Organic Broadcaster

Cover crops can be ‘secret weapon’ to help vegetable farmers improve soil

By Linda Halley, MOSES

Shovel in hand, I set a pace across the farm yard and to the top of the knoll, 30 freshman trailing behind me. With a sweep of the arm I say, “Welcome to Gardens of Eagan Organic Farm. From here you get a great view of our crops and livestock.” Students who aren’t scrolling on their phones look around, puzzled. Rolling out before them are fields of kale, cucumbers, tomatoes and cover crops, not an animal in sight. I start to dig.

“The livestock are down here, under the clover, hundreds of thousands of them. They are doing just what cows do, eating, digesting, pooping, but on a scale you can’t see. Like cow manure, the microbe exudate contains nutrients available to feed the plants. These microbes are critical to healthy crops. And, just like four-legged livestock, we feed and care for them.”

The surprising livestock metaphor was often a good way to get the phones turned off and the students engaged, but I’m going to carry it a bit further to help explain a new way I think about cover crops. Most farmers already are aware that insect, disease and weed pressure can be addressed through skillful use of cover crops. Additionally, we can essentially “grow” nitrogen with leguminous covers. Now, think about cover crops as a way to “feed and care for your underground livestock” and you will have one more good reason to dedicate yourself to fitting cover crops into your vegetable rotations.

It’s a widely known fact that plants make energy out of sunlight through photosynthesis. What isn’t as widely known is the fact that plants pump up to 50 percent of that energy, in the form of sugars, into their roots. There they become root building blocks. But, believe it or not, up to 15 percent of that sugar is pumped out into the soil. It is used by the roots to lure soil microbes into their root zone for a sugary snack.

Most soil microbes are decomposers, physically and chemically breaking down organic matter and minerals. All that digesting is a hard way to make a living—what microbe wouldn’t want to stop by the soda shop in the plant root zone for a quick sugary energy drink? (And, to think it took humans 200,000 years to invent Coke! Plants had it figured out eons ago.)

It just makes sense that the more you feed your soil, the more soil life you have—that can be your secret weapon as a vegetable farmer. A healthy microbial population below the ground can help your soil recover from the destructive things a farmer does to grow a crop. Cutting edge research aside, vegetable farmers pretty much have to till and cultivate to produce a high-yielding crop.

Disturbing your soil through tillage introduces oxygen into the soil and super-charges microbial respiration. Lots of organic matter gets consumed and microbes die off. It can be an ironic fact that the cleaner your fields are, the less alive your soil is—unless you are diligent about cover cropping. Tillage also compacts, slices and dices soil. And, in a worst case scenario, moves it into a whole new growing zone.

Luckily, some microbes aren’t just eating and pooping, they are engineering their environment, minimizing the destruction done by soil disturbance. Soil fungi exude a “glue” called glomalin which is used much like mortar in building a brick house. These soil houses—soil aggregates in more proper terms—are the structures that give soil its “good tilth.” Glomalin-rich soil has plenty of pore space for air and water infiltration. And, glomalin-rich, microbial-rich soil is better able to retain and rebuild its porous structure and rebuild the microbial herd.

Okay, so I have convinced you that you need a healthy population of soil microbes to help build good soil structure, and digest and release available nutrients for healthy plants. You might ask, if actively growing roots feed soil microbes, why doesn’t my cash crop serve as sufficiently as a cover crop to maintain soil health? Simply put, a cover crop has so much more organic matter mass, more roots below and more plant matter above, that a cash vegetable crop can never match the benefits of a cover crop. Besides, the portion of the cash crop you harvest is not insignificant.

Vegetable plants, quite distinct from cover crops, are bred to devote a greater portion of their energy on the very part of the plant you remove—exaggerated flower buds or multitudes of sugary fruits, for example. Plus, all that bare ground you maintain as weed-free is also a root-free zone, which is short on sugar and long on dry heat. In short, there’s no substitute for all the benefits provided by cover cropping.

John Peterson, owner-operator of Angelic Organics in Rockton, Illinois, is a farming systems geek devoted to cover cropping. He considers it an essential part of fertility and weed management, growing two years of cover crops followed by two years of cash vegetables.

“It is expensive, at first glance, to have fields out of production half the time,” he said. “Seeding, mowing, applying compost—all take money and time. But that expense is more than offset by yield increases when the fields are in production.”

His go-to cover is a legume mix which he mows every 10 days. “I thought that this regular mowing would encourage foxtail to go to seed on shorter and shorter plants. But our foxtail problem is almost non-existent, and it used to be our biggest weed problem.”

Even during the two cash crop years, Peterson fits in a quick summer pea cover following any short-duration veggies that finish up by mid-August. “Seeding cover crops is not something that we do only if we have time,” he explained. “We do it, no matter what. It is as important to seed the cover crops as it is to plant and harvest the vegetables.”

A word of caution: just like every other aspect of vegetable farming, there’s a learning curve to growing cover crops. Buy yourself a cover crop resource book or two. Several excellent books are listed at the end of this article. MOSES also has a good number of articles about cover crops on its website —just enter “cover crops” in the search bar at the bottom of the screen.

Additionally, get yourself fired up to plant some cover crops yet this season by listening to Allen Philo and Chris Blanchard, soil and farming systems consultants, dissect cover cropping on the Farmer to Farmer Podcast. I LOVED it!

Finally, I’d like to share just two of my favorite ways to incorporate cover cropping into vegetable rotations. One you can do this season, one you can put on the back burner for next spring. If they don’t work on your scale or with your crop mix, a little creative adaptation is always appropriate.

Broadcast into Late-Standing Brassicas –
After the final cultivation, or pass with the hoes, broadcast white Dutch clover. The clover may be sparse, depending on foot traffic and canopy, but will return in early spring to fill in and create a green carpet. I flail chop the brassica after the last harvest, slowly and with high rpm, so the chop is fine, since I am not incorporating into the soil.

Oat-Pea Mix on Early Open Fields –
I hate to see bare ground, and since I don’t routinely double crop, a good share of my ground doesn’t get planted until late June or well into July. If there is no overwintered cover crop, I have the drill ready with a relatively cheap oat-pea mix. Peas are the fastest, cheapest legumes in our climate, and oats love cool weather, so it’s never too early to plant this mix. I know April and May are busy months, and the windows of opportunity might be few, but if your seed is in the drill, and the plan is in place, it’s more likely you’ll get it planted.

Linda Halley was the farm manager at Gardens of Eagan for many years. She currently serves as the interim executive director at MOSES.


Resources on cover crops for vegetable farmers


Farmer to Farmer Podcast Episode 69


Soil, Cover Crops, & Systems MOSES Webpage

Resource Books:

Crop Rotation and Cover Cropping, NOFA

Crop Rotation on Organic Farms, Mohler and Johnson

Managing Cover Crops Profitably, SARE


From the July | August 2016 Issue

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