Recent Reserch Work

New Zealand Journal of Agriculture, Volume 94, Issue 2, 15 February 1957, Page 129

Recent Reserch Work

POTASH TOPDRESSING

THE study of potash deficiencies occupies a large place in field experiments by the Extension Division of the Department of Agriculture. At present 380 observational trials and many measurement trials on pastures, lucerne, and potatoes are being conducted.

DEFICIENCY SPREADING

THIS large number of experiments is necessary to keep up with the potash deficiencies which are developing or are believed to become more acute in district after district.

For instance in southern Hawke’s Bay very slight growth responses to potash were observed in field trials as far back as 1935, but hardly any potash is used in that district. Recent evidence points to a quite pronounced deficiency in some fields and it is hoped that for the sake of increased production potash topdressing will become more general.

At Marton Experimental Station quite good growth responses to potash applications are now obtained on pastures and on potatoes, though earlier no growth response was obtained. The soils represented at the Marton station cover an area of about 200,000 acres. Outside Marton on the same or similar soils 9 out of 14 observational trials indicate slight to very good responses to potash topdressing.

In Northland there are large areas of molybdenumdeficient soils which have not been farmed intensively in the past but which are expected to require potash topdressing a few years after pastures have been improved. Accelerated pasture improvement is believed to take place owing to the discovery of molybdenum deficiency, the widespread use of aerial topdressing, and the ability of farmers to finance improvements. Soils on which the development of potash deficiency is being studied by field experiments are the so-called “semi-volcanic” soils and the poor clay soils derived from sandstone. These soils alone occupy a very large area, well over one million acres, a small proportion of which is being brought into high-class

pastures every year.

RATES OF APPLICATION

Since potassic fertilisers are costly, accurate knowledge of rates of application for maintenance and initial dressings is essential.

uesiraoie rates 01 application vary wiui suu type and with the kind of management a field receives. A good hay crop, for instance, may withdraw the equivalent of IJcwt. to 2cwt. of muriate of potash per acre. It is not unlikely that this loss should be replaced in full by additional topdressing on certain soils already deficient in potash. There is considerable experimental evidence that on severely potash-deficient fields on the friable soils derived from volcanic ash in Taranaki, Waikato, and south of Auckland dressings of 2cwt. of muriate of potash or even more are much superior to lewt. per acre. How far these rates are economic will have to be proved by farming experience. On very deficient fields an initial dressing of lewt. of muriate of potash per acre may be inadequate, and some trials appear to indicate that higher rates of application are more economical. There is a possibility, however, that some economy can be achieved by using occasionally common salt with potash or salt alone on certain soil types in the Waikato basin and the Bay of Plenty. Further experimental work is needed before definite advice on the use of salt can be given.

TIME OF • APPLICATION

A change of severe deficiency to adequacy of potash as a result of topdressing is usually accompanied by temporary clover dominance. This may cause a high incidence of bloat. Potassic fertilisers should be used by farmers, or at least tried by them, before pasture deterioration has gone too far. If this is done, many years of low production may be avoided, the need for using high rates initially may not occur, and clover dominance may not become troublesome. Instructors in Agriculture are able to say which fields are likely to be or to become potash deficient; in this they are helped by the soil testing service.

What limited experimental evidence is to hand points to spring as the best season to apply potassic fertilisers.

SOUTH ISLAND

In the South Island as a whole potash deficiency is of far less economic consequence. Nevertheless the use of potash is being found to be essential on a number of soil types. Replicated mowing trials

are being conducted at Winchmore, near Ashburton, and at Invermay, near Dunedin. The only measured response occurred at Winchmore from lewt. of muriate of potash per acre on irrigated lucerne. The soil type is Lismore

stony loam. Of the 150 observational trials only 10 per cent, have shown growth responses to potash. The soils most likely to be deficient in potash seem to be: Soils containing much granite, particularly associated with high rainfall or much cropping. These occur in the Nelson, Buller, and Takaka districts and are represented by the Karamea, Sherry, Buller, Hokitika, and Ikamatua soils. All have a high potential production in districts where the area of such soils is very small. More use should therefore be made of the advice of the local Instructors in Agriculture in pinpointing potash-responsive fields.

There is little evidence that potash is important in Canterbury and Marlborough, except perhaps for special crops, such as irrigated lucerne on the light, stony soils of Canterbury. In Otago and Southland potash deficiency seems widespread only on the Edendale soils, though there is evidence from individual field experiments that it may exist on the related Drummond and Tisbury soils. These types occur over quite a large area of flat to undulating land near Invercargill. It is not unlikely, therefore, that in the near future in Southland, as in the North Island, potash deficiency may become more severe on areas so far only slightly affected.

—C. DURING