VALUE OF FARMYARD MANURE 109 



manure. Even when the plant does not entirely fail it has 

 been often noticed that, if a spell of hot weather comes in the 

 early part of the season, the plant on the dunged plots will be 

 growing vigorously when that on the other plots is still 

 struggling for existence. Later on, when all the plant is 

 established, the differences are not so marked, and in favourable 

 seasons the crop on the plots manured with artificials rivals 

 the crop grown with dung, if due allowance be made for the 

 larger amount of nitrogen actually supplied to the dunged plots. 



The right-hand portion of the diagram Fig. 14, page 101, 

 shows the effect of the successive additions of other nitrogenous 

 manures to dung. 



Considering first the crops on Plot 1, in each series (see 

 Table XL., page 98), we find that notwithstanding the large 

 amount of nitrogen which the dung supplies, and its accumula- 

 tion in the soil, yet dressings of quickly-acting nitrogenous 

 manures will still bring about an increase of crop. The 

 amount of nitrogen annually supplied to Plot 1 O is much 

 greater than is removed by the crop, hence there must be a 

 considerable accumulation of nitrogen from year to year in the 

 soil of this plot. Nevertheless, these reserves cannot become 

 active quickly enough for the needs of so rapidly growing a 

 plant as the mangold, hence the increase which is seen when a 

 further addition of active nitrogen in the shape of ammonium- 

 salts or of nitrate of soda is made. 



When more than 86 Ib. per acre of nitrogen is added, as on 

 Plot 1 C, which receives 98 Ib. of nitrogen as rape cake, or 

 as on Plot 1 AC, which receives 184 Ib. of nitrogen as rape 

 cake and ammonium-salts, no further increase of crop is seen, 

 the average remains stationary at 24 tons per acre. The crop 

 has, in fact, attained its maximum, and is limited, not by the 

 amount of nitrogen and other plant food available, but by its 

 restricted period of growth, or by a scarcity of water, sunlight, 

 and other factors of development. 



Turning to a comparison of Plots 1 and 2, some interesting 

 results are to be seen ; both receive a similar dressing of 



