Know Your Manure!

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The gardening season is about to shift into high gear as the last threat of frost for the foothills fades. Most veteran gardeners understand that one of the keys to long term success is good soil management. ‘Feed the soil not the plant’ is a gardening mantra that continues to hold true. However, this green thumb axiom does come without a caveat – know the source of everything that you put into your garden. Some sources of organic materials including hay, grass clippings, and manure can be good sources of organic matter and nutrients but they can also be the carriers of undesirable contaminants such as weed seeds, heavy metals, and certain herbicides.

It is not an uncommon call for the Cooperative Extension Service to receive every year.

“My tomato plants have suddenly begun to turn yellow and die. This ‘disease’ is making the leaves curl up and twist with swollen stems. What can I do?”

Following a cursory investigation an agent can often discern herbicide damage. The gardener is almost always very surprised, often reaffirming their efforts to grow chemical free vegetables. Unfortunately, herbicides can be unknowingly brought into your garden if you don’t investigate the source of added soil amendments.

Many livestock producers in our area struggle to grow vigorous grass forages while keeping many broadleaf weeds in check. Herbicides are a crucial technology to grow healthy pastures while eliminating poisonous weeds that can harm or kill livestock. Likewise, many homeowners spray (or use a lawn service that sprays) products to kill chickweed, clover, dandelions and other turf invaders. One of the most common herbicides on the planet, effective at killing many broadleaf weeds without harming grasses, is a group of herbicides that mimic the plant hormone auxin. These growth hormone herbicides include 2,4 D; dicamba, picloram, clopyralid, and aminopyralid. Auxin mimic herbicides, sometimes called growth regulator herbicides, cause many non-grass weeds to grow in uncontrolled and damaging ways. Uncontrolled cell division can block the water and nutrient transport system of the plant. The plant’s internal hormone system controlling key processes becomes completely disrupted and plant death follows within a week or two. While many pesticides are completely broken down within a few weeks in sunny, warm, and moist conditions; some auxin mimic herbicides are very persistent. In some cases, these residues can have some activity for years. Additionally, some herbicide residues can remain on hay consumed by horses and livestock. These residues are almost entirely passed through the animal in urine and manure, remaining active in some cases even after composting the manure before use.

The only way to protect your garden from a chemical contamination disaster is to always ask about the herbicide history of manure (hay and pasture eaten), grass clippings, hay, and compost. If in doubt about the material you can utilize a simple bioassay test. Fill five pots with regular potting soil and five identical pots with potting soil mixed with the questionable organic material (be sure to take small amounts throughout the pile and mix well for a representative sample). Plant 10 bean seed in each pot and evaluate any differences over a three week period. Be sure to keep the pots separate enough that drainage water cannot mix between pots. Stunted growth or death of bean plants grown with questionable organic material is a good indication of contamination. Suspect material can be spread on other turf, pasture, or grass crops but do not risk using it in a vegetable garden, even after long term composting. Adding more soil organic material is an effective remedy for many garden problems. Take the time to investigate sources of these materials and avoid unintended herbicide contamination. For more information on specific herbicide products of concern read this article on-line: Be sure to visit the Catawba County Cooperative Extension Service website and sign up for our monthly update of events, workshops, training courses, and fun activities with 4-H, local food, gardening, agriculture, pest management, Eat Drink and Be Local and more: