Erosion, irrigation, & growth: comparing organic & conventional

By Max Marriott

Assistant Vineyard Manager, Gibbston Valley Wines School House Vineyard

The latest field day at the Central Otago Focus Vineyard on Wednesday the 7th of March followed on from a series of rainfalls in January and February. Almost half of the annual rainfall for the region occurred over six weeks, with three particular events in the middle of January, end of January and end of February. At the field day (after 10mm of rain that morning) participants were able to witness the level of erosion and lack of water penetration that is occurring on the conventional blocks, particularly on the sloping top block.

Silt from runoff in conventional vines

The photos illustrate this, with a clear run-off occurring through the lower headland areas off the conventional blocks. This contrasted with the organic blocks, where the worked soil of the undervine cultivation and increased ground cover have been responsible for greater water retention. This was a little at odds with people’s perception of the gutters that tend to be created from the undervine mounding, however the blading essentially fills these in and as long as the cultivation doesn’t occur prior to such a rain event, the effects are negligible.

Irrigation was a sub-theme for the day, and despite very little need for irrigation over the past two months, water analyses by Tony Davoren at Hydro Services showed that there is significantly greater water penetration into the subsoil profile of the organic blocks already (particularly after the larger, soaking rain events). He cited that it is perhaps too early to simply pass this off as due to organic management (organic matter build-up and soil structure enhancement takes years), though the fact that the soil had been worked and that “seal” broken was clearly contributing to the effects. It has been interesting to note over the course of the growing season that smaller rain events are having less impact as that topsoil ground cover helps take up the available water.

Colour differences in organic (left) and conventional (right) vine rows

It has also been evident as the growing season progresses – and the photos illustrate this as well – that the organic blocks have shown a slightly slower growth pattern than the conventional blocks. The water availability – and irrigation regime – has been largely comparable for both blocks, which indicates the slight reduction in vigour is a result of the change in regime. The colour of the canopy between the two blocks is also noticeably different, with the organic block a slightly lighter green. On discussions with the likes of Bart Arnst and Blair Deaker, who have been involved with established organic vineyards for some time, they indicated that this was a symptom that they also noticed in the first years of organic conversion; almost like a stress mechanism, potentially due to the root-cutting effects of the undervine cultivation.

The second fertigation round was applied in January and Kristalon Orange was applied to the conventional block instead of calcium nitrate to help with replenishment of micronutrients. Veraison began in early February and was stalling somewhat, however the rain events helped to kick it along. All blocks were trimmed once in February, though it was only the hollows and sections on heavier soils that really needed the work. Another round of undervine cultivation was done on the organic blocks and another round of weed spraying on the conventional blocks prior to the nets going on. Apart from some net-mending and green thinning, all is ready for harvest in mid April.

Vineyard manager Grant Rolston shows the conventional block’s runoff levels, pushing a finger into silt

March 7th field day


Conventional canopy

Organic canopy