Ill POWER OF THE SOIL TO ABSORB SALTS [chap. 



the crop can be recovered from the top 9 inches of these 

 Rothamsted soils. It is clear, then, that soils well 

 provided with calcium carbonate, as the Rothamsted soil 

 is, will precipitate very near the surface any soluble 

 phosphoric acid applied, and retain it for a long time in 

 a form easily redissolved and obtainable by the plant. 

 It follows, therefore, that superphosphate, the most 

 soluble of the phosphatic manures, can be applied to 

 normal soils in the winter or early spring without any 

 fear of the phosphoric acid being washed out. 



The Composition of Drainage Waters. 



Further evidence of the fate of the various substances 

 applied as manures, their retention or otherwise by the 

 soil, can be obtained by studying the composition of the 

 water flowing from land drains. 



The drainage from the continuously manured wheat 

 plots at Rothamsted, each of which possesses a tile drain 

 running down the centre at a depth of 2 feet to 2 feet 

 6 inches, has been collected from time to time and com- 

 pletely analysed by Voelcker and Frankland ; in addition, 

 systematic determinations of the nitrogen contents have 

 been made for many years. In a general way, the chief 

 constituent of the various drainage waters is lime, either 

 as bicarbonate, sulphate, chloride, or nitrate; soda is 

 the only other base present in any quantity, very small 

 amounts of magnesia, potash, and ammonia pass into 

 the drains. Of the acid radicles, chlorine and 

 sulphuric acid predominate according to the manuring, 

 and the proportion of phosphoric acid is minute ; but 

 the amount of nitric acid varies according to the 

 manure applied and the season at which the water is 

 collected. 



The following table shows the complete analysis of 

 the drainage water from twelve of the plots : — 



