114 



THE ROTHAMSTED EXPERIMENTS. 



Ammon- 

 ium-salts 

 unsuitable 

 for legu- 

 minous 

 crops. 



Nitrates 

 uncertain. 



Continuous 

 cropping 

 with beans 

 a failure. 



The reason 

 why. 



Amount of 

 nitrogen in 

 bean crops. 



crop 3292 lb. It will be seen further on that the average 

 annual yield of nitrogen was also nearly as great over the 

 last 4 years of crop as over the first 8 years. 



It may be observed that nitrogen supplied as ammonium- 

 salts to the highly nitrogenous leguminous crop seldom gives 

 any increase, and is sometimes injurious in the year of 

 application ; though some benefit may afterwards result from 

 the residue after the ammonia has been converted into nitric 

 acid. Even nitrates, however, directly applied as manure, 

 are very uncertain in their action, and at any rate yield very 

 much less increase of produce with the highly nitrogenous 

 Leguminosse than with the Graminese, and crops of other 

 Orders yielding produce of low percentage of nitrogen in 

 their dry substance, and accumulating comparatively little 

 nitrogen over a given area of land. 



It is specially to be noted, that whilst the cereal crops may 

 be successfully grown for many years in succession on the 

 same land, provided only that mineral and nitrogenous 

 manures are liberally supplied, this leguminous crop — beans 

 — gradually fails when so grown ; and although characteristi- 

 cally benefited by mineral manures containing potash, neither 

 these alone, nor a mixture of mineral and nitrogenous manure, 

 has sufficed to maintain even fair growth for a number of 

 years in succession. The result is, however, not entirely due 

 to deficiency in the supply of constituents within the soil, 

 but is also in a considerable degree dependent on the fact 

 that, by the continuous growth of the crop, with its special 

 habit and range of roots, the surface-soil acquires a close and 

 unfavourable condition, and a somewhat impervious pan is 

 formed below. The improved result in the later years with 

 the intervention of fallow, further illustrates the fact that the 

 previous failures were not wholly due to exhaustion. 



The next Table (36) shows the amounts of nitrogen in the 

 bean crops, the produce of which we have been considering. 

 The table is on the same plan as that relating to the produce ; 

 the upper division giving the averages for the four 8-yearly 

 periods, and for the total period of 32 years, and the lower 

 division those for the years of crop only, within each period ; 

 and, as in Table 35, the results for the total produce only 

 (corn and straw together) are given. 



Eeferring to the figures in the upper division of the table, 

 it may be observed that, notwithstanding there were 6 blank 

 years, and one year of wheat, out of the 32, and notwithstand- 

 ing that the produce declined much, and gave on the whole 

 much less than the average obtained under ordinary agri- 

 cultural conditions, yet the average yield of nitrogen in the 

 crops grown without any supply of it was much more than 



