A cousin in the UK contacted me recently concerning a major problem that UK organic gardeners and farmers are experiencing. I would like to draw your attention to it because a similar problem could occur here.
The cause of the problem is a hormone-based herbicide (weed killer) Aminopyralid, which is an ingredient in several brands of herbicides produced by Dow Agrosciences. Aminopyralid has become popular because it is only effective against broad-leaf weeds and does not kill grasses. However, the herbicide binds to woody tissue in grasses and remains active in the grass, hay and silage fed to animals. The herbicide survives passage through mammalian digestive systems and remains active in manure produced by animals that consume contaminated feed.
Consequently, the herbicide affects a range of vegetable crops planted where contaminated manures have been added to beds, or where contaminated hay or straw is used as mulch. As soil bacteria begin to decompose the mulch or manure, the herbicide is released into the soil and absorbed by roots of broad-leafed plants.
Symptoms of affected crops are dying seedlings or curled leaves and gross deformity of plants and produce. Susceptible crops include peas, beans and other legumes, carrots and parsnips, potatoes and tomatoes, and lettuce and similar crops. The affected beds remain unusable until soil bacteria have completely broken down the herbicide. As you can imagine, there are a lot of angry gardeners in England and Wales where this problem is endemic, and there are calls to for this product to be banned.
The best Dow Agrosciences can offer is: the levels of amylopyralid in crops “are unlikely to cause a problem to human health”, although the Dow website says: “As a general rule, we suggest damaged produce (however this is caused) should not be consumed.”
Originally the Royal Horticultural Society (RHS) advised allotment gardeners that contaminated soil would not be usable for two years. However, more recent information indicates that it can be used as soon as soil bacteria break down the herbicide, so the faster you can get them to do this, the better. Aerobic bacteria that break down organic matter faster than anaerobic bacteria require moisture, air and nitrogen for fast consumption. They work faster in a soil pH that is close to neutral. If soil is acidic, raise soil pH to close to neutral (6.5-7) by watering in agricultural lime. Start with a generous handful per square metre, and mix it through the topsoil. This will also aerate the soil to provide air for the bacteria. Don’t use hydrated or slaked lime (builder’s lime) as this will lose nitrogen from the soil, and the bacteria will work more slowly when soil is low in nitrogen. Check soil pH every six weeks because manures produce hydrogen ions as they break down (making soil more acidic), and turn the topsoil regularly to maintain good aeration. Keep soil just damp. If uncontaminated mulch can be sourced, it can be applied to the bed surface to maintain consistent moisture content and keep soil warmer through the colder months, especially if mulch is applied in the middle of the day when the soil is warmest.
Other herbicide problems
Other herbicides, including glyphosate, increase the risk of soil diseases by damaging the good fungi in soils that keep soil diseases under control. Recent research published by the United States Department of Agriculture (USDA), found that herbicide-resistant GM crops are causing an increase in “Fusarium” diseases, and predicted that there would be an epidemic of soil diseases and a food crisis, before long. Food crops from plants badly affected by fusarium diseases can also contain the disease fungi, causing a range of health problems, or death, as happened in Mexico some years ago when tortillas made from affected corn were eaten.
Herbicides cause more problems than they solve but many people would rather believe the advertising hype put out by chemical companies because weedkillers look as though they will save the gardener some work. (See post ‘Wilting diseases’ in the Pest-free Gardening category.)