solution and reduc- 

 tion by the organic matter and carbonic acid, followed 

 by redeposit on evaporation, is a matter requiring 

 further investigation. The formation of the material 

 is only noticed in clays very poor in calcium carbon- 

 ate and liable to waterlogging through insufficient per- 

 colation. 



Owing to their coolness, their retention of moisture, 

 and comparative impermeability to air, humus tends to 

 accumulate in the clay soils ; both arable and pasture 

 soils show a higher proportion of organic matter and 

 of humus than is found, as a rule, on the lighter 

 lands; the effect of manures like farmyard manure is 

 also more lasting. The use of more slowly acting 

 nitrogenous manures is therefore not so desirable 

 on the clay soils; on the other hand, sulphate of 

 ammonia is often unsuitable because of the want of 

 calcium carbonate, and nitrate of soda, which often 

 gives the best returns, is apt to affect the texture 

 injuriously. 



The clays are very generally deficient in calcium car- 

 bonate, often to an extreme degree, much to the detri- 

 ment of the texture of the soil. The use of lime is of 

 the utmost value to all clay soils, improving the texture, 

 making them drier and therefore warmer and earlier, 

 and rendering available the supplies of nitrogen and 

 potash with which they are often liberally endowed. 

 The excess of magnesia and unoxidised iron com- 

 pounds which also characterise many clays is corrected 

 by the use of lime. 



Many clay soils also show a considerable deficiency 

 of phosphoric acid, and respond freely to dressings 



