254 CAUSES OF FERTILITY AND STERILITY [chap. 



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 

 when it has become dissolved as bi-carbonate in the 

 soil water. 



Amendments of the Soil. 

 Many soils, without being absolutely sterile, carry 

 very poor crops until their physical character has been 

 altered by the admixture of some considerable quantity 

 of one or other of the constituents of a normal soil 

 that may happen to be originally wanting. These 

 amendments of the soil by the mixture of other soils 

 date from the time that enclosures first began to be 

 made ; they were perhaps at their height during the 

 early years of the nineteenth century, after the middle 

 of which they rapidly diminished as it began to be 

 less and less remunerative to "make" land, until at 

 the present time the fall in the prices of produce and 

 the rise in the cost of labour have put an end to all 

 such operations. Among other causes of this neglect 

 may perhaps be set down the increased use of artificial 

 manures ; men began to take too exclusively a chemical 

 view of the functions of the soil, and shirked expendi- 



