WEATHER INFLUENCE ON FARM CROPS.

Otago Witness, Issue 2493, 25 December 1901, Page 6

WEATHER INFLUENCE ON FARM CROPS.

Frost -v— As farmers and gaideners invariably speak of temperatures below 32deg «k so many degrees of frost, and as such ! ow readings have often a distinct influence on plant life, it may be well to devote a few paragraphs specially to their consideration. In the first place, there are many kinds and degrees ot frost, each of which has a different effect on plants which are not peifectly hardy. For instance, there is the moderate fio,4 of short duration, which. . t s a mle, acts beneficially by giving a whole(■ome check to growth during a dangerous period of the year. There is, the prolonged winter fiost, which, if not too severe, does more .good than harm to vegetable life, while greatly benefiting the soil by its disintegrating effect. On the other hand, iheie occasionally occurs a fiosl extending over many weeks, when extremely low readings are registered, during which all but the hardiest plants suffer severely, while many of the more delicate species are killed outright. Then there are wind frosts, which, although not generally so low in temperature as in the ordinary radiation frosts, are much more trying t.< vegetation, as not only are the upper surfaces oi. t^ie leaves chilled, but the whole plant is enveloped in a swift-flowing curvent of cold air. A frost in winter nas to be very' keen indeed to inflict any serious injuries, but late spring frosts are greatly to be feared, as most plants are tjien in active growth, and covered with young and tender foliage. If their leaves be dry, plants will frequently escape injury from a frost which, if it had been preceded by rain, would have greatly crippled them. In protecting delicate plants in a severe winter fiost, the covering requiies to be very substantial to be of any real service, but it is surprising how slight is the protection necessary in most instances in the case of late spring or early autumn, frosts. As often as not it is the exposxire of the plants to sunshine on the morning succeeding these brief frosts which is answerable for the injuries attributed to them. The effect of veiy low temperatures on vegc- , tation, and .also of quick thawing, is thus explained by Dr Vines in his "Student's Text-book of Botany" : — "When a part of a plant which contains a large proportion of water is exposed to a low temperature, a portion oi the \i ater contained in the cells escapes from them, and becomes frozen on their surface, the whole tissue at the same time contracting; the water does not freeze in the interior of the cells. The water which has t-Ims> escaped and frozen forms an incrustation consisting of a number of elongated ice crystals, arranged side by side. This ice is very pure, for the substances in solution in the cell-sap rema'u behind in a more concentrated form. It has 'been ascertained that this formation of ice },s not necessarily fatal in all cases. If the frozen part 'be slowly thawed, the cells may gradually reabsoi'b the water and so return to their normal condition. If, however, the frozen part be quickly thawed, the cells cannot absorb the water sufficiently rapidly ; it therefore either collects in the intercellular spaces, causing discolouration and decay, or it runs off "md evaporates, so that the part diies up." Ram. — The influence of rainfall on vegetation stands second only to tha.t of temperature, for each kind of plant requires not only a certain degree of warmth, but also a certain degiee of moisture, in order to perform its various functions in the most perfect manner. There are two classes of farm crops, the requirements of which, as regards lainfall, differ considerably. Taking one j ear with another, the fall of rain in this colony more than meets the wants of the cereals, while there is often too little and seldom too much for the root and grass ciops. In times of di ought, plants have to rely on the water in the subsoil, and consequently the deeper-rooted corn jrops are less injuriously affected, when once established, than the comparatively shallowrooted turnips and anany of the grasses. This leads to the consideration of the passage of rain into the "-oil, known as percolation. Of the moisture received from the ck-uds, the greater part during the winter half of the year descends peimanently into the ground, whatever be the crop growing upon it. But it is otherwise during the summer half of the year, for even on uncropped ground only aibout one-fourth of the total rainfall for these six months finds peimanent lodgment in the subsoil, the other three-fourths being carried into the atmosphere by evaporation. On land thickly covered with vegetation, like permanent pasture, it is only in times of exceptionally heavy rains that at this season any water at all ha* a chance of penetrating the soil to any depth, the whole being taken up by the herbage or evaporated. It may be mentioned in passing that it was found .it Rothamsted that by the application of nitrate of soda to pasture land the gi.T-s became deeper rooted, and was thus enabled \o withstand without injuvy e^en the long spring and eulv summer drought of 1870. The question of neicolatio'n becomes one of consi deivible importance, when we consider how the fertility of land is lessened w hen th< drainage >s excessive. This is ev:dencec by the Rothamsted experiments, where th( soil ha^- ibeen shown to be richest in ni t rales after the penod of laa&l percolation— thdfc i* to siv. in the late summer and earlj autumn months ; and poorest after tin winter, when the drainage is greatest — sa;* the earliest months of spi-ing. The greater lobs Of t'he«e valuable nitrates consequently occurs during unusually wet autumn am winteis, whereas dmincj a dry ar>i Minny spiing ;mu! Summer, th soil incre^s-es mos-l in feilP'ly. 1 vi a^ foimeily supposed thai land Id f.Jlow during tho winter was benefits thereby, but it :•- now known thai the foi .retains its nitrates oiucb more efiectuall; if covered by a. crop of i-oin-e kind, and mor particularly* will this be the case if th winter proves a wet or.c. To put th mdtter in another wjv. if it were cert.ii th.it any umtei would be d'v\ it migh bo de&irable to .ipplv nitrogenous maniac u> vii&at ££ug* iv tiie auluiiui, bu r . \i cci

tain to be wet their application should un« . doubtedly be deferred until the following spring. Winter rains may not be necessary for the immediate wants of either farm or garden crop*, but they are invaluable in allowing a plentiful supjjly of water to be stored up m the uibioil fJr the fm ure well-being of these crops— to say nothing of the requirements of fruit and other trees and shrubs, with their deeper roots and dense foliage. It is during the late spring and early summer, when most plants arc growing more rapidly than at any other period of the year, aad evaporation is greatest, that the want of sufficient rain ie most keenly felt. On the other hand, continued wet during the same period is also undesirable, principally because a long continuance of rainy weather at this season generally means also a lonir continuance of un.>ea«onably cold, humid^ and sunless wea.ther. The great difficulty in treating of tlus question of rainfall consists in the different effect it has on different soils and subsoils. To take tyro extreme cases, heavy clays require but a* moderate amount of rain, whilst deep, sandy or gravelly soils can scarcely at any; season have too much. It must not be forgotten that the character of the rain- < fall in any locality or at any period of I the year is a matter of almost equal moment with the quantity. For instance after a dry period the fall of an inch of rain on a single duy during the first week would not be nearly so beneficial as if fonr falls of a quarter of an inch each were equally distributed over it. Snow. — A deep fall of snow during the winter months is always welcomed by farmers who have made provision of stock food for the contingency and by gardeners. It keeps the ground comparatively warm and steady in temperature, while affording the most efficient safeguard against severe frost to all plants covered by it. A slight coating is also often of considerable service, even to such a hardy plant as young wheat, owing to the pro* tection it affords from keen frosts and biting winds. Sunshine. — It has been truly said tEafc heat, water, and soil can be artificially supplied to plants, but no light &o suitable as sunlight. Almost every crop is more dependent -upon sunshine at some particularperiod of the year than at any other. For instance, corn at the time of ripening, the grass crops in the spring, the roots in the autumn, the fruit crops when approaching maturity, and so on. In the spring months it is sunshine rather than rain thab is wanted — sunshine to warm the ground, for without this rain is of little service, either for the germination of seeds or for . the .growth of plants. Again, it has been found at RotLamsted 'that, while certain, reasons may favour the mixed herbage of grass lands, in so far as luxuriance of growth is concerned, it is in others, no doubt during the more sunny ones, that this herbage is more nutritive and becomes properly matured, so that in the latterease, although the yield may not be sogreat, the quality is very superior. Corn crops are likewise similarly influenced, some seasons favouring growth, while others favour more particularly the perfecting of the grain and its maturation. TLere has been found to be a very close connection between the quantity of watery vapour exhaled by leaves and the degree of sunshine to which they are exposed. This being the case, and knowing the important part played by this fimction of transpiration as it is termed, in building Uj7 the structure of plants, it will be reauily understood how much the character of their growth must be dependent upon whether any particular bummer is unusually sunny, or the reverse.