The good earth: Thorpdale Red Ferrosol and chip potatoes

Australia has some of the world’s most ancient soils, many of which grow delicious produce. In this series, “The good earth”, soil scientist Robert Edis profiles some of those soils and the flavours they bring.

Given that soil types aren’t commonly top of mind for consumers, it’s a rare one indeed that features in commercial subterfuge. Yet that’s what happened recently when farmers were caught trying to boost prices by dusting their potatoes with Red Ferrosol.

Why the ruse? Well, Red Ferrosol (once known as Krasnozem) happens to be a soil with a big reputation. Tasmania elected a Ferrosol as their state soil, such is their affection for this lovely earth, and one variety was a serious contender for state soil in Victoria.

Ferrosols are old soils but still full of life. At Thorpdale the Ferrosol has formed on early Tertiary basalt (lava flows about 60 million years ago) uplifted by ongoing local tectonic instability. Over millions of years the complex reactive minerals have dissolved and simpler clays and oxides of iron and aluminium formed.

Ferrosols are deep, deep red on red soils with red bits, and under a suitable climate are unparallelled for growing potatoes. In Thorpdale in West Gippsland, Victoria, potato growers produce top quality potatoes for making chips (French fries). These soils are used for growing potatoes in other cool areas, such as pockets of Northern Tasmania, and some treasured areas near Ballarat, but most frying potatoes come from around Thorpdale. Russet Burbank potatoes are good for frying because of their low moisture content (requiring well drained soils) and high starch content (requiring a long season and plenty of available water).

There are three main reasons why these soils are so good for growing potatoes. They are well structured and fairly resilient soils - potatoes require lots of cultivation, even harvesting is a cultivation action, and these soils can stand up to a lot of activity. The large amount of iron oxides (giving the red colour) help to stabilise the soil aggregates and maintain friability (or crumbliness) and, most importantly, drainage even in the face of such abuse (within reason!).

They have a clay to clay-loam texture, with good structure. This structure holds water in a way that makes it easy for plants to get at it. It also stores nutrients reasonably well.

Additionally, Ferrosols are quite acidic. This is normally a bad thing, but in the case of potatoes the acidity helps to suppress pests and diseases.

Of course there are a few problems. Dr Mark Imhof from Victoria’s Department of Environment and Primary Industries emphasises that if the soil gets compacted by machinery or tillage, particularly when wet, its structure will deteriorate and the soil will get too strong. Strong soil reduces growth of the plant and the tuber.

Iron oxides in an acid soil develop strong positive charge and this leads to a strong affinity for phosphorus, making it hard for crops to extract the phosphorus in the soil. To grow a good potato crop, around 100kg/ha of phosphorus may have to be applied to this Ferrosol. This is not only an operational cost, and a loss of a valuable resource, it is also a danger to phosphorus-sensitive water resources (watch out Gippsland Lakes!) when the soil is eroded and unused phosphorus washes away. The clearing of the original enormous Eucalyptus forests in this area has also accelerated land slippage.

Thorpdale’s picturesque country-side, pleasant climate and its proximity to Melbourne will likely put future pressure on potato farming from more “lifestyle” land uses (think: grapevines), so now’s the time to have another Thorpdale chip.

Robert Edis receives funding from ARC, ACIAR, Incitec Pivot, IPNI, Mansfield Council