FARM AND STATION

Press, Volume XC, Issue 27410, 24 July 1954, Page 5

FARM AND STATION

(By

H.S.)

CATCHMENT OF SELWYN RIVER

Detailed Survey Of Conservation Needs

A scheme to control the Selwyn river and its tributaries from the western edge of the Plains to the mouth has been worked out by the North Canterbury Catchment Board, which recently announced details of the work to be done. To complement this work, conservation activities in the catchment of these rivers will eventually be needed, and the board’s staff under Mr R. D. Dick, assisted by members of the staff of the Soil Conservation Council, have completed a survey of the 140,000 acres which make up the catchment of the Selwyn river system. The work is described as a reconnaissance survey, but contains a great wealth of detail.

Two objects were in mind in making the survey. One was to outline methods of retarding the run-off from the catchment to reduce the peaks of floods, and the other was to suggest means of land' use which would at the same time maintain and increase fertility and thus production. In effect the survey is a. statement of the assets and liabilities of the area as farm land and as a soil conservation problem. Obviously the two considerations merge at many points. The survey has been made along lines used with success for many years in the United States, and modified for New Zealand conditions. In this system the land is classified into eight main “capability classes” according to soil, slope, vegetation, climate, and a number of other considerations, and to the use that is being made or could be made of it under good farming practices.

To carry through the plan for protection of the catchment area would cost about £240,000. This is the sum of all the money estimated to be required to treat each of the 191 properties which are wholly or partly within the 140,000 acres surveyed. The biggest single item in this cost by a long way is fencing, which totals over £llO,OOO. Realignment of present fences accounts for a small part of this, and fencing of gullies and other places to be planted in trees takes the bulk. Liming, topdressing, and seeding are estimated to require £Bl,OOO, terracing, contour furrowing and similar work to require £21,000, and purchasing of trees for protective afforestation, £19,000.

All the work suggested is thoroughly practical, and in fact, some is being done by individual landholders in the area who have already planted gullies against erosion, contour-furrowed sloping ploughable land, topdressed and seeded hill country from the air, planted windbreaks, put in drainage and so on. The estimate of £240,000 is the cost of applying these practices generally to the whole catchment. The area surveyed includes the whole of the catchment area of the Selwyn system from the Big Ben range eastward to the point at which the Plains meet the foothills. It includes three ranges of foothills and runs from under 1000 feet above sea level to 5436 feet at the top of Ben More. Rainfall varies from about 30 inches in the front country to 45 inches on the higher land to the west. About 43 per cent, of the area is flat or in slopes of less than 20 degrees, and therefore ploughable, and about 50 per cent, is too steep to plough. Land Claims Farming practices and other considerations were used in dividing the land into its various capability classes. Of the eight main classes established for New Zealand, the area contains seven. Class one, first grade cropping land free from flooding, well drained and not subject to more than very slight erosion no matter how it is treated, is absent from the area. There are 17,000 acres of Class II land, which is not quite as good as Class I, but is still good land from all points of view, but for cropping requires care with cultivation on Slopes. Some of it may be naturally badly drained, and some may have soils liable to wind erosion. Control measures required for this class are trees to prevent wind erosion; contour furrowing and so on to retard run-off; liming and topdressing to maintain good pastures and keep up fertility for cropping; and drainage. Treatment is estimated to cost about £16,000, of which £7500 is for terracing and so on, and £BOOO for fencing to protect trees. About 26,000 acres are included in Class 111 and its two sub-classes. This is land suitable for cultivation only with intensive soil conservation practices and with highly skilled farming. It may require extensive drainage, or may have a stiff clay subsoil liable to give a high run-off, or have a very porous sandy soil liable to dry out.

This land will require the full range of conservation practices against wind

and water erosion; treatment of scrub and gorse; topdressing to maintain pasture and possibly special pastures; drainage; and realignment of fences to conform to the capabilities of the soil. The total cost of the work envisaged is £87,000 on this Class 111 land. Once again the bulk of it, £60,000, goes in fencing for trees. Liming and topdressing is estimated at £lB,OOO and contour work at £9OOO.

About 9000 acres are placed in Class IV, which is land considered to be suitable only for occasional cropping. Its slope is generally greater than Class 111 land, some is too dry for dependable production, and some is badly naturally drained soil. It is good grazing country under longrotation pastures. The usual intensive conservation operations are needed to make it safe for occasional cropping, and it needs the best pasture and grazing management. The cost of protecting this area is estimated at about £48,000. Tree planting in gullies, fencing these trees, and realigning fences again make the largest section of this expenditure, about £27,000. Liming and top-dressing account for £18,750. and contour work £3125.

Only 381 acres of the area falls within Class V. This is flat or gently sloping land and because of wet, or stoniness, or inaccesibility, cannot be cultivated, and is therefore grazing land. The largest area falls into Class VI, which accounts for 58,629 acres. This is good grazing land, and in the Selwyn catchment, much of it is good silver tussock country. It can be grazed with only small limitations usually due to steep slope or shallow soil. Conservation practices require control of burning and careful use of fire to improve the grazing; the use of cultivated or burnt firebreaks to control fire; fencing to allow the best use of blocks; a good ratio of cattle to sheep; water for stock; top-dressing and seeding to improve pastures; and control of scrub, gorse, and other weeds, and animal pests.

The cost of control of this Class VI country is set at £69,000, of which £37,500 is estimated to be needed for control of weeds and for topdressing and seeding, and £30,000 for subdivisional fencing and realignment of fences.

In the next class, Class VII, there are 17,215 acres. Much of this land is sreep, rough, eroded, or highly susceptible to erosion, and should be grazed only with care. Much of the fescue tussock of the area is included in this class.

Top-dressing and Weeds The cost of protecting this Class VII land is estimated at £12,325, of which

top-dressing, seeding and weed control at £6875 and new subdivisional fences and realignment of fences at £5200 are the main items. The remainder of the catchment area falls into Class VIII, which embraces 2703 acres of the snow grass country at the top of the Big Ben range, which is beyond practical measures of remedy at present; and 6066 acres of native forest, which is the responsibility of Government departments. The over-all cost for the work is equivalent to £1 14s 3d an acre, and ranges from 14s 4d an acre on the Class VII land, to £8 17s 6d an acre on the 3500 acres of strongly rolling, badly drained land with soils particularly subject to sheet, rill and gully erosion, and poorly drained naturally, which form part of Class IV. The next step is to translate the general survey into surveys of individual farms. There would be a fairly .stiff bill for some farmers to meet, but expenditure for others would be light. The Soil Conservation Council has in the past subsidised work of all the types indicated on a few. individual demonstration farms by at least £ for £.

The survey is interesting in that it is among the first in which the whole catchment of a New Zealand river has been considered as a unit, and in which a conservation plan for the whole area has been worked out. The soil and land capability maps which have been produced in the course of the survey contain a great deal of detail which will undoubtedly be of great value outside their immediate purpose.