Scientific A fjricuUure. 



42 



[July, 1909. 



nitrogen it requires, but it is very 

 shallow-rooted and must be supplied 

 with an abundance of phosphoric acid." 



A Complex Subject, 

 Yet this explanation does not begin to 

 exhaust the possibilities of what is an ex- 

 tremely complex subject. "Many plants 

 do not exhibit such idiosyncrasies as 

 are shown by wheat and swedes, but 

 require a general fertilizer, the com- 

 position of which is determined more by 

 the soil than the plant. Indeed, no 

 theory of manuring can be based upon 

 the plant aloue, but must also take the 

 soil into account, so that a fertilizer 

 may be regarded as rectifying the 

 deficiencies of the soil as far as regards 

 the requirements of the crop in question. 

 What those special requirements are can 

 only be decided by experiment. Mr. 

 Hail comes to the conclusion that the 

 best general point of view of the action 

 of fertilizer is, perhaps, obtained by 

 extending the " law of the minimum" 

 originally enunciated by Liebig (accord- 

 ing to which the yield of a given crop 

 will be limited by the amount of the one 

 particular soil constituent which may 

 happen to be deficient) and extending 

 it to "all the factors affecting the yield 

 as well as to the supply of plant-food, 

 e,g., to such matters as the supply of 

 water, the temperature, the texture of 

 the soil. On poor soils the water supply 

 is very often the limiting factor, on 

 very open soils because the water 

 actually drains away, on extra close soils 

 because the root range is so restricted 

 that the plant has but little water at 

 hand, and the movements of soil water 

 to renew the supply are very slow ; in 

 either case the plant will be sure to 

 have as much nutriment as. is required 

 for the small growth permitted by the 

 water present. It is only when the 

 water supply is sufficient that the 

 resources of the soil as regards all or 

 any of the constituents of a fertilizer 

 are tested, and may become in their 

 turn the limiting factors in the growth 

 of the crop. Hence it follows that 

 fertilizers may often be wasted on poor 

 land where growth is limited by the 

 texture of the soil, by the water supply 

 or some other factor." Points are made 

 by the lecturer of the huge amount of 

 plant-food actually present " though in a 

 highly soluble condition," in soils— "The 

 soil of the manured plot on the Rothams- 

 ted wheat field contained in 1893, after 

 54 years' cropping without fertilizer, 

 2,570 lbs. per acre of nitrogen, 2,950 lbs. of 

 phosphoric acid, and 5,700 lbs. of potash" 

 — and of " the law of diminishing re- 

 turns " by which the first expenditure of 

 fertilizer or other factor of improvement 

 is the most effective, each succeeding 



application producing smaller and 

 smaller returns, until a further addition 

 causes no increase in the yield ; and a re- 

 markable proof is given of the direction of 

 the movement of soluble salts in the soil. 

 It appears that in the sod all reactions 

 are extremely localized, since they take 

 place in the thin film of water normally 

 surrounding the soil particles, in which 

 movement of the dissolved matter 

 takes place very slowly and mainly 

 by diffusion. At Rothamsted two grass 

 plots have received for 52 years in 

 succession very large amounts of soluble 

 fertilizer (in the one case 550 lbs. per 

 acre of nitrate of soda and in the other 

 600 lbs. per acre of ammonia salts), and 

 though these plots are separated only by 

 an imaginary line from others receiving 

 either no fertilizer or characteristically 

 different one, the distinction remains 

 perfectly sharp " and the rank herbage 

 produced by the excess of nitrogenous 

 fertilizer on one side does not stray six 

 inches over the boundary." A test of the 

 soil to a depth of seven feet (in 1893) in 

 the Rothamsted wheat field — in which 

 the fertilized plots are separated from 

 each other by unfertilized strips only 

 12 inches;in breath— showed clearly that 

 the amount of nitrates found " was in 

 each case characteristic of the supply 

 of nitrogen to the surface of the plot, 

 and right down to the lowest depth 

 there were no signs of the proportions 

 approximating to a common level as 

 they would have done had any consi- 

 derable amount of lateral diffusion taken 

 place." As the treatment has been 

 continued in all cases for at least 40, 

 and in some instances for 50 years, Mr. 

 Hall seem-s justified in his conclusions 

 that the evidence indicates that " the 

 movement of the soluble salts in the 

 soil are confined to up-and-down motions 

 due to percolation and capillary up- 

 lift, and take place laterally only to an 

 insignificant extent." 



The "Poison" Theory. 

 Having come to these general conclu- 

 sions, Mr. Hall proceeds to consider 

 Messrs. Whitney and Cameron's theory 

 of the action of fertilizers. This would 

 appear to be inspired by a real difficul- 

 ty — indeed, the Rothamsted expert ad- 

 mits it to be " fundamental "—which 

 may be illustrated by the behaviour 

 of phosphoric acid when applied as a 

 manure. It is a fact that a soil may 

 contain enormous quantities of plant- 

 food and yet be by no means notably 

 fertile— for instance, 2,500 lbs. per acre of 

 phosphoric acid may be present, and yet 

 swedes will not do well unless supplied 

 with an additional dressing of 50 lbs, 

 per acre of soluble phosphoric acid— and 

 " it is usually assumed that the effect of 



