208 CHEMICAL FERTILIZERS 



sub-tropical regions, irrigation may enable the cultivator to 

 produce two or three crops in a year; on such lands irri- 

 gation is extremely profitable. In the British Isles irrigation 

 has presented too many difficulties to be undertaken on any 

 large scale ; some slight efforts are made in the Fenland 

 districts, where excessive water is pumped up into high-level 

 canals and a deficiency of water is remedied by merely 

 reversing the mills. The difficulty of lack of water is gene- 

 rally surmounted by making the most of the water in the soil. 

 Organic manures, such as farmyard manure, increase the 

 retaining power of the soil, and therefore spread the water 

 supply over longer periods. Systems of cultivation on the 

 surface prevent excessive evaporation of water. The use of 

 stimulating manures like sulphate of ammonia and nitrate 

 of soda at critical times does much to mitigate the evils due 

 to a lack of water. To some extent manure may be said to 

 take the place of water, since plants that are well manured 

 do not require as high a proportion of water transpired to 

 plant tissue formed. The soluble fertilizers, such as super- 

 phosphate, sulphate of ammonia and potash salts, are those 

 that have proved most useful on light lands afflicted with 

 drought. Some light soils, however, are deficient in lime, 

 when it may be better to use basic slag instead of super- 

 phosphate, but lime and basic slag must be used sparingly 

 on light dry soils. On light, dry soils, lime destroys the 

 organic matter which is commonly deficient, and modifies the 

 physical conditions till the soil becomes still drier. In such 

 cases it is better to use ground limestone. The method 

 commonly adopted to consolidate these light soils is to grow 

 a green crop to be eaten by sheep, so that the soil becomes 

 firmer and very much more retentive. Whilst much has 

 been done during the last hundred years to overcome the 

 wetness of soils, it cannot be said that the difficulties of 

 overcoming dryness have been tackled quite so fully. 



Sulphate of ammonia, when used on light lands, may 

 produce results similar to those discovered at the Royal 

 Agricultural Experimental Station at Woburn, wheie, by 

 its excessive use, lime has been so completely removed from 



