January 20, i9ibj 



NATURE 



;8: 



hen, that the biochemical decompositions in the soil 

 must proceed smoothly and rapidly. 



New difficulties arise as soon as one begins to de- 

 lop this principle. Reverting to the scheme just 



Mven, it is seen that the decomposition of proteins 

 inav proceed in two ways, either ending with nitrate or 

 with nitrogen. Now the nitrate ending^ is desirable 

 enough, but the nitrogen ending is highly undesirable. 

 Vet this happens directly the process is speeded up too 



nuch. The more intense the cultivation becomes the 

 more serious are these losses ; they are bad on the 

 prairies, but still worse under conditions of intense 



Pot No. 47 55 63 72 79 



Ftg. 3. — Tom.itoes supplied with increasing doses of nitrate of soda. 



Pot 47. — No nitrate. Pots 55 to 79. — Increasing dressings of niirate. This increases 

 the amount of growth up to pot 72, but it depres.ses growth in pot 79, where too much is 

 euen. The middle pot, 63, is best for fruit. Phosphates and salts, potassium, calcium, 



, were given equally to all pots. 



ditions 

 pots 72 



vultivation. To some extent this is inevitable; it is 

 equally true of engines, but just as the engineer has 

 increased the efficiency of engines, so the agricultural 

 chemist has to Increase the efficiency of the nitrogen 

 utilisation processes. 



L'nfortunately, the purely chemical work on the 

 decomposition of protem has not gone far enough to 

 enable a full working hypothesis to be mapped out. 

 The decomposition does not stop at amino-acids ; under 



bacterial action there is a further change to bases and j Di 1 r +u 



acids. These are under investigation in several chem- G-.tjctaI RcldtioT) bttwtci) At)y pai'ticixlAT UctoT a.nd Hint UTPOwtl). 

 ical laboratories, but the results have not yet helped 



plant is not a mere passive bucket into whict ihe pro- 

 ducts of the reaction are drawn ; each plays an active 

 part, aisturbing both the reaction and the distribution 

 of the products. 



The recognition that the plant is a living thing has 

 broadened our conception of the factors necessary for 

 plant growth. In much of the literature of the 'seven- 

 ties and 'eighties it is tacitly assumed that ihe whole 

 art of crop production is a question of manuring. 

 Whitney's investigations on American tobacco, how- 

 ever, led to the recognition that the type of the soil 

 is an important factor in crop production, which has 

 had some extremely interesting' developments. 

 Mechanical analyses to determine the type 

 became an indispensable part of the routine 

 of soil analysis. The persf>ective was re- 

 stored, and the fact emphasised that the plant 

 not only wants food, but also proper water 

 supply, air supply, and temf>erature. 



Now there is a very simple rule that ap- 

 plies to all these factors. Plant growth in- 

 creases with increasing supply of any one of 

 them, but this only happens so long as the 

 supply of every other factor is adequate. 

 When anything' is lacking the increase in 

 growth is not kept up, and additional sup- 

 plies give no extra crop. Finally a stage is 

 reached when extra supplies may do harm, 

 either by direct injury or by cutting out 

 another " indispensable substance. This is 

 shown in the tomato experiments of Fig. 3, 

 where successively increasing doses of 

 sodium nitrate are" applied in the four pots 

 55, 63, 72, and 79, although no further 

 growth is obtained in 72 and 79 because 

 of the insufficient w-ater supply. The con- 

 fer favourable growth are all present in 

 and 79 excepting only this one, but it effectu- 

 ally prevents the plant from taking full advantage 

 of the good conditions. All this is expressed in a 

 generalised form in the curve of Fig. 4. which thus 

 represents our third principle of crop production. It 

 has only recently been revived in agriculture, although 

 the fundamental idea is old; it can be found in the 



us much. Here, therefore, we have an economic 

 problem of the first importance waiting for the solu- 

 tion of a chemical problem which, at first sight, seems 

 rather academic and remote from practice. 



These biochemical changes, important as they are in 

 crop production, do not end the matter. The soil 

 comes directly into the reaction. The calcium 

 carbonate neutralises acids produced during the de- 

 composition. The clay and some of the other con- 

 stituents possess colloidal properties, so that all these 

 reactions proceed on a jelly-like surface and not in a 

 fluid medium, and they are liable to be affected by all 

 the complications produced by surface actions. 



The plant plays an even more active part. Its roots 

 absorb some of the products — the nitrates, the phos- 

 phates, etc. — and might therefore be expected to hasten 

 the whole process ; but this does not happen. On the 

 contrary, the growing plant appears to retard it, and 

 nitrate is always formed in higher quantities on fallow 

 than on cropped land, even after allowing for what 

 is taken by the crop. 



Whether the growing plant affects the nature of the 

 change or only the rate is not yet known. The essen- 

 tial point is that, so far as plant nutrients are con- 

 cerned, neither the soil nor the plant plays an entirely 

 passive part. The soil is not an inert medium, and the 



\0. 2412 VOL. 96] 



Limiting Facton. 



lT)ci'etT)ct)l of Fact Of. 



Fig. 4.— Increases in amounts of the various factors necessary for plant 

 growth do not cause indefinite increases in growth. After a time some 

 other factor becomes insufficient and operates as a limiting factor. 



writings of the political economists of the Malthusian 

 school; it was used in a si^ecial form by Liebig in his 

 "Law of the Minimum"; it was developed by Horace 

 Brown and F. F. Blackman. In its full generalised 

 form it is proving extremely useful. 



Problems of soil fertility generally have to be ap- 

 proached from this point of view. Whenever a case 



