Miscellaneous. 



150 



[February, 1910. 



exhaustion of its mineral constituents, 

 as will be explained later. Thus must 

 the general history of a soil be con- 

 sidered first, when the question of 

 the application of green dressings is 

 under debate and, in the event of a 

 favourable conclusion being arrived at, 

 whac follows here is then, and only 

 then, a matter that can have relation to 

 that particular soil. 



Turning, now, to the effects of the 

 buried plants on the soil, the subject 

 may be viewed conveniently from the 

 aspect of the uses of such plants when 

 they are applied in this way. These 

 uses may be grouped under two heads : 

 that of the prevention of the loss of 

 plant-food already present, and that of 

 the provision of additional plant food. 



In the first connexion, it may be 

 remarked that the very important effect 

 of green dressings in preventing the loss 

 of usefnl soil constituents is very often 

 given much less recognition than it 

 deserves. It is well-known that nitrates, 

 owing to their solubility, are very likely 

 to be lost iu drainage water, and that 

 the prevention of such loss is a matter 

 of supreme importance to agriculturists. 

 Green manures are especially efficacious 

 in this direction, for they take up those 

 bodies and form stable combinations 

 with them, which are subsequently 

 rendered available by bacteria in the 

 ordinary way. A similar action takes 

 place in regard to other food-consti- 

 tuents, such as phosphoric acid, potash 

 and lime; not, however, because, like 

 nitrates, they are liable to be lost in the 

 wash-water, but because the green dress- 

 ing unites with them in such a way as to 

 render them more directly available. 

 These maintenance effects follow the 

 use of any kind of green dressing, but 

 they are augmented, in the case of legu- 

 minous plants, by the fact that the 

 presence of the latter helps to lessen the 

 number of those organisms which cause 

 soils to lose nitrogen, 



The second use of the practice which is 

 under consideration, that of the pro- 

 vision of plant food in addition to what 

 is already existent in the soil, is one 

 which, so far as has been satisfactorily 

 demonstrated up to the present, solely 

 connected to the ability of leguminous 

 crops to add nitrogen to the soil. It is 

 not the purpose of this article to deal 

 with this aspect of the subject ; if further 

 information is required, attention is 

 directed to the recent one on Soil 

 Inoculation, Agricultural News, Vol. 

 VIII., No. 184, of May 15, 1909, to which 

 reference has already been made. It 



will not be out of place, however, to 

 draw attention, at this stage, to the 

 influence of former leguminous crops on 

 the later ones. The tact that a soil has 

 already had the benefit cf nodule nitri- 

 fication hastens succeeding efforts in 

 this direction, for the reason that such a 

 soil already contains an amount of 

 nitrogen sufficient to stimulate plant- 

 growth, and because the fact that 

 nodule bacteria have already been raised 

 in it hastens the infection of the later 

 legumes with those useful organisms. 



The kind of soil to which green dress- 

 ings are applied must, naturally, be an 

 important factor iq regard to the results 

 ou such application. The action in light, 

 open soils must be very different from 

 the one which will take place in those 

 which are heavy. Their effectiveness 

 is generally far greater in the former 

 case than in the latter. The lack of 

 plant food in sandy soils, their openness, 

 their small capacity for holding water, 

 and their lack of humus all contribute 

 to the great change for the good which 

 often follows the application of plant- 

 material. There is certainly the fear 

 that, in well-watered soils of this 

 kind, acidity may result from the 

 practice ; but this tendency may be 

 controlled by judicious action in the 

 matter and by the use of lime as a 

 corrective. It is quite another matter 

 in the case of heavy soils. These do not 

 require applications of green dressings 

 as often as they are demanded by the 

 lighter ones. They hold Avater well, the 

 activity of the bacteria which cause 

 decay is smaller, they retain nitrates 

 to a much greater degree, aud, under 

 good conditions of drainage are much 

 Jess likely to suffer a loss of nitrogen 

 from the action of bacteria. None the 

 less, such applications are eminently 

 desirable from time to time, because 

 they have the effect of quickly f reeiug 

 otherwise slowly accessible stores of 

 potash and phosphoric acid, and because 

 of their improvements of the condition 

 of the soil. 



Sufficient has already been said to 

 indicate that care must be exercised in 

 the use of green dressings. Damage to 

 the soil will obviously cause harm to 

 the plants which such dressings were 

 intended ultimately to benefit. By 

 what criteria shall we chiefly judge in 

 particular instances, whether benefit or 

 harm will accrue? The answer is, as 

 has been stated differently already : 

 By those in which the first consideration 

 is given to the conditions regulating the 

 bacterial life, both in the buried plants 

 aud in the soil. 



