5 August 1996 Let's protect Canterbury's high class soils for food production

Recent predictions of an expanding population in Christchurch to about 350,000 people in the next 20 years have intensified debate on whether the City has sufficient land to accommodate such growth.

An expanding city inevitably encroaches on land in its periphery. Christchurch was founded 147 years ago on land containing good soils. Areas adjacent to the Christchurch City contain some of the most versatile land and elite (i.e. high class) soils in the country. Should these soils be used for urban subdivisions and development instead of food production? Is there a case for the protection of high class soils for food production in Canterbury and in New Zealand?

Proponents supporting the use of high class soils for urban development argue on economic and technological grounds. Rising demand for housing and high section prices were cited as justifications. Another argument was that the application of new technology such as irrigation can make formerly poor quality soils more productive, thus expanding rather than shrinking the stock of good agricultural land. For example, stony soils, generally regarded as poor soils, have been found to be best for growing grapes.

The Environment Minister Simon Upton mentioned at last year's Planning Institute Conference that there was no longer any justification for protecting high class soils, other than from land degradation. The Minister did not believe that the provisions in the Resource Management Act 1991 for "sustaining the potential of natural and physical resources to meet the needs of future generations" or "safeguarding the life supporting capacity of soil..." provide a reason for protecting a unique and finite resource for primary production purposes. In his view, "the justification needs to be a great deal stronger than simply saying that soil is suitable for food production and therefore should not be used for anything else".

Furthermore, the Minister suggested that "if people are worried about the productive value of soils ... they should reflect that the market is very good at determining whether it will be used for that purpose".

Such arguments are not unexpected because they are largely defined from an urban perspective, giving equal or less weight to food production compared to urban uses. However, it shows a lack of appreciation of the limitations and potential of soils and the different specialised requirements for different agricultural and horticultural crops. Although stony and shallow soils are suited for growing grapes, they are unsatisfactory for growing cereals, fruit trees or root vegetables and wine cannot be a substitute for staple food.

Irrigation may improve crop and pasture yields on poor soils but water is a finite resource deserving conservation. Large-scale irrigation has seriously impaired soil quality such as induced salinity in many parts of the world. In the past in New Zealand soils were selected for irrigation because they could easily be reached by water races from the nearest river. Many of these soils are shallow and stony prone to excessive water losses and nutrient leaching.

High class land has the unique ability to support actual and potential production of a wide range of crops on a sustained basis because the land is relatively flat, easily accessible and manageable by farm machinery, less erodible and contains high class soils. These soils possess high inherent fertility, active soil biological activity and excellent intrinsic soil physical attributes such as good water retention, soil aeration, adequate soil depth and friable consistence to sustain good plant top and root growth.

Some of these desirable attributes (e.g. adequate soil depth) cannot be replaced in practice by technology without considerable expense in both money and energy. Furthermore, high class soils maximise energy-use efficiency in primary production as they require lower inputs (e.g. fertilisers) per unit of output compared to poor soils.

The present world population stands at 5.6 billion and with 15 people born every 5 seconds there will be one billion more mouths to feed in the year 2005. Thus, the demand for food will continue in the long term.

In New Zealand, the present differential rating system for urban and rural land does not provide any protection to high class soils. It only fuels the land market and subdivisions of farmland. The approach of urban development usually raises the price of land on the urban fringe in anticipation of zoning changes to urban use. This makes farming activities more difficult to sustain and at the same time, offers the owner of the farmland a handsome capital gain, should the farmer decide to sell out. There is every incentive to sell. This reflects the profit motive syndrome driven by our free enterprise market forces. It is very difficult to visualise how present market forces can predict the needs of the future generations.

Urbanisation cannot be dismissed lightly as a competitor for land as there is a real limitation on the supply of high class soils. These soils do not exceed 3% of the area of New Zealand. The Canterbury region has about 294,000 hectares of versatile land, scattered throughout the region. The present rate of conversion of rural land to urban use is not precisely known although Christchurch City is estimated to convert annually about 100 hectares of land to urban uses.

It is generally recognised that once this land is used for urban development, its loss is irreversible and permanent. Some may argue that this loss can be reversed. I have yet to see cities and towns being dug up to grow food for their inhabitants. Even if previously used urban soils are excavated, they are probably too contaminated with toxic residues of vehicle fuels (e.g. lead) and other urban wastes to be safe for food production. In many land-scarce countries such as Singapore and Japan, food is almost totally imported or grown in water­ culture technique with chemicals known as "hydroponics". This system is not only very costly, but introduces a strong feeling of "chemophobia" to its consumers.

Christchurch City is fortunately endowed with a wide range of soil types with close proximity to each other. Within the city urban fence a large area of soils of low value for food production occur. These soils are stable, well-drained with non-swelling clays, reasonably level with good access and are best suited for urban development. These alternative sites should be given priority for urban development.

One may argue that to restrict future urban development to areas of relatively poor soil is to deny the urban population the opportunity of developing and maintaining a "garden city" environment. Surely not all the present best gardens in Christchurch are on high class soils. After all, a good garden is often the result of the deep devotion, patience, energy, innovativeness and time spent by the gardener on a relatively small piece of land compared to the farm.

The New Zealand Soil Science Society President, Dr Les Basher, should be commended for recommending recently to the Environment Minister the introduction of a National Policy Statement for future management of high class soils. However, is another policy statement or Act sufficient to protect this finite natural resource?

The intent of the Town and Country Planning Act (1977) was to give protection to land "of high actual and or potential value for food production". Although this Act has achieved a number of instances of significant protection of such land, the loss of first-class soils to subdivisions and lifestyle blocks continues in Canterbury and other parts of the country as evident to any observer who cares to take a drive to the urban fringes of Christchurch.

I believe we need more than Acts and policy statements to protect our high class soils. Perhaps, a regional or national 'land bank' be established to purchase and protect high class soils in Aotearoa from being taken out irreversibly from food production.

Kuan M. Goh is the Professor of Soil Science (Personal Chair) at Lincoln University, Canterbury, Fellow of the Royal Society of New Zealand and is the author of the popular garden book "An Introduction to Garden Soils, Fertilisers and Water".