IX.] BAD TILTH DUE TO FERTILISERS 293 



phosphate, or a mixture of nitrate of soda and sulphate 

 of ammonia should be used as a nitrogenous manure, 

 because the two manures will act upon the soil in 

 opposite ways, the nitrate of soda as an alkali and the 

 sulphate of ammonia as an acid. Dressings of soot are 

 also effective ; not only does it assist the soilmechanically, 

 but also the small percentage of sulphate of ammonia 

 it contains possesses some power of flocculating the 

 clay. 



Other fertilisers which give rise to an alkaline 

 reaction in the soil are sulphate of potash, common 

 salt, and other soluble salts of sodium and potassium, 

 which, as has already been noticed (p. 254), interact 

 with calcium carbonate in the soil, and give rise to a 

 little soluble alkaline carbonate. The injurious effects 

 of sulphate of potash upon the tilth of the heavy soil 

 at Rothamsted is very evident on the mangold field, 

 where the plots receiving this fertiliser every year 

 become excessively sticky and clinging in wet weather, 

 and dry with a hard caked surface. It has often been 

 noticed that applications of potash salts and common 

 salt have depressed instead of increasing the yield ; this 

 may probably be set down to the deterioration of tilth 

 that ensues when the soil is heavy and also contains 

 calcium carbonate. Some fertilisers, on the contrary, 

 aid in the flocculation of clay soils, the most effective 

 being superphosphate, which is acid and contains gypsum, 

 an effective flocculating agent. The ammonium salts, 

 which give rise to free acids, in consequence of the 

 withdrawal of ammonia by the moulds, etc., living in 

 the soil, act as very potent flocculators, and at 

 Rothamsted, for example, give rise to an open and 

 friable soil, as compared with the neighbouring plots 

 receiving nitrate of soda. Lime, which is the chief 

 flocculating agent employed in practice, is only effective 



