and Magazine of ihe Ceylon Arp iciUturul Society. 



189 



f HE PRINCIPLES OF MANURING. 



It is a philosophical common-place that we 

 can only apprehend the external world through 

 the medium of thought ; indeod, it may be 

 maintained that we know nothing of the ex- 

 ternal reality, but merely the workings of our 

 own minds. But without pushing our scepti- 

 cism to such a point, we must recognise in all 

 our science, i.e., our attempts to reconstruct 

 nature in terms of our mind, a tendency to 

 rest content .with explanations which fit in 

 with our habits of thought, and to substitute 

 simple, compact theories for the very complex 

 operations of things in themselves. In other 

 words, it is easier to let our minds work on 

 straight-forward " logical " lines than to puzzle 

 out things as they are. 



These somewhat trite reflections have been 

 prompted by reading a recently-published work 

 on manures by Dr. A B Griffiths,* the author 

 of a long series of communications on ques- 

 tions connected with the nutrition and com- 

 position of plants, which have not always re- 

 ceived a very cordial recognition. In this book 

 we find developed at considerable length two 

 ideas which we have every reason to consider 

 fallacious, but which for the last half-century 

 have exercised a powerful influence both on 

 agricultural and horticultural practice. These 

 two fallacies are, firstly, that the composition 

 of a particular plant, as ascertained by an 

 analysis of its ash, affords 



A GUIDE TO THE MANURE IT SHOULD RECEIVE ; 



secondly, that the sulphate or other compound 

 of iron is a desirable ingredient of a fertiliser, 

 because it will give colour to fruit or flowers. 



The first opinion is due to Liebig, who, put- 

 ting aside for a moment the question of the sup- 

 ply of nitrogen, laid down the general prin- 

 ciple that, after ascertaining what a given crop 

 is accustomed to take away from the soil, it is 

 only necessary to add the same materials before- 

 hand in order to satisfy all the requirements 

 of the plant. Now, such, a point of view is at 

 first sight eminently logical ; so conformable, 

 indeed, to our ways of thinking that it has 

 taken 20 or 30 years of experiment and hard 

 controversy to demonstrate its insufficiency ; 

 in agriculturo the idea has practically dis- 

 appeared, but in horticultural matters it is 

 constantly recurring, owing to the lack of sys- 

 tematic experiments on the nutrition of gar- 

 den plants. The cardinal defect of Liebig's 

 theory is that it takes no account of the soil ; 

 it might be true if one started to grow a plant 

 in a pot of pare sand containing no nutriment 

 but that which one added to it, but since the 

 plant is placed in soil containing enough food 

 for at least a hundred crops, though mostly in 

 a more or less dormant condition, manure is 

 only really wanted to make good certain defi- 

 ciencies which are special to the soil and the 

 crop, and these can only be ascertained by 

 actual experiment. Different plants have very 

 different powers of getting hold of particular 



* Manures for Fruit and other Trees. By A. 

 B. Griffiths, Ph.D., pp. 264, xxii. London : 

 Sutton, 1908. 



elements of nutrition, and experience shows 

 that the analysis of the ash of the plant does 

 not enable one to predict in which direction 

 the difficulty will lie. We must 



DRAW OUR EXAMPLES FROM FARM CROPS, 



because it is only in regard to them that there is 

 a sufficient weight of experimental evidence, and 

 we will simply instance Wheat and Barley 

 among cereals and Turnips and Mangolds 

 among root crops. In the growth of Wheat it is 

 very rarely that any other manure than a nitro- 

 genous one will be required ; nitrogen is what 

 ville calls the dominant for Wheat, and all 

 field experiments, confirmed by the working ex- 

 perience of two generations of farmers, have 

 demonstrated that under normal conditions, if 

 Wheat be supplied with from one to two hun- 

 dredweight of nitrate of soda or 6iilphate of 

 ammonia, no profitable increase of crop will be 

 obtained by adding phosphates or potash salts. 

 Yet the aualysis of the Wheat plant does not 

 show that it takes much nitrogen from the 

 soil ; in fact, it takes more potash ; only under 

 the usual conditions of Wheat-growing does 

 the plant find a special difficulty in making use 

 of the reserves of nitrogen in the soil. As far 

 as the ash goes, Barley has much the same 

 composition as Wheat, yet Barley requires much 

 less nitrogen than Wheat, and it is very de- 

 sirable to supply it with some phosphoric acid. 

 The analysis of Swede Turnips would show that 

 the crop takes away from the soil about 100 lb 

 per acre of nitrogen, 30 lb of phosphoric acid, 

 and perhaps 120 lb of potash: yet there can- 

 not be a shadow of doubt but that the proper 

 manuring for a Turnip crop is primarily 60 to 

 80 1b of phosphoric acid, then from 10 to 20 

 lb, but not more, of nitrogen, and only potash 

 in rare cases. Mangolds do not differ greatly 

 in composition from Swedes, yet Mangolds 

 must be manured with nitrogen and potash, 

 phosphoric acid counts for very little, and may 

 often be omitted. Now, these are facts which 

 have been demonstrated to weariness ; they may 

 be paralleled for Clover, for t'otatos, for Beans, 

 and for all our other farm crops ; they form the 

 basis oE the practice of farmers in every civi- 

 lised country. And yet Liebig : s theory — fallacy, 

 we prefer to call it— still is supposed to hold 

 for garden plants, and Dr. Griffiths 1 book is made 

 up of a series of analyses of the ash of shrubs — 

 elude that an exceptional amount of iron in 

 the soil accounted for the brilliancy of 

 colour in its products. But the least exami- 

 nation will show how very defective the chain 

 of reasoning is, and whatever experimental 

 evidence exists is entirely adverse to the view 

 that the heightened colour is to be attributed 

 to a direct effect of the iron upon the plant. 

 In the first place, however great the part it 

 may play, the amount of iron taken up from 

 the soil is very small, 4 or 5 lb. per acre at 

 the utmost, whereas the amount present in 

 the soil is enormous, 20,000 to 30,000 lb. per acre 

 in the top 9 inches only of soil, and that, 

 too, in forms which could easily be appropriated 

 by the plant if it wished to obtain more. 



Secondly, the red soils in question are not, 

 in fact, exceptionally rich in iron ; most soils 

 —and we have the analyses of many scores 

 before us— yield from 2 to 3 per cent of oxide 



