168 EXPERIMENTS UPON GRASS LAND 



The reason for these differences in the action of lime is 

 to be found in tlie previous manuring of the plots. On Plots 

 6, 7, and 15, potash has been applied every year, so that there 

 was a large accumulation of potash residues in the soil. On 

 Plots 3 and 8, on the contrary, no potash had been used ; 

 and as Plot 8 had been receiving phosphoric acid, the store 

 of available potash originally in the soil must have become 

 considerably exhausted. As we have also seen from the 

 effect of mineral manures with and without potash on the 

 other plots, that the development of leguminous plants is largely 

 dependent on the supply of potash, it is obvious that the effect 

 of lime had been mainly due to bringing into action the 

 residues of potash accumulated from the previous manuring ; 

 the lime only acts where there is such a residue of potash, 

 and has chiefly stimulated the growth of leguminous plants, just 

 as a direct application of potash would do. 



The long-continued use of manures like ammonium-salts, 

 which are in effect acids, has altered the reaction of the soil 

 and made it som' on some of the plots. This is very palpable 

 on Plot 5, which has received a very heavy dressing of 

 ammonium-salts alone, and on which, as has before been 

 mentioned, there is now a large amount of Sorrel, except on 

 a small portion where chalk had been applied. A dressing 

 of lime is, without doubt, necessary for grass land on most 

 soils, in order to neutralise the acidity produced by decaying 

 vegetation, and to enable the manures to exert their full effect. 

 Thus although the liming at the rate of 2000 lb. per acre above 

 mentioned was extended in 1887 to cover the whole of the 

 experimental field, yet a fmther dressing of lime in January 

 1903 to the halves of the plots had an immediate effect upon 

 the following crop. As the results of only one or two years 

 are available as yet, they need not here be considered. 



VII. Changes in Herbage following Changes in Manuring. 

 On two of the plots, which had received ammonium-salts and 

 nitrate of soda respectively until the herbage consisted entirely 



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