PRINCIPAL CONSTITUENTS OF ROTATION CROPS.

Otago Witness, Issue 2173, 17 October 1895, Page 4

PRINCIPAL CONSTITUENTS OF ROTATION CROPS.

The practice of rotation is admitted to bo Iho foundation of the improvement in the agricullure of all civilised countries ; it is of great importance, therefore, carefully to consider both in what tho practice ifcaelf consists and how the benefits are to ba explained. Defined in few words, it may be said to consist in the alternation of rootcrojs and of leguminous crops with cereals. Such alternation is iv fsctflie basis of all the various rotations adopted, and wilh the view to ascertain how the admittedly very beneficial (iFt-cts of alt ernata, as distinguished rom continuous, cropping are to be explained, what may be termed a complementary serie3 of experimeLts oa an actual course of rotation were commenced at RothamsUad in 1848 and are still continued. There are many deviations from the regular rotation practice referred to, but the simple four-courae ous conducted on a self-BupportiDg baßis— that is to say, in which only certain cereal and animal products are sold or removed, and no compecß»tion made by minurefrom external eources — was selected for investigation at Rolhamstead. As the maintenance of the fertility of the soil is of primary importance in farm managemett, tbe purpose o? these remarks is to refer to the evidence relating to Feme of the more important con^ti-tut-u t, f.-.nml m fie fliif- m b cr<>\>s of (lvi fourcou 1 " c ot.i'i'-n, aa lh- information \si'^e. ve to in-'.i.ico i irn erj as Io the cla-is of manuios which fuits the varitus crops grown.

Oi pbo6|h'-rio acid, supplied in phosphates, the c. real croi* take up as much a?, or more tban, any crops of the rotation excepting clover, and the greater portion of what they take up is lost to the farm in the Baleablo product — tho grain. The remainder — that in the straw as well as that in the roots and tho leguminous cro^s — ia supposed to be retained on the farm, excepting the Btnall amount removed in mcvb and milk. Of potash each of tho crop} takes uo very much

mure than of pfcoipfcoric acid ; bub* much less potash than phosphoric acid is removed in the coresl grains, inuoh more being retained i« tho strnt? ; whilst the othor products of the rota.ion—t he rots and thelegum'uoco — whioh are aUo anpp 'fed to be re!a : ned on tho farm, contain very much m>iro potash thau the cereals, and very li'.tle of it is txj.,o tod iv inoati and mnk Tho general result is that the whol-5 of the crops (if rotation take up very muc^ more of potash than of phosphorc acid, whilst probably even less of it is eventually lost to the land. Of lime, very little ifl fc*keu up by th-j cereal crops, aud by the roots much less than of potash ; mow by tho leguminous than by the other crops, j»nd by the clover especially — sometimes much more thm by all the other cropß of the rotation put togeih-r. Of (ha lime of the crops, however, very little goes in tho saleaHle products of the farm undtr the conditions supposed of a self-supporting rotation. There is, however, frequently a coutiderable Uws of lime in land drainage. Although the facts relating to other mineral constituents" are not without R ; g .ifi.jance, nf-rence can be mad a lore to only one tt'.of of tfco n— iiiinely, the silica. The root and legmniiuns ci". pi tU« up ecarcely imy eilici, but fch-i ctroal* take up a very Urge amount of io. The large amount, indeed, ti-tilicft taken up by thfsj crops when grown uudor ordinary con iv ions is a* characteristic a chemical phenomtnoa of rt t t oa as is tho very large amount of I me taken up by olovsr and other leguminous plants. Vt-iy lutk'Vi'ica, hjvrever, is lost to Iho land in the astura d saleable products — ibo straw being retimed. Thu% alth' u^h diff-r.^ub and sometime very large amounts of these principal mineral constituents are t*keu up by the various crops of the rotation, th^re is no material export of any iv the 6aleable prolucfcs, exctptiug of phofpharic acid aud potash ; acd so far at !chs ; as phosphoric acid ii concerned, experieuco has shown that it may bo nlvaufcageeutly supplied in purchased manure?. But, although the tveotual losa to 'the load of mineral constituents is, in a solf-mpporting rotation, comparatively so small, the fact that the different crops require for their growth not only very different amcunts of individual constituents, bur. r-qnire these to bo available within the soil in veiy diff rei.t coudi'K-iM of couub nation and of distribution, points to the cniclu.-ion that, in any c-xplanation of the bentfits cf an alternation of crops, the position aud the <6l«> of the miucral coi.s*itueiiti cannot be overlooked ; aud the lefs can it bo to when tle'c connection with that very important element — the nitrogen of the crop" — i-« considered. As to the nitrogen, although very cha-ftc-tcrhtically beutiiteu by nitrogen* n\ manur. s, the cereal crops tako up and retiin much less nitrogen than any of the cropi alterutvtrrd with thorn. In fact, the root cropi may contain two or more times as muoh nitr gu as either of the cereals, and tho lcgumnous crops, especially th« clover, nvieh more than the root crops. Th'i grea'or pert of tho nitre gn of the cc oils is, however, s-ild off (he farm ; but perhaps not more than 10 cr 15 per cent, of that ot ciiher tho root crop or tho clovee or other forage leguminous crop i« sold off in animal increase or milk. Thus, most of the nitrogen cf the straw of the cereals and a very large proportion of that of the much more bi fc hl> iiitrojfon-jielding crops returns to Ihe land as mniure for the benefit of future cereal and otn«r croi'g. As to the source of the nitrogen of the eocalled rcitcra.livo crops, certaiuly in the case of rovtj it is nos aa bas been sometimes assumed, that such pUnts take up nitrogen from the »ir by viitue of their extt-nde^ leaves. Both common expoiieu^o aud direct experiment demonstrate that they are as dependent as any crop that is grown ou available nitrogen within the soil, nh'ch is generally supplied by tho direct application of nitrogenous m»nuro» — natural or artificial. Under 6uch corditions of supply, however, the rcob crops, 8) to speak, gross feeders as they are, and distributing a very largo amount of fibrous feeding root within the soil, avail them el yes of a much greater quantity of the nitrogen supplied than the cereals" would do under similar ciroumstaucoj ; tbis result being perbly due to their period of accumu'atiou and growth extending even months after the period of collection by tho ripening cereals h»s terminated, and at the season when nitrification within the ioil is the most active, and the accumulation of citrates in it is the greatest. Lastly, fall supply of both mineral constituents and nitrogen being at command, these crop 3 assimilate a very large amount of carbon from the atmosphere, and produce, besides nitrogenous food-products, a very large amount of the carbohydrate sugar as respiratory and fat-forming food for the live stock of the farm. Ihe still more highly nitrogenous legntninous rrors, on the other hand, althongh not characteristically benefited by nitrogenous manures, nevertheless contribute much more nitrogen to the total produce of the rotation than any of the other crops comprised in it. It is also certain that at any rate a large proportion of the nitrogen of thsse crops is obtained from the soil and subsoil ; though recent investigations bave proved thatr come of their nitrogen, and sometimes much of it, may bo derived indirectly from the free nitrogen of the atmesphrre, b-ouyht ii-to combination under the influence of r.iicru-org.Mii'Uis wi bin the nodules o.i Iho roots of the p'anti. It is the leguminous fodder crops, and amongthem especially c'orer, ■which h-.s a mush more extended period of growth and a wider range of salectioa within the soil and aub<oil than any other crops of the rotation, that yield in their produce the largest amount of nitrogen per acre. Much of this is doubtless taken up as nitrate, yet the direct application.of nitrate of soda has comparatively liltlo bentfidal influence on their growth. The nitric acid is probably taken up as nitrate of lime, and probably as nitrate of potash alflo.

tii 1 ; ib in »igniilu.mb ihafc tha high nitrogen* yielding clover t*kes up, or ah least; returns, very little soda. The general result .tlaeu is that, a'though tha clover takes up a good deal ot its nitrogen as nitrate, this would seem to be dorivel fri-m combinations within tho soli, whi'ih aro brought i.»to suitabla conditions of ■otbn kHoa and distributed through' a wide rag'a g' « f ' •'oil and subt-oil.