The fate of pesticides in soil

[Feldspar]

I'm not quite sure where this thread should reside. However, given the creation of this new buzzword laden subsection, it seemed rude not to put something in it.

Increasingly within the discussions of regenerative / conservation / hollistic / verging on organic agriculture there seems to run an undercurrent of assumptions about the effects of pesticides upon the soil and upon soil biology. I wanted to fact check those assumptions and discuss and understand the extent to which those effects are positive or negative.

Knowing my predilections you will be unsurprised to read that I think a discussion on this topic is going to have to draw quite considerably from academic literature. However, I'm sure @Clive will be able post up pictures from his peerings down the microscope tube to add a on-farm dimension.

So, to start, what do most people think they know about the effect of pesticides on soil and soil biology?

I think the common ones I've heard of are these:

1. Glyphosate makes fusarium worse (probably coming from Don Huber).
2. Residual chemistry creates a toxic soup which is bad (in some unspecified way).
3. Insecticides (particularly Dursban) wipe out soil life including beneficials.
4. Fungicides kill beneficial fungus living within the soil which vital for a healthy soil (Elaine Ingham).
5. Fertilisers massively upset the soil biology balance and this is not good.
6. Ferric phosphate is much better than metaldehyde as it doesn't kill slug predators like ground beetles.
7. Some pesticides hang around for ages and this is bad.

 

[Andrew K]

Metaldehyde doesnt kill ground beetles IMO.
Methiocarb does[or did rather]

 

[The_Swede]

Potentially a very useful thread here - great to get information (or at least opinion / experience) all in one place as such...

 

[Feldspar]

This is going to come in a very piecemeal and as-and-when fashion, but here goes: one of the first things that I've found are that some pesticides can very considerably increase the amount of particular types of soil biology as opposed to decreasing it (http://www.sciencedirect.com/science/article/pii/0038071794902739). Whether that's a good or bad thing is the obvious next question.

 

[SilliamWhale]

Look at the EIQ of each active ingredient. And the Eiq of other non pesticide agricultural practice's too

 

[Simon C]

Agree with points 1 to 7.

It is purely my own opinion, but I would have a bet that herbicides, including glyphosate, are a major cause of the ever increasing problem of blackgrass. I think they are modifying the balance of soil biology to perfectly suit the weed. Most weeds only germinate when the right conditions are present in the soil for them to proliferate and produce the maxim amount of seed, criteria would include moisture, temperature, nutrient status and biological balance, (bacteria/fungi ratios etc).

As with so many of out arable inputs, the more you use, the more you need. Examples that we sort of know about are nitrogen wiping out free living N fixing bacteria, insecticides killing predators, fungicides stripping wax and defensive B and F from plant leaves and even all chemicals that modify our plants optimum nutrient balances, rendering them vulnerable to insect and disease attack. Given time I could think of a few more examples, but you get the idea.

Another piece of evidence that I have seen on this farm to back this up is that sometimes, when I apply some really good compost which gives the soil a big biological boost, blackgrass almost disappears. Biological conditions become completely unsuitable for it.

So it seems to me that is almost inevitable that the short term cure for blackgrass is actually the long term cause.

 

[Feldspar]

I'm going to also throw out one question / entirely unrehearsed idea which relates to Simon's point about black grass. One of the first bits that I read on this topic was this paper: http://www.researchgate.net/profile...s_a_review/links/00b7d52dd2f2329e8c000000.pdf

Whilst it doesn't entirely get to the heart of the idea, there was enough in there to stimulate a possible line of inquiry. The idea is that digestive ability of soil to breakdown pesticides increases with increasing amounts of the applied pesticides and also increases with a more biologically active soil.

So, just as an example, it might be that when you first apply a residual BG chemical it is entirely new to the soil and so there are no microbes that can break it down. It therefore remains in the soil (in various guises as that paper explains) for enough time to kill most of the BG. Once applied though this 'food' (because it is a type of food) stimulates the growth of microbes that break the chemical down into an inactive form. The more chemical you put on, the more this stimulates the relevant microbial growth, and the quicker the chemical is broken down.

My question is what is the longevity of these digesting microbes in the soil, and what is the extent to which they affect the half life (or whatever the right measure is of longevity of active / useful chemical in the soil) of the residual pesticide. Do they, for example, remain around long enough that when you come to grow a second wheat they're still there and at a higher level than in the unprimed soil. If so, do they then breakdown the pesticide more quickly so that it doesn't work as well a second time around.

It's often said that residual chemistry doesn't work as well on 'meadow soil' or on soils with high SOM. Might it be that these soils with high levels of biological activity have a heightened ability to bread down and degrade residual chemistry such that it reduces its effectiveness?

This thought also extends to fungicides. If you look here: http://www.sciencedirect.com/science/article/pii/0038071781900110 you see that these fungicides did indeed wipe out some of the fungi in the soil. I wonder though, if the soil's stomach contains the right bacteria, could these fungicides be broken down fast enough that it would be possible to use fungicides without having a major and lasting effect on soil fungi and the fungal : bacterial ratio?

 

[SilliamWhale]

And some counter arguments against eiq

http://weedcontrolfreaks.com/2015/07/an-evaluation-of-the-environmental-impact-quotient-eiq/

 

[Simon C]

I'm going to also throw out one question / entirely unrehearsed idea which relates to Simon's point about black grass. One of the first bits that I read on this topic was this paper: http://www.researchgate.net/profile...s_a_review/links/00b7d52dd2f2329e8c000000.pdf

Whilst it doesn't entirely get to the heart of the idea, there was enough in there to stimulate a possible line of inquiry. The idea is that digestive ability of soil to breakdown pesticides increases with increasing amounts of the applied pesticides and also increases with a more biologically active soil.

So, just as an example, it might be that when you first apply a residual BG chemical it is entirely new to the soil and so there are no microbes that can break it down. It therefore remains in the soil (in various guises as that paper explains) for enough time to kill most of the BG. Once applied though this 'food' (because it is a type of food) stimulates the growth of microbes that break the chemical down into an inactive form. The more chemical you put on, the more this stimulates the relevant microbial growth, and the quicker the chemical is broken down.

My question is what is the longevity of these digesting microbes in the soil, and what is the extent to which they affect the half life (or whatever the right measure is of longevity of active / useful chemical in the soil) of the residual pesticide. Do they, for example, remain around long enough that when you come to grow a second wheat they're still there and at a higher level than in the unprimed soil. If so, do they then breakdown the pesticide more quickly so that it doesn't work as well a second time around.

It's often said that residual chemistry doesn't work as well on 'meadow soil' or on soils with high SOM. Might it be that these soils with high levels of biological activity have a heightened ability to bread down and degrade residual chemistry such that it reduces its effectiveness?

This thought also extends to fungicides. If you look here: http://www.sciencedirect.com/science/article/pii/0038071781900110 you see that these fungicides did indeed wipe out some of the fungi in the soil. I wonder though, if the soil's stomach contains the right bacteria, could these fungicides be broken down fast enough that it would be possible to use fungicides without having a major and lasting effect on soil fungi and the fungal : bacterial ratio?

It's been known for a long time that the more pendamethalin you use, the quicker it breaks down due to increase in bacteria that like eating it. Same would apply to all chemicals, I expect.

 

[The_Swede]

Another case (not that it was needed) for a diverse and wide rotation it would seem.

 

[Feldspar]

It's been known for a long time that the more pendamethalin you use, the quicker it breaks down due to increase in bacteria that like eating it. Same would apply to all chemicals, I expect.

Ah, but would it? That's something to find out. More digging required. It's plausible at the very least that some are more digestible than others.

 

[Andy Howard]

At Simon Chiles open day there was a company there that had list of chemicals and there effects. On a train at the moment so can't find the company

 

[shakerator]

[QUOTE="Simon C, post: 1786374, member: 319]

So it seems to me that is almost inevitable that the short term cure for blackgrass is actually the long term cause.[/QUOTE]

Have always thought the cure for blackgrass is to let blackgrass grow uninterrupted for 3 years and let it heal the soil, fighting it is like fighting cancer, hope to kill first before healthy cells (ie crop)

SU herbicides are diabetic drugs ( Google sulfylureas and you'll find herbicides play second fiddle to big pharma)

Big pharma + Big Farmer = good business model for BASF BAYER DUPONT etc

 

[Feldspar]

We actually had one extended overwinter stubble field which we failed to spray in May meaning the black-grass (and there was quite a bit of it) was allowed to seed. I thought it was going to be a disaster having that level of seed return. However, we left that field as the same ELS option for another year and let the black-grass grow again. After spraying it off in time the next year in May we then direct drilled spring wheat into it the following spring and it's been no dirtier than next door fields which didn't go into extended overwinter stubble.

 

[shakerator]

We actually had one extended overwinter stubble field which we failed to spray in May meaning the black-grass (and there was quite a bit of it) was allowed to seed. I thought it was going to be a disaster having that level of seed return. However, we left that field as the same ELS option for another year and let the black-grass grow again. After spraying it off in time the next year in May we then direct drilled spring wheat into it the following spring and it's been no dirtier than next door fields which didn't go into extended overwinter stubble.

Try oats
http://thefarmingforum.co.uk/index.php?threads/blackgrass-allelopathy-via-wild-oats.25306/

 

[Feldspar]

Try oats
http://thefarmingforum.co.uk/index.php?threads/blackgrass-allelopathy-via-wild-oats.25306/

Who do you sell them to? Also, do the oats have an allelopathic effect upon the wheat?

 

[shakerator]

Who do you sell them to? Also, do the oats have an allelopathic effect upon the wheat?

Yes

Spring wheat no tilled after oats drilled on green into oat volunteers. Big rooting impact on wheat due to acids, but even more on small seeds. Result better than herbicide stack. Should have been beans really. But I've got greedy and gone into winter wheat this year, results look promising! Some BG but not worth Atlantis check. May use spring axial but better for soil to let wild oats seed IMHO as cover crop then into beans next yr (More alleopathy no real WO resistance to decent accase or kerb)

Oat contract with glencore to morning foods. Linked to LIFFE not sure if much tonnage left

 

[Feldspar]

Yes

Spring wheat no tilled after oats drilled on green into oat volunteers. Big rooting impact on wheat due to acids, but even more on small seeds. Result better than herbicide stack. Should have been beans really. But I've got greedy and gone into winter wheat this year, results look promising! Some BG but not worth Atlantis check. May use spring axial but better for soil to let wild oats seed IMHO as cover crop then into beans next yr (More alleopathy no real WO resistance to decent accase or kerb)

Oat contract with glencore to morning foods. Linked to LIFFE not sure if much tonnage left

Did you use any residuals on the winter wheat?

 

[shakerator]

Did you use any residuals on the winter wheat?

No

 

[newholland]

There is a lot of modern food, fruit and veg which are in reality "empty calories" - as in they do not contain the correct / full range of minerals.
Do you see much black grass on a correctly managed, 20 year experienced / professional organic farm? We are organic and some of our crops are considerably cleaner than our conventional neighbours, but we have plenty of other disasters happen.

I have enjoyed seeing this thread and will read with interest. I have very little knowledge, so can't really help, but am very interested.