Many observers are concerned urban sprawl is sterilising prime vegetable-growing land in the peri-urban areas around our major cities. For example, The Age’s editorialist, alarmed by reports the Bailleau government is about “to extend suburban Melbourne to vegetable growing areas”, is seriously worried about food security.

The paper says agricultural land in two outer suburbs in particular — Werribee and Casey — is “in the front line of urban expansion now that the Baillieu government intends to review the growth boundary every two years.” The paper quotes RMIT planning academic Michael Buxton:

We’ve already built over the best soils in this state …Why would you keep building over it and subdividing it when in the next 50 years we’re facing an era of incredible uncertainty and major changes to climate, to food supplies and to energy markets?

Food security is an important issue in its own right but before we can begin to address how it relates to sprawl we need to get some base information about the importance of agriculture in peripheral-urban areas. As with most political discussions, the other side of the argument is often neglected.

Here are some pertinent figures, mostly drawn from an article I wrote nearly two years ago on this issue:

Moreover, according to this report, between 1976 and 2009, the area of land in Australia devoted to farming and grazing declined by 33%, while the population grew from 13 million to 22 million. Most of the reclaimed land was shifted into a “conservation and natural environment classification”.

At the same time, the productivity of agriculture in Australia has increased markedly — by 2.8% per annum over the past 20 years, double the rate at which the wider market economy grew.

Looking specifically at Victoria and Melbourne, it can be deduced from a detailed study of the value of agriculture production in peri-urban areas by Peter Houston, that the area of land used for agriculture in Melbourne’s green wedges comprises just 1.7% of all agricultural land in Victoria.

Most of the expansion of Melbourne is not in any event at the expense of prime agricultural land. That’s because the major part of the land in Melbourne’s vaunted green wedges is used for non-agricultural purposes. These purposes include airports, sewage works, prisons, sporting facilities, quarries and more.

The major uses though are extensive areas of protected natural bushland, particularly in the east, as well as semi-rural uses like so-called hobby, lifestyle and part-time farms.

Less than 10% of land in the Mornington Peninsula, northern, and Sunbury green wedges is used for productive agriculture. In the Yarra Valley it’s 12%. Only the western and south-eastern green wedges have a significant proportion — about a third — devoted to productive agriculture.

With non-agricultural uses occupying two thirds or more of the land in the green wedges, it seems likely most of the impact of outward expansion will fall on hobby, lifestyle and unproductive “farms”.

It can be argued that agricultural land in the green wedges is extremely productive — although it accounts for only a very small proportion of all farming land it accounts for about 12% of the state’s agricultural production. Houston argues that’s true of peri-urban areas around all capital cities in Australia, so it’s unlikely to be because of some special or unique attribute like soil quality.

Rather, the higher productivity is probably due to its proximity to the metropolitan area. Nearness appears to increase agricultural productivity by providing better access to skills, capital, technology and markets (including tourists).

The implication is the peri-urban zone of higher productivity will expand with the city as it sprawls outward. That’s what’s happened historically — well before existing peri-urban agricultural areas were established there were highly productive market gardens and orchards closer to the city centre that were swallowed up by urban development long ago.