September 15, 1904] 



NA TURE 



489 



ijarclens no amount of manure will enable one kind of fruit- 

 tree to flourish on a spot from which another tree of the 

 siime species has been recently removed, and all farmers 

 practically evince, by the rotation of their crops, their ex- 

 perience of the existence of the law." He attributes to 

 Macaire the demonstration of the fact that all plants part 

 with a f.'Ecal matter by their roots. These excretions he 

 held to be poisonous, maintaining that, although plants 

 generate poisonous secretions, they cannot absorb them by 

 their roots without death, concluding that " the necessity 

 of the rotation of crops is more dependent upon the soil 

 being poisoned than upon its being exhausted." He 

 indicated the lines along which investigation might with 

 jidvantage proceed, one of the questions put forward being 

 " the degree in which such excretions are poisonous to the 

 plants that yield them, or to others." 



In 1833 botanists and agriculturists had not the advantage 

 ■of the knowledge that is at our disposal through the con- 

 tinuous growth for a long series of years of certain crops 

 at Rothamsted, but consideration of the fact that some 

 crops (as, for example, pure forests of beech, silver fir, 

 ■Scots pine and other trees, as also permanent pasture) may 

 be grown for hundreds of years on the same ground without 

 any evidence of poisoning might have led to the conclusion 

 that the law, as it was called, was not of general appli- 

 cation. It is, of course, true that rotations are an 

 advantage, and it is a matter of experience that certain 

 crops — e.g. clover and turnips — cannot be grown con- 

 tinuously on the same land, but the cause is not now 



. associated with excretions. The reason for the failure of 

 clover, or the cause of land becoming " clover-sick," as it 

 is called, is still a debated point ; but I may hazard the 

 conjecture that it is due to the fact that organisms or 

 enzymes inimical to the vital activity of the minute living 

 bodies, that exist in symbiotic relationship with the clover 

 plants, increase with great rapidity when the living bodies 

 that they affect are present in abundance. Red clover is 

 the species that is usually associated with the term clover- 

 sickness, but it would appear that a precisely similar pheno- 

 menon is exhibited in the growth even of wild white clover. 

 It is a matter of common observation that on certain classes 

 of land white clover is stimulated to such vigorous growth 

 liy the use of phosphatic manures that for one year at least 

 it monopolises the area to the almost total e.xclusion of 

 other plants. But such rank luxuriance is not of long 

 <luration. In a year or two the clover disappears to a very 

 large extent, and cannot at once be restored by any process 

 with which we are acquainted. The land has, in fact, be- 

 come sick to white clover. But given a period of rest, 

 ■during which the inimical agents will disappear, and it 

 again becomes possible to stimulate white clover to vigorous 

 growth. We have, it seems to me, an analogous state of 

 things in the case of certain insects. On the Continent 

 the caterpillar of the Nun Moth (Liparis monacha, L.) 

 periodically proves extremely destructive to certain conifers, 

 and it is found that in the first year the insects are 

 moderately abundant, in the second they are excessively 

 abundant, while in the third the visitation begins to decline, 

 and usually terminates quite suddenly. The causes of this 

 cessation have been thoroughly worked out, and are found 

 In the great increase of parasitic insects, and insecticidal 

 fimgi, including bacteria. I believe it will be found that 

 the almost sudden cessation of our periodic visitations of 



' the diamond-back moth is due to a similar cause. 



The failure of turnips is apparently largely, if not entirely, 

 ■due to the increase of insects and parasitic fungi. 



The subject of harmful excretions has recently obtained 

 renewed attention through the work being done at the 

 \\'oburn Fruit Station. No point has received more striking 

 •demonstration there than the harmful influence that growing 

 grass exerts on fruit-trees. It has been shown that this 

 prejudicial influence is not due to the w-ithdrawal of moisture, 

 to the curtailment of supplies of plant food, to interference 

 Avith aeration, or to modifications of temperature. In Mr. 

 Pickering's opinion.' " the exclusion of all these possible 

 ■explanations drives us to believe that the cause of the action 

 of grass is due to some directly poisonous action which it 

 exerts on the trees, possibly through the intervention of 

 liacteria, or possibly taking place more directly." It is 

 1 The Effects of Grass on Apple Trees." ournal R.A.S.E. Vol. Ixiv. 

 p. 36s. 



NO. 1820, VOL. 70I 



satisfactory to know that the subject, which is of consider- 

 able scientific and practical importance, is likely to be 

 vigorously followed up. 



In the early 'forties attention was being directed to a sub- 

 ject that even now has a great attraction for agriculturists, 

 namely, the stimulating and exhausting effect of artificial 

 manures, especially nitrate of soda. The principle that 

 " stimuli lose their full effect upon living matter when 

 frequently repeated " was generally held to account for the 

 want of response that crops exhibited to repeated dressings 

 of nitrate of soda ; but Prof. Daubeny in 1841 ' pointed out 

 what is now generally accepted as the true cause, namely, 

 the exhaustion of the soil of other substances. This, he 

 said, can be counteracted by giving other manures, of which 

 he instanced bone meal. His suggestions for future investi- 

 gations have been largely followed, though, as we now 

 know, they are of theoretical rather than practical import- 

 ance. He proposed the alternatives : 



(i) Analysis of the soil, discovery of the amount of avail- 

 able plant food, and the application of the substances found 

 to be deficient up to the probable measure of the crop's 

 requirements. 



(2) Discovery, by analysis of the yield, or estimation by 

 calculation, of the amount of plant food removed in the 

 produce, and the application to the soil in the form of 

 manure of what was withdrawn by the crop. 



Daubenv suggested that manuring should be undertaken 

 on a svstem of book-keeping — on the one side being entered 

 all the items of plant food taken out by crops, and on the 

 other all that is applied in the form of manures, the two 

 sides of the account being made to balance. This theory 

 of manuring is distinctly suggestive, and often fits in rather 

 remarkably with actual practice, though the comparative 

 agreement between theory and practice is due to causes 

 that the author of the theory probably hardly contemplated. 

 Take, for instance, the case of wheat. An average crop 

 removes from an acre about 50 lbs. nitrogen, 30 lbs. potash, 

 and 20 lbs. phosphoric acid. This loss would be restored 

 by the use of some 3 cwt. nitrate of soda, 2 cwt. kainit, and 

 i^ cwt. superphosphate; and on many soils wheat could, no 

 doubt, be grown continuously for many years on such a 

 mixture, aided by good tillage, without the yield suffering 

 materiallv. But we now know that much of the plant food 

 offered in manure never enters the crop at all, so that the 

 balancing of the account is due almost as much to chance 

 as to calculation. This becomes more apparent when we 

 regard such a crop as meadow hay, which in actual practice 

 is often grown for a long series of years on the same land. 

 To balance the withdrawal of phosphoric acid by an average 

 vield of this crop only about J cwt. of superphosphate per 

 acre is theoretically necessary, but on most soils an average 

 vield would not be maintained by the use of so small a 

 quantitv. 



During the 'fifties the volumes of the Association contain 

 several important contributions from the two distinguished 

 Englishmen to whom the world's agriculture owes so much, 

 Lawes and Gilbert. Their first contribution was made in 

 185 1, and dealt with Liebig's mineral theory, a subject with 

 which their names will always be associated. They drew 

 upon their rich store of experimental data to prove that the 

 yield of wheat is much more influenced by ammonia than 

 by minerals, and they gave it as their deliberate opinion 

 that the analysis of the crop is no direct guide whatever as 

 to the nature of the manure required to be provided in the 

 ordinarv course of agriculture. With the reservation " in 

 the ordinary course of agriculture," the dictum cannot be 

 questioned, though in the circumstances of the continuous 

 growth of wheat, as has been pointed out, conclusions 

 indicated by the analysis of a crop happen to accord, at 

 least approximately, with manorial practice. 



Field experiments or demonstrations, which have been 

 such a prominent feature of the educational work of the 

 past decade, appear to have been first introduced at the 

 meeting of the Association in iStii by Dr. \'oelcker. 



While agricultural subjects have claimed a considerable 

 share of the time of the .Association, forestry has not been 

 altogether overlooked. As early as 1S38 we find attention 

 being directed to what has of recent years come to be a 

 burning question — namely, the maintenance of our timber 

 supplies, .At that early date, when the industrial develop- 

 1 " On Manures considered as StimuU to Vegetation." 



