Killer compost is a term given to compost and manure mixes that contain high enough levels of herbicides to negative affect non-target plants.

It is a worldwide problem, and has been in existence for a long while (any internet search into the terms "killer compost", "weedkiller damage", "toxic herbicide", "weedkiller in manure", or "aminopyralid") will render articles from bloggers, news hubs and universities for over least the last decade.

The herbicides, one can only hope, land up in the soil mixes unintentionally, possibly via a lack of understanding and/or labeling as to what can or should be done with the herbicides left over, the plants that the herbicide was used on, etc.

1. Pasture/Hay from treated areas being dumped or sold to green waste, or compost processing facilities.
   Herbicides such as aminopyralid, picloram, or clopyralid are often marketed for use on pasture as they target unwanted weeds without having a negative effect on the pasture grasses, or fodder brassicas.
2. The manure and urine from animals that graze on pastures treated with those herbicides are being added to compost at compost processing facilities.
   Herbicides such as aminopyralid, picloram, or clopyralid do not break down during digestion and are deposited in manure and urine, and even after going through an aerobic composting process, can still be active for up to 3 years in clumps or clods, or even a few specks of soil dust.

What about home use weedkillers?

While I initially thought that these herbicides were not available to the home user since they were banned for public use a while ago, I believe you can simply log in to a few websites and purchase them without having to prove you are a farmer, parks worker or such. I also have read articles where these herbicides are still being used as they were found in the back of the shed long ago, and people simply don't know they are not to be used on anything that may land up in central green waste plants (where a lot of compost is made).

Labeling

An article I read during my research into this claimed that products that contain these herbicides have prominent labeling indicating that the plant material on which the herbicide was used should not be fed to animals, used as mulch or deposited with green waste.

Two problems with this:
1. No one reads labels anymore

2. I found a product today on line which contains picloram which has nothing on the their product leaflet or information brochure saying that it should not be used in green waste, as animal feed, or mulch... They do say that it negatively affects other plants and that you should not use the same sprayer that had the herbicide to water your food plants... but that's not enough...

How do they work?

The hormone mimicking family of auxin herbicides is huge, and the three main sub-families of phenoxy, benzoic acid, and pyridines, all have different modes of action. In the simplest terms they are growth regulators, impacting plant processes such as vascular tissue, meristem differentiation and leaf initiation.

Basically, as far as I understand it, because the herbicide is mistaken by the plant as a growth hormone, and the affected plants can't control the use of said hormone, they grow in all sorts of overstimulated and confused ways, and eventually deformed by this, dies.

Non target collateral damage plants

Including, but not limited to:

• Solanaceae (tomato, potato, eggplant, peppers, chillies tobacco, etc.),
• Leguminosae or Fabaceae (beans, broad beans, peas, clovers, etc.),
• Umbelliferae (carrots, celery, parsley, parsnip, etc),
• Compositae or Asteraceae (artichoke, lettuce, sunflower, jerusalem artichoke, dandelion, daisies, calendula, echinacea, dahlia, cosmos, zinnia etc.),
• Vitaceae (grapes),
• Rosaceae (roses, as well as berries such as blackberry, loganberry, raspberry etc.
• and even mushrooms

For more on this, and how I've been dealing with it, see this article on Herbicide Damage, and follow #TonoliBioAssay2019 on Facebook and Instagram

Before

If you are buying soil from a bulk yard, always ask if they can confirm the path of supply did not include any herbicides or manures from animals who may have eaten herbicide sprayed pasture. They probably won't know, so this won't help...

Second best is to do a bioassay (in soil grow test) to confirm the presence of not of herbicides. The problem with this is if you do it off site, by the time the test results come back, a whole new load of compost/mix is now at the yard and may not be from the same source or batch as the lot you tested.... so not the sharpest plan either...

After

Effective Microorganisms (EM) and Soil Biology: While I investigated this a lot of ideas came forward, one being that soil microorganisms will deal to it, and this is true, but unfortunately, even if you add liters of effective microorganisms, you are probably not going to expedite the process. A clever colleague mentioned that it had already gone through aerobic composting for months while it was being made, so adding a few liters of extra microorganisms is like adding a drop in the ocean.

Biochar, Activated Charcoal: The idea is that just like activated charcoal attaches to bad things like poisons in your body, charcoal in the form of biochar will lock in the herbicides rendering them useless in the soil. I'm not sure this will work, and have not tried it, but might give it a bit of an experiment just to see. Can't find too much information on it from proper studies being done, but will see if I can find more scholarly articles.

Time: The good news is that the herbicides do break down. The bad news is that this may take up to 3 years. And in those years you should mix it up and cultivate it quite extensively, spreading it as thin as possible in order to expose it to as much direct soil, air and water. So it will take time.

IMPORTING PARTING NOTE

If you grow things in the killer compost that are not affected by it (e.g. cabbage, corn, squashes, grasses, rye etc.), do not put any of that plant material back into your own compost system. And do not feed any of your fresh produce to animals whose manure you would use in your compost or garden (chickens, quail, bunnies, guinea pigs etc)