Cultivating Questions A Bio-Extensive Demonstration


Concerning the Bio-Extensive Market Garden

by Anne & Eric Nordell of Trout Run, PA

We are honored and excited to feature a research report written specially for the Small Farmer’s Journal. George Kuepper’s article illustrates so well the successful implementation of the bio-extensive system. It also shows how bio-extensive management can be taken to the next level by minimizing fertility inputs through the use of compost teas, biochar and long season cover crops. Of particular interest to us is the experimental work with cover crop mulches and reduced tillage for organic vegetable production. Most important of all, the holistic, market garden-scale system that George Kuepper and interns have developed over the last seven years at the Kerr Center represents exactly the type of long term, site specific research that makes the most sense for understanding and advancing sustainable agriculture.

A Bio-Extensive Demonstration at the Kerr Center in Oklahoma

by George Kuepper of Poteau, OK
Horticulture Program Manager


The Kerr Center for Sustainable Agriculture is a non-profit foundation, located on a 4000-acre ranch in the foothills of the Winding Stair Mountains, in Southeast Oklahoma, about one-half hour from the Arkansas border. The ranch and Center are the legacy of Robert S. Kerr, a pioneering oil man who served both as Governor and U.S. Senator toward the middle of the twentieth century. Dedicated to sustainable agriculture since 1986, the Kerr Center has involved itself in various food and agricultural efforts including the preservation of heirloom livestock, education on rotational grazing, organic horticulture, and programs such as Farm-To-School and Buy Fresh, Buy Local.

In 2007, we initiated a new horticulture program in tandem with student intern training to make Kerr Center a destination for sustainable education. The horticulture program revolves around a six-acre demonstration area called the Cannon Organic Horticulture Site, which is dedicated to demonstrating organic management principles. In addition, we use it to support beginning farmer trainings, demonstrations of small market farming technologies, heirloom variety trials and seed production. The dominant feature is a four-field rotation that has been under bio-extensive management since ground was first broken in 2007.

A Bio-Extensive System

Two management directives led us to a bio-extensive design. First, because our staff is small, we required a system that would provide inherently good weed control. Bermuda grass was a particular concern. Bermuda is a southern species that we welcome in pastures but curse in organic horticulture. Bermuda grass dominated the pasture ground that became the Cannon site.

Our second directive demanded a reduced dependence on outside fertility inputs, particularly industrial poultry litter. Many, if not most, of the organic market farms in our region depend on broiler or layer litter for annual supplies of nitrogen and organic matter. We wanted an alternative that would be more independent and sustainable.

Cultivating Questions A Bio-Extensive Demonstration

The bio-extensive design that emerged from these directives is based on a sound rotation with generous and creative use of cover crops. The summary of that rotation is shown in Figure A. You will note that fully 50% of the land is in green fallow (year-round cover crops) in any single year — the main characteristic that defines it as bio-extensive. Our system addresses the management directives in several ways, through systemic weed management, growing our own organic matter and nitrogen, and by supporting disease and insect pest management.

Cultivating Questions A Bio-Extensive Demonstration
Photo shows growth of sorghum-sudan around July 1st, prior to first mowing or grazing.

Systemic Weed Management

Bermuda grass is a highly aggressive and invasive summer grass. It boasts fast-growing above-ground stolons with equally aggressive underground rhizomes. It shrugs-off cultivation and thrives under mowing. Its Achille’s heel, however, is its low tolerance for shade. Therefore, when we rotate a field to green fallow, we plant vigorous, tall-growing, dense-canopied cover crops. If we maintain them throughout the summer season, they smother Bermuda grass, essentially eliminating it before the next growing season. We rely primarily on annual sorghums — especially sorghum-sudangrass — as our main green fallow smother crop. Planted in late April or early May, annual sorghums commonly grow to the height shown in Figure B. If bush-hogged or grazed to leave several inches of stubble, the crop readily regrows and reaches a similar height by the middle or end of August, just in time to prepare the ground for a winter cover crop.

The short- and long-term effects of smother crops in rotation cannot be understated. In our opinion, the impact is profound. In addition to Bermuda grass control, we have reduced populations of amaranth and other summer annuals. Perennials like horse nettle (a solanaceous weed) and yellow nutsedge are present each summer, but their populations are small and appear to be dwindling.

While we haven’t fully evaluated them, we believe pearl millet, sunn hemp (crotalaria), or sesbania might work as well as annual sorghums. Lablab (hyacinth bean) and late cowpea varieties, like ‘Iron & Clay’ also show promise, though all would likely need different management than sorghum.

We supplement our systemic weed suppression with mulching using plant residues, crop canopy management, and living mulches, when possible. Some cultivation and hand-hoeing is required each year in the vegetable fields, but this is minimal — a fact well-appreciated by our student interns, who would bear much of the burden.

Organic Matter

While we have never tried to calculate biomass yields, Extension Specialists have told us that our sorghum-sudan stands produce 6–7 tons of above-ground biomass per acre. Whether that is an accurate estimate or not, it confirms that we have a serious plan for returning organic matter to the soil to support a vibrant soil food web.

Cultivating Questions A Bio-Extensive Demonstration

To enhance soil biological activity further, we spray cover crops — especially green fallow cover crops — with compost teas or extracts and other organic stimulants. Figure C presents our annual planned schedule for green fallow management and the application of compost sprays. We rarely use compost sprays on the fields when they rotate to vegetables, to ensure no violation of National Organic Program regulations. (Being conservative about food safety, the NOP currently treats compost tea in the same manner as raw manure, requiring 90 to 120 days to lapse between application and harvest of food crops. ) The abundance of cover crop biomass provides an added bonus in the form of available mulch. When necessary, we harvest from our green fallow fields and move it wherever needed. In some instances we use organic ‘no-till’ methods, mechanically-killing the cover crop and leaving the residue in place as mulch.


Winter cover cropping is standard practice here. We value it for the usual benefits of erosion control, nutrient capture, and biomass. However, we are especially interested in the legume nitrogen we can grow in the off-season.

Our usual winter cover includes an annual cool-season grass. In our region, the obvious choices include ryegrass, wheat, grain rye, triticale, or oats. We usually choose grain rye when we want a large amount of crop residue — such as when using no-till methods (discussed later). When we are planning for early vegetables with more conventional tillage, we need less residue. We then choose a spring oat variety which readily winter-kills. In either case, the grasses cover and protect the soil early in the fall, and serve as nurse crops to nitrogen-fixing legumes, which we seed at the same time. Our most reliable winter legumes are hairy vetch, Austrian winter peas, and crimson clover, though we are evaluating arrow-leaf clover as well.

We pull a few plants each season to check for nodulation — our assurance that nitrogen fixation is, in fact, underway. We have continued to inoculate legumes every year, though this is probably no longer necessary. Every field has hosted peas, clovers, and vetches within the past five years.

To date, we have not observed nitrogen deficiencies in our vegetable crops. Yield and quality have been reasonably good. However, we wonder if both might well be better with a bit more nitrogen in the system, so we are planning to tissue test crops in the future. In the meantime, we are also evaluating summer legumes that might do well as green fallow. Some of these — sunn hemp, lablab, cowpeas, sesbania — have already been mentioned. In due time, we will learn whether these might best be grown as companions to annual sorghums, or be grown in pure stands.

Pest and Disease Management

Every good vegetable grower knows that crop rotation can control a wide number of plant diseases and insect pests. The length of the rotation, sequence, and the diversity of crops and cover crops determine effectiveness. Our four-year rotation suppresses many of the common problems encountered in systems where planned crop-rotation is not practiced, like early blight in solanaceous species and black rot in cucurbits. Through careful mapping, we further ensure that crops do not re-visit the same piece of ground more than once in eight years. This addresses more persistent diseases such as blackleg, fusarium, clubroot, and white rot.

While several insect pests are also managed through rotation, we feel that a significant level of control also comes from the habitat for beneficial parasites and predators that our cover crops provide.

Cultivating Questions A Bio-Extensive Demonstration

It is simplistic to understand organic pest control in terms of the traditional management tool box (Figure D). In this outdated reductionist approach, growers identify the pest and then try to match them with controls, as if all options are equal when they are not. It ignores the ecological benefits that emerge in well-designed organic systems. Systemic practises such as planned rotation, the strategic use of cover crops, and organic soil building activities represent a foundation and paradigm in which specific cultural practices and commercial products should then be evaluated and selected (Figure E).

Cultivating Questions A Bio-Extensive Demonstration

Reduced-Tillage Innovations

Tillage and cultivation can un-do the best efforts at soil building. They disrupt soil biology, increase erosion, and burn up organic matter leading to both carbon and nitrogen losses. Most of our organic systems are dependent on tillage and cultivation and, therefore, are largely compensatory — manuring, managing residues, and fertilizing to fix the damage and replace the losses we incur through constantly stirring the soil. A bio-extensive system does not necessarily reduce tillage, though we believe it limits the need for cultivation through weed suppression. Its main counterbalances to the effects of tillage and cultivation are the return of large amounts of biomass and biologically-fixed nitrogen through the use of cover crops. Still, we believe that more can be done.

Growing and using our own organic mulch is the first and most straight-forward means. When you grow a large amount of winter and summer cover crops, it is relatively easy to harvest on-site. We simply cut, field dry, and move it to wherever it is needed. More, we feel, can also be done by adapting reduced-tillage methods — what many are calling “organic no-till.”

Our reduced-tillage methods mimic those being trialed elsewhere in the country. They involve mechanically killing cover crops to create in situ mulch, which we plant or transplant into. We have had our best successes to date with main-season tomatoes and fall pumpkins.

Cultivating Questions A Bio-Extensive Demonstration
We are still evaluating the crimper/roller as an effective tool for organic no-till. Note the cleated steel wheels and wheel weights on the tractor.

Reduced-tillage Tomatoes

While we are evaluating the crimper/roller (Figure F), we have found that close-mowing is the most reliable means for mechanically killing most cover crops. We have used flail, rotary, and sicklebar mowers, mounted on a BCS walkbehind tractor. While some “no-till” proponents recommend the flail mower, the sicklebar is the optimal choice for our circumstances (Figure G). Among the factors that affect our preference is the fact that we expect to re-arrange and concentrate the mulch, and it is easier to rake and gather when the stems are intact. Intact stems also breakdown more slowly, extending the effectiveness of the mulch.

Cultivating Questions A Bio-Extensive Demonstration
The sicklebar does not shred the cover crop, thus prolonging effectiveness as a mulch. It does however, leave gaps. We use hay rakes and forks to re-arrange the mulch cover as needed.

We prefer rye and crimson clover as a winter cover crop for reduced-till work, though combinations of wheat and/or triticale with arrowleaf clover might work just as well. Austrian winter peas and vetches are vining and difficult to mow with the BCS sicklebar.

Cultivating Questions A Bio-Extensive Demonstration
Cover crop residue is raked to the side and a rotary plow used to build robust planting ridges. Drip tape is laid before the mulch is raked back over the ridges and tomatoes transplanted.

To ensure a good kill, timing is important. Winter grains like wheat, rye, and oats must be well into the boot stage (when the flowering head has moved partway up the leaf sheath). Annual winter legumes should be cut between one-half and full flower to get effective kill and to take full advantage of nitrogen fixation.

Cultivating Questions A Bio-Extensive Demonstration
Photo shows two planting ridges. Drip tape is laid before the mulch is raked back over the ridges and tomatoes transplanted.

If our soil drained better, we would transplant directly into the mulch. Since it is not, however, we rake the row area clear and use a rotary plow (Figure H) to build planting ridges (Figure I). Drip tape is then laid on the ridge and the mulch pulled back into place before transplanting. While this requires a tillage operation, we disturb only one-third to one-half of the soil surface, and the tilled soil remains exposed for only a short time before being mulched (Figure J).

Cultivating Questions A Bio-Extensive Demonstration
Photo shows two rows of heirloom tomatoes planted on ridges created using a rotary plow. Sunflowers and buckwheat provide beneficial habitat.

Reduced-tillage Pumpkins

The procedure we use for growing reduced-tillage pumpkins is similar to that for tomatoes. However, in this instance, we are mowing a summer cover crop, and direct seeding rather than transplanting. If we follow the same guidelines for timing cover crop kill, a number of species appear promising including crotalaria (sunn hemp), sesbania, pearl millet, sesame, and southern peas. So far, the soybean variety we tried — ‘Viking’ — as well as buckwheat, produce too little biomass to leave very much mulch. Japanese millet also proved unsuitable.

In our part of the Mid-south, we can plant all-but-the longest-season cucurbits on, or shortly after, July 1st, and expect them to mature before frost. In most years, early July coincides with optimum kill-times for many summer cover crops. We get an added bonus by delaying pumpkin and winter squash planting until July. By this time, overwintering populations of squash bug have starved out and pest pressure is reduced. Of course the delay only helps when you are relatively isolated from others’ gardens and cucurbit fields, and forego planting early squash yourself!

We direct seed pumpkins, gourds, and/or winter squash directly into the mulch using a jab-type planter. This works well because the in-row plant spacing is wide. A jab-planter would be too slow for closely-spaced or drilled crops, and we are not aware of a small-scale planter that would allow drill-seeding into high residue.

Supplementing Fertility and Soil Biology

As indicated earlier, we utilize compost sprays to supplement soil fertility by bolstering the soil biology. The feedstocks for our compost and vermicompost come primarily from bull and horse pens on Kerr Center’s cow-calf operation, crop residues, and from on-site food wastes. Since these sources are limited, the amounts of compost and vermicompost we produce are small. Compost sprays are a means for extending their value. Composting, vermicomposting, and compost sprays are among several practices we investigate and use to free our system from an over-dependence on external inputs. Biochar is another of these.


Biochar results from a process called pyrolysis, which is the same as that which yields charcoal. Heating wood or other organic materials to high temperatures with little-or-no oxygen drives off wood gases and leaves a stable, carbon-rich material that we use to enrich the soil.

Don’t confuse biochar with wood ash. Wood ash results from burning under air/oxygen-rich conditions — all the carbon is consumed leaving only mineral residues.

Cultivating Questions A Bio-Extensive Demonstration
Making biochar in a double-barrel nested retort.

We produce biochar using a double-barrel nested retort (Figure K). The capacity of these units is small — suitable for small gardens. We have yet to explore larger retorts that might be more appropriate, while still remaining affordable.

While we could apply biochar directly to our fields, we use ours as a compost and vermicompost feedstock. There are two reasons for this. First, since the pure material is very dusty, it is physically easier to distribute the char in compost. Second, fresh biochar acts much like a dry sponge — drawing water and nutrients away from plant roots. Once “charged” with water and nutrients through composting, it provides immediate benefits to plants.

Closing Note

In late summer 2013, I had the great privilege of visiting Beech Grove Farm near Trout Run, Pennsylvania. Anne and Eric Nordell have been National leaders in demonstrating the practical benefits of bio-extensive management. They have been a major source of information and inspiration as we developed the Cannon Site. Much of the descriptive language we use traces back to Anne and Eric’s articles and video. I was anxious to see what land under three decades of bio-extensive management could look and feel like.

I was not disappointed. The quantity and quality of crops, as well as the condition of the soil, was what I’d hoped to see, but it was the extremely low weed pressure that really impressed. The long-term benefits of rotation and cover crops were clearly in evidence and I satisfied myself that we are on the right track at Kerr Center, following a bio-extensive approach.