Cultivating Questions

Concerning the Bioextensive Market Garden

by Anne and Eric Nordell of Trout Run, PA

Grassfed Potatoes

In the Spring 2002 photo essay we began documenting the cover crop-tillage combinations we use for each planting window in our short growing season. We first looked at ridge-tilling and no-tilling winter-killed oats for the earliest planted cash crops of the year, then, in the Summer issue, we discussed the use of mulch-tilled oats-and-peas, and lister planting, for May planted crops under drier conditions. The Winter 2003 photo essay covered the use of ridged rye for summer crops planted the last half of June and the first half of July, as well as the benefits of mulch-tilling a mature stand of hairy vetch and rye for fall crops established in July and August. That leaves the late May/early June planting window wide open!

Actually, the cover crop-tillage combination we use for late May and early June crops is one of the first we developed and have already described in detail in previous articles and the video.

Typically, we skimplow an overwintering cover crop of rye and vetch the beginning of April in preparation for this planting window. Shallowly undercutting the live cover crop at that time of year prevents the rye and vetch from removing the winter accumulation of moisture from the ground and preserves the soil structure created by the cover crop by leaving most of its root system intact.

In recent years, we have tried to enhance soil quality even more for this late May/early June planting window by substituting italian ryegrass and clover for the rye and hairy vetch. Although it is too early to tell if this shallowly plowed grass-legume sod provides a significant advantage for demanding crops like May planted storage potatoes, we think this new cover crop-tillage combination adds another measure of diversity and resiliency to the overall market garden rotation.

1. We planted the italian ryegrass and clover with a nurse crop of buckwheat in field 11 the middle of June, 2001. Our eleven-year-old neighbor, Kim Hartley – with some help from Becky and Buster – clipped the flowering buckwheat the last day of July to prevent it from going to seed.

2. The grass-legume mix really filled in after the buckwheat was clipped, providing 100% ground cover and excellent weed suppression. We decided to use italian ryegrass in the mix instead of the more common annual ryegrass because this winterhardy biennial does not go to seed the first year, guaranteeing that it won’t become a weed. Italian ryegrass also has the reputation for improving aggregate stability in the soil more than any other short-term cover crop. As for the clover, we used a blend of medium red, Alice white, and yellow blossom sweet clover to add more diversity to the cover crop mix both above and below ground. We have been using the Green Spirit blend of italian ryegrass from Barenbrug Seeds, a Dutch company, which caters to pasture-based livestock producers. Generally, we find that forages recommended for grazing livestock are also ideal for feeding the microherd below ground level. The prolific, fast starting Alice white clover also comes from Barenbrug – it is expensive but only a couple of pounds per acre is necessary in a pasture mix. The other clovers we purchase from Seedway mentioned in the last column.

3. Plowing down the grass-legume sod in field 11 early Spring of 2002. The italian ryegrass is like an iceberg this time of year in that the frosted topgrowth is a poor indicator of the massive root system underneath. Although this extensive root system is just what we are looking for in terms of improving soil structure, it can very quickly soak up the entire winter supply of moisture once this overwintering cover crop begins growing again in the spring. For this reason, we like to plowdown the sod just as soon as the frost goes out of the ground and before the grass has a chance to green up. Usually that is around the first week of April in our area, but due to an unusually dry and mild winter in 2002 we plowed field 11 in the middle of March to lock in as much precious moisture as possible.

4. Turning under the sod before it breaks dormancy usually means working the soil while it is still pretty wet. One reason we can get away with working the moist ground is our two horsepower tractor does not cause a whole lot of compaction. Also, the old walking plow turns the furrow over so slowly and gently that there is little damage to the soil’s integrity – especially compared to more intensive forms of tillage like discing or rotovating in muddy conditions.

5. But the main reason we can plow just as soon as the frost goes out of the ground is the sod’s massive root system literally holds the soil together as the furrow is turned. Due to the superior aggregation created by the grasslegume roots and associated bacteria and fungi, we end up with a loose, crumbly seedbed that feels spongy underfoot – not the tight, crusted soil you would expect from plowing the ground wet. Note how the undercut sod leaves a nice crumbly furrow floor despite the muddy conditions, insuring good infiltration and capillary action later on in the growing season.

6. As an added measure to preserve soil structure and moisture, we plow this short-term sod just as shallowly as practical. In the case of this vigorous grass-legume mix, we adjust the old walking plow to go 3-4” deep, an inch deeper than the skimplowing technique we normally use for a rye-vetch cover. We find that a minimum plow depth of three inches is necessary to thoroughly undercut the tough sod and completely slice through, or pop out, the clover’s taproots. At the same time, limiting the plow depth to three or four inches seems to provide the same benefits as skimplowing, such as not drying out the soil too deeply, and preserving soil structure by keeping the bulk of the sod’s massive root system intact below plow level. You might say that shallow plowing the ryegrass-clover sod is tilling the tip of the iceberg.

7. Reducing the depth of tillage simply by plowing shallowly results in a clean seedbed fit for planting and cultivating with traditional equipment. Unlike conventional moldboard plowing, this shallowly plowed sod keeps a good bit of moisture and raw organic matter near the soil surface as you can see from the furrow we are opening up here to start planting potatoes the third week of May.

8. Covering the planting furrows with the sweeps on the cultivator.* The shallow plowed sod produces a loose, crumbly seedbed. But just beneath the three inch plowed layer the ground is still almost saturated with winter moisture. If we had waited until now to plowdown the sod, the ryegrass and clover would probably have already used up the winter store of moisture.

9. First hilling on June 22. Despite preserving a good bit of moisture with this cover crop-tillage combination, we only harvested half a crop of grass-fed potatoes in 2002 due to one of the hottest summers on record and relentless pressure from both flea beetles and leafhoppers. We ended up with just enough quality tubers from field 11 for our customers at the farmers market, but not nearly enough field run taters to meet the growing demand from families who want bulk quantities of root cellar potatoes after the fresh market season is over.

10. A crop of grass-fed potatoes enjoying more temperate weather and a lot less insect pressure in 1996. In this case the shallowly plowed sod was a pure stand of clover. We can’t help wondering if this heavy feeding crop prefers high nitrogen legumes over the soil building grasses so we intend to trial varying ratios of italian ryegrass and clover in the future to try to determine just the right mix for grass-fed potatoes.