EXPERIMENTS AT ROTHAMSTED 85 



the nourishment of wheat by a previous crop of clover, are 

 facts which have been long known to farmers ; the cause of 

 these facts is now made clear by the evidence supplied by 

 scientific investigations that leguminous plants assimilate the 

 free nitrogen of the air. We have, in fact, to regard plot 7 as 

 manured with nitrogen from the atmosphere. 



\Yhen 400 Ibs. of ammonia salts are applied, as well as the 

 ash constituents used on plot 7, the crop rises to 51 cwts. as 

 the average of the first twenty years, and to 60-7 cwts. as the 

 average of the second twenty years, including the autumn 

 growth. The crop on plot 9 is entirely different in character 

 from that grown on the unmanured plot, and from that grown on 

 plot 7 with a manuring of ash constituents. The unmanured hay 

 contains only 65-70 per cent, of true grasses, about 7 per cent 

 of clovers and vetches, and 20-25 per cent, of weeds. The 

 produce on plot 7 may contain 23 per cent, of clovers and vetches, 

 the proportion varying in different seasons. The produce on 

 plot g contains no clovers or vetches, the weeds are much 

 diminished, and the true grasses form about 90 per cent of the 

 hay. The action of ammonia salts is thus to develop the 

 grasses to the exclusion of all other forms of herbage. 



Plot 10 received the same supply of ash constituents and 

 ammonia salts as plot 9 during the first six years, but since 

 then the sulphate of potash has been omitted. It will be seen 

 that the omission of potash occasions a diminution in the crop 

 amounting to 8 cwts. of hay per acre. 



On plot 4' 2 we have the produce of superphosphate with 

 ammonia salts. The omission of all alkali salts from the manure 

 occasions a falling off" of 20 cwts. in the crop of hay obtained. 

 Alkali salts have thus an importance as a manure for grass far 

 greater than is usually observed in the case of other crops ; the 

 short roots of grass doubtless limit its power of obtaining ash 

 constituents from the soil. 



\Yhen ammonia salts are applied alone, as on plot 5, the 

 increase obtained over the unmanured plot becomes very small, 

 amounting to only 8 cwts. of hay per acre in the first ten years, 

 and to nothing as the average of the fourth ten years. Thus, 

 on the Rothamsted old park land, ammonia salts were without 



