Gardening with Xpress: Soil bacteria and nutrients ensure successful bean plants

SOIL SUPERHEROES: Beans, peas and other plants in the same family grow nodules on their roots that allow them to partner with soil-borne bacteria to turn atmospheric nitrogen into plant-available nitrogen, or nitrate. Photo by Adobe Stock

Happy summer, mountain gardeners! It’s finally gotten hot for real, encouraging midday breaks by the banks of shady creeks and streams. Hopefully, y’all are caring for your bodies, along with your plants, as the season ripens. Send in any gardening questions that have sprouted up to, and I’ll be happy to share my thoughts next month.

Heading off problems with bean plants

My green beans are looking a bit yellow and not growing well. I thought beans generally grew well, even in poor soil. What’s going on?

It’s true that beans (along with other plants in the Fabaceae family, like peas, clover, alfalfa and vetch) have a fabulous ability to thrive, even in soils that are poor in nitrogen. These “nitrogen fixers” have a symbiotic (cooperative) relationship with soil-borne bacteria (Rhizobium leguminosarum) that turn atmospheric nitrogen — dinitrogen, which plants can’t use — into plant-available nitrogen, or nitrate.

How it works is that the plants grow nodules on their roots that house the bacteria and feed them with sugars made through photosynthesis. In return, the bacteria “fix” nitrogen into a form the plants can use.

It’s a beautiful example of how the living world generally thrives through complex cooperation rather than crude competition as many of us have been taught to believe. There are some nonleguminous plants that also fix nitrogen through relationships with different microbial helpers, but in the garden, legumes are the group we can depend on for this collaboration.

The magic of this symbiosis is only possible, however, when the specific kind of bacterium that can perform it is present in the soil. If you’re starting a garden in a new area or are growing in containers with a soilless medium, you may not already have thriving populations of Rhizobium leguminosarum ready to greet your bean seeds.

But don’t worry! It’s simple to add in these good guys that support bean growth by coating your seeds in an inexpensive and readily available legume inoculant powder (Exceed is one popular brand). In fact, some folks suggest inoculating bean and pea seeds every time you plant, even if you know that your soil is hosting these bacteria already. To do this, soak or moisten seeds with water just before planting, sprinkle on some of the inoculant, stir the seeds around so they’re evenly coated, then plant them.

The lack of this bacterial buddy seems to me the most likely culprit for poor bean growth, though there could be other reasons. Nitrogen isn’t the only nutrient beans and all veggies need to thrive. Other macronutrients that they need a lot of are phosphorus and potassium. And along with these, there are at least 10 micronutrient minerals that are like vitamins for plants.

A soil test is the best way to find out if your soil is deficient in something. However, simply using an all-purpose fertilizer that also contains micronutrient minerals could help.

If voles or other critters are nibbling the roots of your beans, those plants will obviously not be loving life, so dig around to make sure that’s not the problem. Finally, fungal and viral diseases can inhibit growth. See last month’s garden feature at for more on preventing and diagnosing these.

Making the most of manure

I have access to some fresh horse manure from a neighbor. Is it OK to put it straight into my garden?

Horse poop is considered a “hot” manure and will burn plants if it’s applied fresh. To use any manure safely, it’s ideal that you either age or compost it first. This is absolutely essential with hot manures, like horse, and optional with rabbit, goat and llama manures. Often, manure sourced from a farm has already been aged, so make sure to ask the farmer how old the manure is and if it has been actively composted.

One way to age manure is to dig it into garden beds in the fall, then wait to plant anything there until spring. Another option is to build a compost pile with the manure and other inputs, such as straw, then turn the pile a few times — periodically spreading it out and then heaping it up again will add air and stimulate biological activity to speed up the process of breaking down the manure.

The final alternative is to just let the manure sit in a heap for four to six months. This option will result in high nitrogen loss due to runoff from rain, so if you choose this low-labor approach, it’s helpful to cover the manure pile with a tarp to reduce the loss of nitrogen.

Another issue with horse manure can be weed seeds. Horses don’t have four-chambered stomachs like ruminant animals such as cows and goats, so weed seeds are more likely to pass through their bodies unfazed. It can be quite disappointing to import beautiful fertility in the form of manure, only to discover that you’ve also introduced hundreds or thousands of seeds from potentially noxious weeds that weren’t previously present in your garden.

Very active, hot composting can destroy some weed seeds, but it usually won’t render them all incapable of sprouting. One extra step that can be worth the effort is to spread out some manure in a pot or flat and water it for a few days or a week, which basically allows you to take an inventory of the weed seed bank it contains.

If, after a week or so, not too many plants sprout up, your manure is fairly clean. On the other hand, if the whole thing is covered in plants like spiny amaranth (my nemesis weed!), it may not be worth the additional nutrients the manure would add to your soil.

Finally, with horse manure especially, you may inadvertently import herbicide residue with it. Much of the hay and forage that horses eat are sprayed with toxic chemicals that kill broad-leaf plants — everything that isn’t grass. If you can find manure from organically raised animals, that’s ideal. If not, it’s still worth inquiring what kinds of herbicides were used on both pasture and hay that the animals have eaten.

One class of herbicides in particular — those containing active ingredients in the pyridine family, especially aminocyclopyrachlor, aminopyralid, clopyralid, and picloram — can be very persistent in manure, even through the composting process. Unfortunately, they are the ones most commonly used on horse hay.

If these chemicals are introduced to your garden in manure, it can result in extremely stunted growth of your vegetables or even a barren garden, and this can persist for years. At the time of this writing, the following products contain these toxic compounds: Curtail, Forefront, Grazon Next, Grazon P&D, Milestone, Redeem R&P and Surmount.

If your horse poop checks out in all these areas, then by all means receive the gift with gratitude and mix it into your soil!


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