A SURVEY OF COUNTRY AFFECTED BY EROSION

Press, Volume LXXIX, Issue 23940, 6 May 1943, Page 4

A SURVEY OF COUNTRY AFFECTED BY EROSION

SOIL CONSERVATION

During the last 40 or 50 years much has been written on the subject of soil erosion. In New Zealand few of these observations have passed the stage of vague generalisations. The tendency especially within the last few years has been to regard the high country of the South Island as one geographic unit, whose ills could all be cured by considering the problem of its erosion as a purely administrative one. Such theorising, with little or no reference to the field details of the problem, is to be deprecated. On the botanical side, however, valuable contributions have been made by two eminent New Zealand botanists. Li Cockayne and Petrie, who investigated the Central Otago area. Their work, in spite of its botanical value, had only a limited applicability, owing to lack of information about the soil pattern. Broad generalisations on such an area as the high country of the South Island, unless qualified by reference to soil types, serve only to darken counsel, and in some circumstances may actually be misleading. The unsoundness of such generalisations can best be illustrated by considering the wide range of climates in the area, bearing in mind the fact that soils are, largely the product of climatic influences, such as rainfall, temperature, and humidity. The driest regions of Central Otago and the Mackenzie basin receive approximately 13 inches of ram annually, and verge on the semi-arid type of climate. As one passes westwards to the Haast Pass region, the rainfall increases steadily until it reaches nearly 175 inches on the Westland side of the pass. Again, in the Mackenzie basin, the climate ranges from semi-arid in the south-eastern portion, where the landscape resembles desert, to polar and sub-polar climates of the mountain summits. The soils developed under these varying climatic conditions are almost as divergent as the climates which produced them; and, in particular, their susceptibility to erosion and the manner in which it takes place, are equally characteristic, for the mechanism of erosion is closely connected with the chemical and physical properties of the soil. , Survey of High Country

Although the basic importance of approaching the problem of erosion through the examination of soil properties has long been realised, facilities for making a soil survey have been available only within the last few years. When an opportunity presented* itself, therefore, in 1940, the Research Council instructed that a soil and erosion survey should be made of the South Island high country. This was carried out by two officers (Messrs H. S. Gibbs and J. D. Raeside) of the Soil Survey Division of the Scientific and Industrial Research Department during the two following .field seasons. Field work, involving examination of soil profiles at high altitudes, could only be carried out during the summer months. The object of the survey was to identify the main soil types of the tussock grasslands and map their boundaries; to map the extent and types of erosion on each of these soil types; and to investigate the original causes of the deterioration and to assess their relative importance on each soil type. Observations on the mechanism of regeneration were made at the same time.

The survey was, therefore, briefly a scientific survey' of the location, extent. and condition of the areas affected by erosion. At present, while statements are being made that State departments are inactive, the maps and

rnv r n RAESIDE, Soil Survey Division, Department of Scientific and ‘ Industrial Research.]

report are receiving their final touches before going to press. This erosion inventory covered the high country from the Awatere river in the north, to Lake Wakatipu and Roxburgh in the south; and extended from the Canterbury! Westland boundary in the west (roughly the summit of Hie Alps), to the foothills of Canterbury in the east. Maps showing soil type and erosion intensity have been compiled on a scale of four miles to the inch, the rating of erosion intensity being modelled after the American system. In applying it to New Zealand conditions the American classification had to be considerably modified, since it was designed to cover areas of arable land, rolling to flat country—altogether different from the steep mountainous nature of our high country. Degrees of erosion arc usually defined in terms of amount of -£gsoil lost on any particular area, tk jnge being from slight erosion, wl]!efe less than 25 per cent, of the topsoil has been lost, through moderate erosion, where 25 to 50 per cent, has been lost, and severe erosion where 50 to 75 per cent, has been eroded, to extreme, where more than 75 per cent, of the topsoil has been removed. A Unique Work

No work of a precisely similar nature to the survey of the New Zealand montane tussock grasslands has been recorded in any other country, so that in approaching the problem, virtually new ground had to be covered. Moreover, the obstacles in the way of regeneration seem to be peculiar to New Zealand. One of the aims of the survey was to outline these problems scientifically.

Among the immediate applications of the work may be mentioned the selection of those areas most in need of some kind of erosion control, not. ably those where erosion is only moderately advanced, and where timely intervention may preserve them from further deterioration, Oft the other hand, many thousands of acres at higher levels are so badly eroded that their reclamation will be a long and uncertain process. The delineation of the soil boundaries has also made possible the establishment of experimental areas; where various methods of erosion control can be tried under careful supervision. Without the soil maps, such experiments in regrassing would be of little value because it would be impossible to say what general applicability a successful experiment might have. A conservation scheme which might be suitable for Central Otago would not be applicable to the Mesopotamia area in the Upper Rangitata, for example. With the soil maps as a background, therefore, experimental plots can be located so that they cover the whole gamut of soil types, and any conclusions drawn will be applicable to the whole of the tussock grasslands. Moreover, the soil maps,, serve to place the information and experience of one runholder at the disposal of others whose runs are on th® same soil type. u Finally, it should be emphasised that spectacular results cannot be expected from soil conservation programmes. The deterioration has been allowed to proceed for more than 60 years, and systematic and painstaking work over a long period will be necessary before complete regeneration can be achieved. The present survey provides a direct and scientific approach to this problem of regeneration, and on the facts assembled during its progress further detailed work can bl based.