Herbicide damage when converting to organics?

UPDATE: (June 2014)
More investigations by the vineyard manager all seem to point to the seaweed spray being associated with the symptoms. The explanation below (that the phosphate in the spray is causing the release of bound glyphosate) is still a possibility, but it is also possible that plant growth regulator-like substances in the seaweed spray are interacting with other environmental factors to cause the shoot tissues to respond, and glyphosate residue is not involved.

However, the exact cause is still a mystery!


Original post:
I recently visited a vineyard where vines were exhibiting classic glyphosate damage symptoms. Not so unusual, you say, except that this wasn't in patches - it was pretty even over the whole vineyard, spanning vines of different ages, rootstocks and varieties... Drift from another property was ruled out as surrounding vineyards were fine...

The vineyard had only recently converted to organics from years as a conventionally managed weed-free strip under the vines, created by the use of glyphosate (e.g. Roundup).

Symptoms appeared towards the end of November, after cultivation under the vines had been done. This is not the first time this sort of thing has happened (organics guru Bart Arnst has observed this sort of thing before). Could there be a link between the symptoms and the cultivation? Conventional thought is that glyphosate is rapidly inactivated when it comes into contact with the soil, so how could this be possible?

Well, that last statement is usually misinterpreted. Yes, it's inactivated, but that's not the same as being degraded and rendered harmless...

A bit of research shows that glyphosate is stable in sunlight, and tightly bound to soil particles (Rueppel et al., 1977), particularly clays and particularly in those that have high levels of aluminium and iron. Soil pH is also important, with higher adsorption rates at lower pH values (see Borggaard and Gimsing (2008) for a review on the topic). 

Soil organic matter has variable effects on glyphosate adsorption, but generally results in no change or tends to decrease it due to its blocking of binding sites in the soil (Gerritse et al., 1996), though studies exist to say the contrary (e.g. Sprankle et al., 1975).

Glyphosate can be degraded by soil micro-organisms, but generally not when it is bound to soil particles. The ability of a soil to mineralise the herbicide seems to correlate pretty well with general soil microbial activity, but few species have been identified that can directly act on the molecule itself (Borggaard and Gimsing, 2008; Forlani et al.,1999).  Because of all these variables, the half life of glyphosate in the soil varies from days to months.

The upshot of all this is that the glyphosate sprayed on the soil doesn't disappear - it is bound to soil particles and can be degraded only under the right conditions. Those conditions include levels of microbial activity, but surprise surprise, there isn't a lot of that near the surface of a herbicide-treated soil (Reinecke et al., 2008; Whitelaw-Weckert et al., 2009). Microbes need moisture, nutrients and organic matter to do well, and none of those are very common near the surface of a herbicide strip.

So this is evidence that glyphosate applied season after season is still present in the vineyard, but why does it become a problem when cultivation starts?

A little more digging (pardon the pun...) reveals that glyphosate and phosphate compete for binding sites in the soil, so adding phosphate to soil could release glyphosate (Gimsing and Borggaard,2001). This is why high phosphate soils can bind less glyphosate in the first place (Sprankle et al., 1975).

So here is a possible explanation! The vines in this case had a soil drench of seaweed extract and fish oil, which could have relatively high levels of phosphates. These could have encouraged glyphosate to de-sorb from the clay particles and be taken up by the roots.

Even then, I suspect there is an interaction with rainfall events as the symptoms appeared to stop developing, but later came back after a significant rain event. It's possible that there is renewed root growth into the herbicide-containing soil, leading to more uptake.

Curiously, reports are that the shoots arising from the base of the vine trunks show no symptoms.

As with every other season, there is at least one situation that makes you scratch your head and go, "Huh???"

If anyone has similar experiences, please leave a comment!!