Ξ February 24th, 2009 | → 2 Comments | ∇ A Day at a Time, International Terroirs, Interviews, Technology, Winemakers, Wineries |
Over the next week I’ll be posting a series of interviews on the subject of vineyard soil. I’ve chosen three prominent figures each operating from a different perspective, that of the soil scientist, a non-interventionist winegrower and a vineyard experimenter. The first is with Zed Rengel, Professor of the School of Earth and Environment, of The Centre of Land Rehabilitation and the Integrated Land and Water Management Program Leader in the Institute of Agriculture, all within the University of Western Australia. The second interview to be posted will be with Oregon’s Jason Lett winegrower for Eyrie Vineyards. The third is with Swiss winegrower Hans Peter Schmidt of Domaine de Mythopia.
These three perspectives offer interesting points of agreement and divergence. However, it is important to stress that owing to the daunting complexity of the subject of soil each gentleman’s remarks are necessarily incomplete, a feature they each insist on. Intellectual honesty requires nothing less. These are interviews, conversations, after all, not technical research papers or philosophical summations of a life’s work. So they must be read as modest gestures, each pointing to important areas of inquiry and reflection. That said…
James E. Wilson writes in his fine book Terroir, The Role of Geology, Climate, and Culture in the Making of French Wines writes,
“The really puzzling question is: how do these mineral elements get from the soil into the plant system? The response that they are taken up by the roots does not explain how they got into the roots. The answer varies from complicated to mysterious to essentially unknown. Any attempt to explain the process has to get down to some pretty fine points – down to the atomic level, as a matter of fact.”
This is a large part of Professor Zed Rengel’s research program at the U of Western Australia (UWA), a program to explain Wilson’s ‘mystery’. Prof. Rengel was first brought to my attention by Prof. John Watling of the Chemistry and the Forensic Science depts. of the UWA. As readers of my two-part piece Forensic Science and Wine Fingerprinting pt 1 and pt 2 know, Prof. Rengel is a member of Prof. Watling’s forensics research group. Being a chemist by training, he deferred to Prof. Rengel’s expertise on a number soil related questions. Hence, I wrote him. Indeed, whereas I enjoyed a telephone conversation with Prof. Watling, I exchanged emails with Zed Rengel.
Admin How do microbial populations and varieties change with respect to soil depth?
Zed Rengel Soil microbes are prevalent where there is organic matter. In Australian soils, organic matter is present in appreciable amounts only in the top couple of centimetres of soil; hence, strong microbial presence is limited to that layer. Little microbial activity can be detected deeper. However, in the fertile, deep soil profiles, intensive microbial activity may be detected even at 1/2 m depth providing there is sufficient organic matter there.
Are there specific soil microbes associated with grape vines? I am thinking of the example of fungi symbionts and trees.
ZR Outside my expertise.
Do the varieties of wild yeasts vary from region to region in Australia?
ZR Outside my expertise.
Is there resource competition among species of soil microbes in a vineyard? And can viticultural practice alter microbe populations?
ZR Resounding yes to both questions. There is a huge number and a variety of microorganisms in soil – it may be approximately the same number of microbial cells in a gram of soil as all the people on the planet (and in some soils microbes will have several orders of magnitude advantage).
Regarding variety of microorganisms (how many different species?), no one knows the answer and estimates may vary widely. Nevertheless, all estimates deal with huge numbers. Different species have different “food requirements and preferences”, so depending on viticultural practices some species and even groups of microorganisms may be either favoured or disadvantaged. However, there is “functional redundancy” in soil microbes, meaning that if some species and groups get knocked out there is plenty of similar ones that can do the same job.
Do insecticides and herbicides have any effect on microbial populations in the soil?
ZR The answer is definitely yes, but which ones and under which conditions would require an extensive literature search.
Do higher and varied concentrations of microorganisms in the soil play a role in the availability of elements a vine takes up?
ZR Yes because microbes decompose organic matter and in the process release nutrients from that organic matter. There is competition for these nutrients between microorganisms and plant roots: there are many facets to this competition, but the bottom line is that microbes are more effective competitors for nutrients than roots. It is only when microbes start running out of food and their numbers start declining that roots have a decent chance to get their share.
It is often said a vine needs to struggle to produce the finest fruit. Often that struggle is facilitated by deficit watering, planting in poorer soils, etc. Now, one would assume that ‘poorer’ soil means not only one of a low organic matter content but also of a correspondingly more modest microbe population. Is it possible that too much organic matter added to a vineyard soil, such as you might find under an organic regime, might actually cause a decline in vine vigor and grape quality?
ZR Yes, because lots of organic matter will cause explosion in microbe numbers, they will mop up all nitrate available, and plants will struggle. It will only be when microbes start declining because they’ve eaten most of the food that there will be nitrate left to plants to take up. This is the balance: it is always up and down (and the magnitude of changes may be substantial) – there is no balance where things stay at optimal without peaks and throughs.
What might be a definition of soil health?
ZR There are lots of definitions or descriptions of what a healthy soil is, but all of them have shortcomings. Healthy soil for an agriculturalist as opposed to an environmentalist is obviously a different thing. We should be thinking along the lines of soil functions (is soil doing what it is supposed to do) and resilience (if we hit it hard, is it going to be impacted a lot or not).
What is the difference between an ‘agriculturist’ and an ‘environmentalist’?
ZR I would like to think NONE, but in reality, agriculturalists want to make money off the land and environmentalists want just to enjoy it for its natural beauty and value.
Is microbial species diversity an index of soil health?
ZR Many scientists would like to believe so. Greater microbial diversity generally means greater effectiveness in soil functions and thus greater soil resilience. However, practical indices that would link microbial diversity and eg. soil productivity are not there yet.
In the US we have what are called Super Fund sites, some are places of extreme soil damage. One of the techniques to leach from the soil certain toxic contaminants is the planting of beets and other root crops. With respect to the ‘fingerprinting’ of wines and their associated vineyards, do vines, over time, alter the concentrations of metals in their soils?
ZR Generally, the total amounts of metals (I mean nutrients like zinc, manganese, copper, iron, etc.) in soils is relatively large compared with plant needs. So, it would take many vegetations without any external inputs for a change in soil metal content to be noticeable. In terms of contaminant metals, a vine is a plant not particularly keen on taking them up (if contaminant metals are present in high concentration, the vine will die). Hence, a vine would not be a plant of choice for contaminated land.
With respect to a vine root’s uptake of metals, how do grape bunch concentrations of trace metals differ from that of the roots, if at all?
ZR Trace metal concentrations tend to be higher in roots than in vegetative or generative (fruit) tissues (with some exceptions in case of trace metal /nutrient/ deficiency). In case of metal toxicity, plants generally try to minimise transport of toxic metals into above-ground parts, resulting in relatively high concentration of such metals in roots.
May a soil scientist, based on a vineyard’s soil elemental profile alone, determine whether it is under an organic or conventional farming regime? If not, why might that be?
ZR I do not think so. Traces of herbicides and other pesticides in soil may be a distinguishing factor. Ditto for the amount of organic matter. However, regarding soil nutrients, I do not believe it is possible to distinguish organic from conventional enterprise.
If you are familiar with what is known as Biodynamic viticulture, what is your evaluation of the practice?
ZR Yes. Scientific backing of these practices is just about non-existent, but I have seen biodynamically managed fields that looked very good, and farmers were making a profit (including one viticulturalist).
What is your take on biochar as a technology of carbon sequestration and, more abstractly, what James Lovelock has called ”our only hope”?
ZR Biochar has a potential to be very useful in C sequestration and also in improving soil properties. There is lots of work currently going on around the world (including in my own Department) on assessing what biochar does and where it sits in the bigger scheme of things re C sequestration and energy balance. So, for me, the jury is still out there (but I tend to be conservative rather than a person who waves the flag on a train that is going into the future without assurances that the railroad has been laid down).
There are other people in my Department that have done a bit already, and they are in the final stages of signing up on a major research project with a national agency. So, more activities in the future, but relatively little so far.
Thank you, Zed.