226 



THE AGRICULTURAL NEWS. 



July 24, 1909. 



constituent — nitrogen — instead of undergoing those 

 changes which would render that element more avail- 

 able. In the second case, an e.^cessive rainfall, or 

 insufficient drainage, will induce a formation of acid 

 substances which will act as a preservative against that 

 decay upon which the material absolutely depends for 

 its effectiveness. 



For reasons which are closely connected wiUi what 

 has just been stated, green dressings should, never be 

 buried deeply. The depth of cover maybe greater in 

 light soils than in those which are heavy. The same 

 considerations govern the procedure when it is 

 a question of apjjlying the dressing whon fresh, or after 

 it has been dried. A soil with a large water-holding 

 capacity is more likely to induce beneficial changes in 

 the dried, than in the gre^n, material, while, on the 

 other hand, a light, well-draincl, sandy soil will show 

 the opposite tendency. Nevertheless, with some 

 crops, and under th.e best conditions, fresh green dress- 

 ings and dry grcL'U dressings show an equal efficiency 

 as providers ol nitrogen. 



Setting aside other consiilerations, as far as the 

 crop v.hich is inte:jded to provide the green dressing 

 is concerned, the oest time to appl}' the Latter is when it 

 Just roaches n.aturity : that is to say, at fruiting time, 

 when about half of the leaves have turned yellow, for 

 at this period the plant has reached the limit of 

 production for that season. An additional reason for 

 not allowing the manuring crop to stand too long is 

 ihat the water-content of the soil may be reduced to 

 such an e.Ktent as to decrease the number of beneficial 

 bacteria to a degree which will inhibit its proper 

 action when it is eventually ploughed in. 



Such a consideration, however, namely that of the 

 age which the manurial dressing should reach before it is 

 applied to the land, is governed by another important 

 factor. In soils in the tropics, where bacterial .action 

 takes place very cpiickly, there is a danger that the 

 food which has been rendered available to plants and 

 which, consciiuently, has be<;ome .all the more soluble, 

 may be largely washed out of the soil before the cn^p 

 which is intended to benefit by it has reached the 

 stage when it is capable of taking it in. Th\is it is 

 unmistakably indicated that, under the clim.atic- condi- 

 tions of the West Indies, land which has been treated 

 with green dressings must be put to use soon after 

 their application, in order that loss due to wastage may 

 be avoided as far as possible. . 



Dependence is often placed merely upon one or 

 two uii.ds of plants for the provision of green dressings. 

 Triis .■should not be so, especially where ihere are 

 i icilities for raising, and experimenting witii, a number 

 of different sons. The diseases and pests to which 

 various plants are liable have very distinct limitations 

 in respect to eai/li kind of plant, anil it is reasonable to 

 conclude that several different kinds of plants, raised on 

 a given area, are likely to give a much better yield 

 than one or two varieties. In the matter of leguminous 

 plants, the consideration is further advanced. A refer- 

 ence to the article on ' Soil Inoculation ' in the Agri- 

 '■altaral Ncivs, Vol. VIII, No. 184, of May 15, 1909, 

 will make it plain that different leguminous plants 

 require diff'erent varieties of bacteria for the purpose 

 of nitrogen-fixation, and that, therefore, the raising of 

 as many kinds of those plants as is possible in a given 

 •area of soil will result in the largest employment of the 

 bacteria which are present for that purpose. 



It requires little consideration to reach the conclu- 

 sion that, as buried jilants are dependent upon the 

 action of certain bacteiia for the production of useful 

 |)lant food from them, any means of increasing the 

 number of th(jsc bacteria in the soil will make the 

 efificiency of such buried plants all the greater. This 

 theoretical consideration receives practical support from 

 experiments which plainly show that the admixture of 

 pen manure with the dressings before ploughing in, 

 even in proportions too small for the manure to have 

 any action, of itself, in increasing the yield, has hastened 

 and directed the decay of the green crop in such a way 

 as to cause the maximum benefit to be derived bv the 

 one which succeeded it. 



The .action of bacteria, then, is the chief determin- 

 ing factor in the changes, e.'entually beneficial or other- 

 wise, which are undergone b}' green dressings. That 

 this action is of paramount importance in the matter 

 \inder review will be rendered all the more evident in 

 the next article which, as has been stated, will deal 

 with the effect of the buried |)l,ints lui the soil. 



An interesting troiiieal ii;uallel to tlu' WL'll-kiiowu 

 ■ clovcr-sickne.ss ' of soil.s in England is instanced in tlie 

 AiiniKif Ri'piiit lit' lilt E.ijjrfiiiiciitit/ U'oi /,• '>t l/ie D/iaiinir 

 Aiiriniltiiriil SfiitinH, 1907 (Department of Agriculture, 

 i'.onibay). This was .furnished by the ground nut, which, r)f 

 i-(jur.-;e, like clover, i.s a leguniinou.-; plant. It was found that 

 the cultivation of tliat crop on the .same land in two 

 successive years has a decidedly injmious effect on the yield 

 in the second year, for the same varieties sown on a piece 

 of land whicli had not been under the groiuul nut in the 

 prei'eding year gave niucli liighor ri'turns. 



