COVER CROP CONFUSION
How Much Soil Organic Matter?
In last week’s blog I kept jumping the fence about cover crops. First I extolled their benefits. Then I wrote that they’re probably unnecessary in my heavily composted ground and possibly to blame for poor growth of some corn and tomatoes. Finally, I wound up stating that I do grow some cover crops anyway. No wonder I caused some confusion.
All this prompted one reader, Peter, to comment with some specific questions that might also be of interest to some of you. I will now try to answer them.
His first question was: “How is percentage of organic matter in the soil determined.” “Organic matter” is carbon compounds; as such they can be oxidized, and when this happens they are lost from the soil as carbon dioxide and water. Microorganisms do this naturally in any soil as they feed on organic matter, in so doing releasing minerals associated with the organic matter in forms that plants can use for nourishment.
The way to test the percentage of organic matter in the soil is to weigh a sample, oxidize the organic matter, then weigh it again to measure the loss. Oxidation can be done by burning the sample at high temperature. Another way is to chemically oxidize a weighed sample using potassium dichromate and then measure how much dichromate is left after the reaction. (The latter reaction is more accurate than burning because carbonates in a soil will also lose carbon dioxide when heated, affecting the final weight.)
Peter, I suggest against determining the percentage organic matter of your soil in your kitchen. Soil labs generally include percentage organic matter along with other test results. (I cover soil testing in my book Weedless Gardening.)
Peter also reported that his soil tests very high in phosphorus, potassium, magnesium, and calcium. Soils to which have been added lots of organic matter typically test very high or excessive levels of phosphorus and potassium. That doesn’t present a problem, possibly because, as I wrote last week, “Conventional soil tests are for mineral soils, not soils that are very high in organic matter.”
As far as the high levels of calcium and magnesium, I’m not sure about that. Excessive applications of dolomitic limestone would cause that, and that would also be reflected in a high pH.
To Cover Crop or Not to Cover Crop, That is the Question
Now for a more difficult question. Why do I plant cover crops if I don’t think they’re of benefit in my soil? I guess that’s because I’m aware of the known benefits of cover crops. If nothing else, the cover crop obviously protects the ground’s surface from the pounding of raindrops and wide swings in temperature. Even if I don’t notice any positive impact, they may be there, more subtly. And then there are the still unknown benefits of cover crops.
The major, consistent negative impact of my late summer of fall planted oat cover crops has been to increase the number of weeds the following spring, specifically henbit (Lamium amplexicaule). For the past couple of years I’ve dealt with that problem by tarping those beds in spring for a couple of weeks or more. Or, not as good, just pulling out the henbit.
Because I only cover crop beds that are no longer needed for vegetable plants towards the end of the growing season, growing the cover crops doesn’t take up space in which I could otherwise grow vegetables.
Buckwheat cover crop in greenhouse
It’s also almost no trouble to sprinkle the oat seed over the bed before laying down the inch of compost each bed gets every year.
And finally, I have to admit that I like the lush green look of the oat leaves, the leaves maintaining that look almost until the first day of winter. And even after that, they flop down on the ground, dying, but then hiding the brown compost beneath a tawny blanket.
Rye cover crop at Chanticleer Garden
Timely Planting
Last question: “When is the latest time to plant the cover crop?” Cooler weather and diminishing sunlight dramatically slow plant growth in autumn. Oats like that cool weather, and will germinate and sprout pretty late in the season. But there comes a time when so little growth will ensue that oats or any other cover crop is not really worth planting.
Here, about half way up New York’s Hudson Valley (USDA Hardiness Zone 5), I figure on the latest date to make planting a cover crop worthwhile is about October 1st.
CLOTHE THE GROUND
Something Good for the Soil
Soil has been called “the skin of the earth.” That “skin” nourishes much of life here, so let’s take care of it. Which is one reason for cover crops, that is, plants grown not directly for us, but specifically to maintain or improve soil health. Typical cover crops include rye, oats, buckwheat, clovers, and other mostly grains or legumes.
The most obvious benefit of a cover crop is the protection it affords the soil from wind and rain, either of which can carry away the most fertile surface layer. Also protection from temperature extremes. Another benefit is that cover crops can suppress weeds. Less obvious is cover crop plants’ ability to grab onto and bring back up to the surface layers nutrients that rain might otherwise wash beyond roots into the groundwater.
Buckwheat cover crop at Chanticleer
Some effects are even more subtle. Substances oozing from plant roots feed microbes and also bind soil particles to create small and large pores to create a good balance of air and water in the ground. Cover crops also can increase levels of all-important soil organic matter, which benefits plants and soils nutritionally, physically, and biologically in both known and unknown ways.
But Does My Ground Need It?
Despite the benefits of cover crops, you’ll rarely see them in my vegetable garden. Mostly, that’s because the beds are usually packed full of growing vegetables from very early to very late in the growing season. There’s nowhere to plant a cover crop! The vegetable plants themselves provide the same benefits as would a cover crop, especially since I plant at very close spacing, close enough, usually, to hide bare ground.
Bed of broccoli & endive
Planting cover crops in my garden to increase soil organic matter would be like taking coals to Newcastle. For decades, every year I’ve laid a one-inch blanket of compost over all my vegetable beds. (My book Weedless Gardening describes the whole system of soil care.) The result is soil organic matter levels, last time I tested, at about fifteen percent. No need for cover crops to boost my soil’s organic matter content.
Fifteen percent organic matter is very high. A typical, very good agricultural soil, such as the virgin Midwest prairies, was about five percent organic matter.
Then again, could fifteen precent be too high? Very high levels of organic matter lead to soil tests registering excessive levels of phosphorous and potassium. Traditionally, phosphorus was thought to be so tied up chemically in the soil that it would stay put, but some recent research indicates that water can push some phosphorus into the groundwater if phosphorus levels are high enough.
Conventional soil tests are for mineral soils, not soils that are very high in organic matter. A soil with around fifteen percent organic matter would better be called a “growing medium” rather that a “soil. Soils naturally this high in organic matter are classified as histosols, commonly called bogs, peats, or moors. Such soils, in contrast to my garden soil, are typically low in plant nutrients.
An obvious way to assess soil management is — duh — to look at the plants and their growth. Everything looks fine.
Problem? Maybe, Maybe Not
Except for a couple of strange goings on in one of my two vegetable gardens. In one bed, planted this year to early turnips and lettuce, now in corn, the corn in half the bed is stunted. (The turnips and lettuce did fine.) 
In another bed, tomatoes are doing fine, but are not as vigorous as they should be as compared with another bed of tomatoes in that garden.
My notes indicate that both beds received their annual blanket of compost, just like all the other beds. Last fall, when the compost was applied, I also sowed cover crops in those beds. Rather than my usual oats cover crop, which winter-kills so integrates well with my no-till system, I sowed crimson clover along with the oats. Why crimson clover? Because it’s pretty when it blooms in spring.
Crimson clover is allelopathic. Allelopathic plants can biochemically suppress growth or germination of nearby plants; the clover could have adversely affected growth of tomato and corn. Of course, only half the corn bed is stunted but clover germination, if I remember correctly, wasn’t uniform in that corn bed. I’ll file all this in the back of my head for future investigation.
Then again, oats, is also allelopathic, as are rye, sunflower, sorghum, and many other plants, including, most famously, black walnut. Allelopathy is often very specific, an allelopathic plant inhibiting the growth of only some species of plants. Said allelopathic plant might even promote growth of other plant species. Time, usually a couple of weeks, after an allelopathic plant is killed or removed, usually rids the soil of any allelopathic effect.
For completeness, I should mention that the corn and tomato bed problem could be traced to erratic behavior, this year, of my drip irrigation system, which has now been corrected.
I’m going to continue occasionally planting cover crops and will monitor more closely any positive or negative effects — in my garden. As they say in so many arenas: “Your results may differ . . .”
SENSUAL THINGS, AND WATER
Heady Nights
It’s difficult to work outside in the garden these days, especially in early evening. No, not because of the heat. Not because of mosquitos either. The difficulty comes from the intoxicating aroma that wafts into the air each evening from the row of lilies just outside the east side of my vegetable garden.
These aren’t daylilies, which are mildly and pleasantly fragrant. Wild, orange daylilies are common along roadways and yellow and hybrid daylilies, often yellow, are common in mall parking lots. (That’s not at all a dis’; the plants are tough and beautiful, and I’ve planted them also.) They’re also not tiger lilies, which lack aroma and sport downward turned, dark red speckled orange flowers with recurved petals.
My fragrant lilies are so-called oriental hybrid lilies, which are notable for their large flowers and strong fragrance. My favorite among those I grow is Casa Blanca. The flowers are large and lily white (what’d you expect?) except for the threadlike, pale green stamens emerging from their centers, with dark red anthers capping their ends.
Casa Blanca would be worth growing just for the look of the flowers; the fragrance, very sweet and very heady make this bulb a must-grow. Not for everyone, though. A few people dislike this fragrance. For some people it’s more than just stinkiness, the aroma causing nausea, dizziness, or congestion.
Casa Blanca’s stems can rise to about four feet tall, their upper portions circled with almost a dozen of those large blossoms in various stages of ripening. Some years, staked, persimmon orange, Sungold tomatoes grow in that bed, and the tomato and lily plants looked very pretty mingling together. (Tomatoes were, after all, once grown as ornamentals.)
This year I’m growing kale in that bed which, besides good eating, provides a frilly base from which the lily stems rise.
In Good Taste
Turning to another of the senses . . . taste. Blueberries. They are among my most successful fruits and, as usual, the plants’ stems are bowing to the ground under a heavy load of berries this time of year.
Not to brag, but the average yield of a blueberry bush is 3 to 5 quarts. My blueberry bible, Blueberry Culture (1966), states that “proper cultural practices can increase the yield to as much as 25 pints per bush.” I average about 18 pints per bush, with some bushes yielding as much as 24 pints. Organically grown, of course.
I credit my good yield to periodic additions of sulfur to maintain acidity of pH 4.0-5.5, timely watering with drip irrigation especially the plants’ first few years, topping up of existing wood chip, wood shavings, or leafy mulch each fall with an additional 3 inch depth of any organic, weed-free mulch, and pruning every spring.
In year’s passed, I also added soybean meal for extra nitrogen to fuel stem growth. Blueberry flower buds develop along growing stems, with flowers open along those stems the following spring. More stem growth means more blueberries, to a point. For many years I have foregone soybean meal because the the plants were overly vigorous, creating a dense jungle that makes getting to the berries too difficult.
One other key to success and topnotch flavor is a net during the summer to fend off birds and — for best flavor — careful picking of only dead ripe fruits.
Water, Too Much or Too Little
So far, the growing season here in the Northeast has been one with both dry spells and wet spells, more than usual of each. Some recent thunderstorms fool many a gardener into thinking that the soil has been thoroughly wetted. But such rains are often only a drop in the bucket.
The only way to know for sure if enough rain has fallen for plants to really slurp up water is to check the soil or measure the actual amount of rainfall. A friend tells me he waters his plants every day. Every day! How much? It could be too much or too little, and probably is one or the other. I like to quantitate things so I measure rainfall or watering, as well as soil moisture, in a few different ways.
First, measuring water added to the soil: The ideal is about a 1 inch depth of water per week, which is equivalent to about a half a gallon per square foot of surface area. For hand watering a young tree, with an estimated root spread of only a couple of square feet, I fill the watering can with a gallon of water and sprinkle it on.
Rainfall, or the water from a sprinkler, could be measured with a straight-sided container. I use a rain gauge whose tapered body can break down the measurement into tenths of an inch, readable from indoors.
Digital moisture probe.
I usually measure the actual moisture in the soil with a handy little meter attached to a probe that slides a half a foot down into the soil. As expected, the meter told me today that the soil is very wet. Not surprising after 3 inches of rainfall, as measure in the rain gauge, two nights ago.
(There’s more about blueberries and water in my books Grow Fruit Naturally and The Ever Curious Gardener.)
