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Adding organic matter with oilseed radishes

Late summer gardenOther than "tomato, tomato, tomato!", the primary topic on my to-do list this month is fall cover crops.  I've learned that although I can seed oilseed radishes anytime between August 1 and September 7, every week that passes lowers the the amount of biomass produced appreciably.  That's why I wander through the garden with an eagle eye throughout the early and middle parts of August, looking for things to be pulled out.

For example, giving away excess cucumbers was a lot of fun in July.  But by August, it seems to make more sense to rip out still-producing cucumber plants and scatter radish seeds in their place.  Sweet corn gets cut to the ground as soon as I pluck the least ear, and next week I'll be pulling out dead butternut squash and watermelon vines.  (I may remove some living summer squash plants too --- we've dried our winter quota and can't eat all of the fruits being produced.)

Oilseed radish seedsI like planting fall cover crops because they produce organic matter at an off-time for the garden.  We've got a few more beds of kale and lettuce to plant along with a lot of garlic, but other than that, any beds that come open from here on out are going to be sitting around twiddling their thumbs until spring.  I'd far rather put a cover crop there and capture winter sunlight than let that solar radiation go to waste.

But how much good do cover crops do?  The amount of organic matter produced by a cover crop will depend on a lot of factors, such as your soil conditions and planting time, but I dug up these figures for optimal dry matter production per acre for my three favorites:

  • Oilseed radish --- 7,000 pounds per acre
  • Buckwheat --- 6,000 pounds per acre
  • Fall-planted oats --- 4,000 pounds per acre

To put that in perspective, adding half an inch of compost to your garden per year requires about 68,000 pounds.  Roughly half of that is water, so the bare-minimum level of top-dressing in an organic garden provides 34,000 pounds of dry compost per acre.  Cover crops don't approach that figure, but providing a fifth of your annual organic matter with no shoveling is nothing to sneeze at.

The big question is --- how much of the organic matter added actually turns into humus and stays in your soil, and how much is quickly degraded?  As you may or may not realize, the soil food web is constantly working on breaking down organic matter, which is a good thing since the output feeds your plants.  But those microorganisms do work counter to my goal of building soil structure.

Oilseed radish seedlingsIn fact, if you don't add anything to your soil (especially if you till), your soil's organic matter content will decline each year.  An interesting study showed that organic matter levels of soil increased from 5.2% to 5.5% when 30 tons of dairy manure were applied per acre for eleven years, stayed steady at an application rate of 20 tons per acre, and dropped to 4.8% at 10 tons of manure per acre. 

Meanwhile, in the cover crop world, experts suggest that woodier debris is more likely to turn into humus (a stable form of organic matter) than succulent plant matter is.  That's why I wait to kill buckwheat until it's in full bloom, and why tough-stalked oats may actually produce more long-term organic matter than oilseed radishes do.  Perhaps pulling mature oilseed radishes and composting them would be a better way of capturing their full potential than letting them rot in the ground?  I may try that eventually, but for now, I'm quite happy with the ever-darkening soil in the beds that I commit to winter cover crops.

Our chicken waterer never spills or fills with POOP.


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I love your cover crop posts. I'm doing alot better about planting some as soon as I harvest something, and the deer are really enjoying the buckwheat I planted over the garlic :-) I switched to daikon and oats when I harvested onions, hopefully that will do better.

You mentioned one time about writing an e-book on cover crops, is that a possible winter project?

Comment by De Wed Aug 15 08:50:08 2012

De --- Thanks for asking about the cover crop ebook! I didn't see much interest in it when I asked on the blog, but it is one of the ones that's closest to my heart, so I may ignore my audience and just write it. :-) I suspect it would be relatively quick and easy, unlike the chicken pasturing book I've been working on for months that keeps getting bigger and bigger and bigger so that I'm constantly further from the end.... :-)

Good luck fighting those deer!!

Comment by anna Wed Aug 15 16:31:05 2012