BURNING WHEAT STRAW

Press, Volume LXXXIII, Issue 25134, 15 March 1947, Page 5

BURNING WHEAT STRAW

EFFECTS ON FARM LAND

LITTLE INFLUENCE ON FERTILITY

Each year about one-fifth of a million acres of wheat, is grown in New Zealand, producing, in addition to the 6,000,000 to 7,000.000 bushels of gram, about 150,000 tons of straw, writes H. P. Schapper, of Lincoln College, in the current issue of the rural education bulletin. The ultimate fate of this straw depends very largely on how thewheat is harvested If it is harvested with a reaper and binder and carted to a threshing mill, most of the straw from that crop would be “walloped” into a stack on the site of the threshing operations. The staqf< may be left for stock for the next winter, and then if there is anything left over, burnt, or it may be baled up and used for bedding. On the other hand the crop may be harvested with a header, leaving the bulk of the straw still standing in the paddock. This straw may be either left, bu.rnt or ploughed in.

In Canterbury—and this is where most of the wheat is grown—about 80 per cent, is header-harvested. It is the straw left standing in the paddock after the header with which we are now concerned. In some districts, as for example, Methven, much wheat is sown down with pasture in the spring. After heading, the straw is just left to be picked at and trampled on by stock. This leaves us. then, with headed straw on fields which have not been sown dpwn with grass. Do we* burn it or plough it in? The answer to these questions depends largely on what is intended to be done with the paddock.

If the paddock is to be sown in a spring crop such as oats, barley or peas, rape or turnips, the straw need not be burnt, because by late autumn or winter when the cultivation work on this paddock begins, the straw may be fairly well broken down by weather and stock. Also, by this time weed and stubble clover growth may be quite considerable, and this, when ploughed in with the straw, will help in the decay. Autumn Wheat But suppose the intention is to sow the paddock in autumn wheat, or lambing green feed, or, say, oats and lupins for winter feed. If it is to be autumn wheat then it is advisable to burn rather than plough in the headed straw of the previous crop, because burning will reduce Hessian Fly and fungus disease risks. Also, if wild oats, wild turnip, or Californian thistle are prevalent, burning will kill some seeds. It will also help other seeds to germinate readily, so that a “kill” by cultivation can be carried out. If the next crop'is winter forage or spring green feed, for the best results it will have to be sown as soon as possible after the wheat is headed. If the job is to be done on time the delays caused by ploughing in should be avoided, and so here again it seems advisable to burn. Not all farmers, however, in these circumstances do burn. Some chop up the straw with discs before ploughing it in; others plough it under direct, using high beam ploughs built specially for the purpose, while vet others plough direct with the ordinary plough, but only with considerable difficulty. So, unless there is special equipment for preparing a seed bed on straw stubble the operation is likely to involve higher cultivation costs, with stoppages and delay and considerable mechanical difficulty.

So far we have considered some of the more obvious reasons for burning. Now let us examine the question in a different light. Assume the necessary equipment to handle the straw is on hand and the farmer is prepared to go to the extra trouble. Is it worth it? Is he justified in doing so? The answer is both yes and no. When the season is abnormally dry and there is only a little stubble clover and weed growth in the wheat crop, the straw, if turned under, will not rot very readily. It may even act as a barrier to the upward movement of moisture necessary for the rapid germination of newly-sown seed, and in these instances the answer is “yes” —burn the straw. Where plant material such as wheat straw is turned in, it cannot be of any value as a source of plant food until it has been converted into humus by the soil micro-organisms. In turning straw into humus these micro-organisms use up

available soil nitrogen and convert it into an unavailable form, thus robbing a newly sown crop of an essential food. Wherever material with a nitrogen content of less than 2 per cent, is added to the soil just before a crop is sown—as in the case of autumn-sown forage on wheat stubble —the growing plant may suffer from nitrogen starvation. The nitrogen content of wheat, straw is well under 2 per cent.—it is only about half of one per cent. Farmers have often observed where a partial burning of the stubble has been carried out and the whole area sown to autumn forage, that a better immediate establishment and growth has been secured on the burnt area. On the non-burnt area there is sometimes seen a paleness and slowness of growth indicative of a lack of available nitrogen. And so, again, the answer is yes, burn the straw. Soil Structure On the other hand, when the straw is ploughed under, rain may not soak in so readily and moisture may evaporate more quickly than if the straw were left on the surface. By ploughing in, that desirable 0 crumbly structure of the soil may be lost. In these instances, then, the answer is no—do not bur.n the straw but prepare a seed bed without turning it under. This is a difficult task and it is doubtful if it is worth the trouble.

Now let us put this wheat straw business in its correct perspective from the soil fertility angle.

When headed straw stubble is burnt, the loss to the soil is the organic matter in the*6traw itself and a certain amount of organic material on the soil surface. Minerals are not lost. Organic matter is valuable, not only in itself but also for its nitrogen. Soil organisms must also be damaged to some unknown degree. On 30 bushels an acre wheat land, wheat is grown on an average only once in seven or eight years and the land is under pasture for three-five years. For the remainder of the period it is in fallow, forage, and other crops. Suppose the straw is burned—what is lost? On the wheat paddock once in every seven-eight years half to three-quarters of a ton of organic matter and 101 b of nitrogen in the form of straw are lost per acre. How does this compare with the build-up by pasture? In this same seven-eight year period the total gain to the soil from average grazed pasture is about four tons of organic matter plus 2001 b of nitrogen. Thus the loss to the soil from burning the straw, when compared with the gain from average grazed pasture seems to be of but little significance. Further, when it is remembered that even on a farm where ■ all the wheat is headed, the farm management economy would dictate the burning, on average, of probably only one-half of the annual wheat area, the actual loss from burning has even less significance than the figures indicate. Crop Rotation Thus, whether the farmer does or does not burn headed wheat straw is determined largely by the crop rotation he has chosen for his farm. If wheat is sown wittv a pasture mixture, there is no burning done. If wheat is followed by spring sown crops he may or may not burn. If wheat is followed by an autumn sown crop he usually does burn. The mein advantages of burning are that it aids the control of fungus disease and Hessian flv. helps to control some weeds, minimises the risk of the following crop suffering from

nitrogen starvation and keeps down cultivation costs.

The main disadvantages are that some organic matter is destroyed (although as already mentioned, it is very slight), and undoubtedly some slight but admittedly unknown damage is done to the top soil.

The question next arises: should the farmer change his crop rotation to avoid burning? I think not. because the returning of headed wheat straw to the soil does not seem to be in general sufficiently advantageous, if at all advantageous, to compensate for the difficulties involved. Further, soil fertility can be increased much more effectively without the added cost of incorporating the straw with the soil, fn general terms, soil fertility—its maintenance and building up—on Canterbury mixed cropping farms, hinges on a satisfactory balance between fer-tility-depleting crops on the one hand, and fertility-restoring crops and the maintenance of high quality clover pastures on the other. A satisfactory balance can be maintained in a mixed farm economy without returning straw crop residues to the land, nor will the return of such residues compensate for a poor crop and pasture balance.

A pen of two-tooths sold at the first Amberley ewe fair on behalf of the Estate of H. O. D. Meares (Shellrock) made 375. The sale was wrongly reported as 275. ! Among the best of the Marlborough I ewes to come into Addington each year are those from Wharanui. and a big line of these at Addington on Wednesday made up to 35s for four-year sheep. They were of extra good quality. and attracted keen competition.