Dateline: 11 March 2017
|Amazing beauty, on a microscopic level|
My new Minibeds-on-Plastic experimental garden will be a no-till vegetable growing system. There will obviously be no tillage under the plastic-covered "ocean" of garden space around the minibeds, but there will also be no tillage in the minibed "islands" themselves. Or, perhaps I should be more precise and say that there will be very minimal tillage. I see this as a critically important aspect of the Minibeds-on-Plastic system. And I'd like to explain why...
It so happens that tillage of soil does not improve soil tilth, which is to say, it does not improve soil structure. Which is to say, it does not contribute to porosity and aeration of soils. Those physical characteristics are a very important and desirable thing in any garden soil.
In years past, when preparing my soil for spring planting, I rototilled my whole garden until the soil was soft and fluffy. The freshly-tilled earth was beautiful. It was a clean canvass on which to create another garden masterpiece (or so I always hoped).
I thought that was the right way to prepare a garden. But I am fully persuaded now that such tillage is not a good gardening practice. It is not good for the soil. It is not good for the plants. It is actually counterproductive. Since I stopped the wholesale annual rototilling of my whole garden a few years ago, I have had more productive gardens. I think there is a connection there.
The reason I now believe no-till (or minimal-till) gardening is worlds better is because I have learned about the importance of soil structure and, in particular, the role of mycorrhizal fungi in developing a healthy soil. This deserves some explanation...
The Mycorrhizal Factor
The picture at the top of this page shows a mycorrhizal arbuscle that has invaded the root cell of a plant. It isn't a hostile invasion. It's a mutually beneficial (symbiotic) association that every gardener needs to understand. The plant benefits gained by mycorrhizal fungi are manifold.
First, you need to know that the plant feeds the fungi. As we all learned in elementary school, plant leaves take sunlight and carbon dioxide from the air and "manufacture" sugars (carbohydrates) in the process of photosynthesis. Some of this photosynthate goes into the plant growth, but not all of it.
It turns out that plants do not just take from the soil when they grow. They actually leak or pump (two words commonly used, though probably not technically accurate) a surprising amount of their photosynthates into the soil to feed various microbial life forms in the soil, and mycorrhizal fungi is one of the major consumers of these leaking photosynthates.
So, the tree-like mycorrhizal structure (arbuscle) in the picture above is taking in photosynthates in order to live and grow. In return, the growing mycorrhizal fungi sends out an extensive network of threadlike hyphae into the soil well beyond the plant roots. These hyphae are finding water and nutrients in the soil that the plant's roots can not access and they are feeding these to the plant.
When you see one of those amazing illustrations of root systems that were painstakingly mapped out by Professor John Weaver in 1927, they don't show the extensive network of mycorrhizal hyphae that may also be at work with those roots. Imagine this Weaver down-view of the root system of a mature corn plant with an equally large, if not larger, network of hyphae acting as root extensions...
Plants that are colonized by mycorrhizal fungi will be better nourished, and better hydrated (drought tolerant) than plants that are not. The difference is clearly visible aboveground.
Mycorrhizal fungi also help defend their host plants from pathogens. And, amazingly, scientists are learning that these fungi are part of a subterranean communications network between plants and soil microbiology.
Aside from helping plants to grow better and be healthier, it turns out that mycorrhizal fungi benefit not just the plant, but the soil structure around it in a profound and very desirable way. This is worth understanding a bit more...
The Glomalin Factor
Mycorrhizal hyphae that spread through the soil produce a substance called glomalin (which was discovered in 1996). Glomalin is a biological glue that helps bind soil particles together. Glomalin-glued soil particles are what gives a soil better tilth. Better aggregation is another way of saying it.
Aggregation creates large and small pore spaces. In a well aggregated soil structure, water is more freely absorbed and drained down, leaving better aeration, which is conducive to the health of all kinds of good soil micro-biota. You can see this so clearly in the video at the bottom of this post.
Now, the problem with typical gardening practices (like rototilling) is that they significantly upset the network of beneficial mycorrhizal fungi, and they destroy glomalin.
With no-till gardening, glomalin is preserved, soil structure is improved, and garden plants are able to receive the full benefits of naturally-occuring mycorrhizal fungi in the soil.
Mycorrhizal fungi and glomalin only work where there are live roots in the soil to interact with. In my Minibeds-on-Plastic gardening concept, much of the garden is covered with black plastic mulch. It is no-till soil under the plastic, but there are no plants with live roots.
This can be viewed as a drawback to the idea, but I am looking at it as a worthwhile compromise. I am forfeiting the maximum benefit to the soil of complete, live-root, no-till in return for easier (much easier) manageability of the whole garden space, with high productivity from focused attention on the minibeds.
Without the plastic mulch, garden weeds will get ahead of me and take over. That has been my past experience with a large area of garden. It's discouraging and it's not productive. And it's always a hassle getting the garden back in shape after the weeds have taken over. This will not happen with a manageable Minibeds-on-Plastic garden (and I won't have to get my rototiller out to make the clean canvass again).
Since I will not be digging the minibeds, all roots, along with all mycorrhizal hyphae and glomalin will remain undisturbed in the soil. These roots (and hyphae), along with other beneficial microbiology will extend beyond the minibed perimeter.
Also, as part of my minibed no-till approach, I will be rotating root-dense cover crops into the minibeds. Rye, in particular, for fall to spring cover cropping, will put enormous root density into the soil.
My cover cropping will not be dug into the soil (as a green manure). The top growth will be harvested, chopped up, and used as mulch on the beds, or incorporated into a compost pile.
So that's my concept of no-till gardening with the Minibeds-on-Plastic gardening idea.
Dan Grubbs has asked about the potential problem of high heat under the black plastic. It may not be an ideal environment for beneficial soil organisms. Yes, that might be a problem, especially in certain growing zones, as I explain in my Minibeds-on-Plastic Report #1 (see Question 15 on page 25). If it turns out to be a problem for me, I will put a layer of wood chips over the black plastic.
Some Fungal Loose Ends
Mycorrhizal fungi are just one of many different kinds of fungi that are in soil. Some fungi work to break down organic matter. Some are pathogenic, but most fungi are good.
Mycorrhizal fungi can and will develop a symbiotic relationship with most plants, but it does not cooperate with the brassicas.
Mycorrhizal fungi are naturally occurring in most garden soils. They will naturally proliferate under the right conditions (no-till).
You can purchase mycorrhizal inoculants for your soil. The effectiveness of these inoculants is questionable.
There are bacteria that also create bio-glues that aggregate soil, on a smaller scale.
Good Soil Structure