Get the Dirt!

Soil aggregates, plant roots, and cover crops

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Summer cover crop black eyed peas, some sorghum

Black eyed pea cover crop cut and removed, only pea stalks remain, notice very few weeds

We're doing a lot more experimenting with green cover fallowing and cover cropping in the garden this year. I'm also enjoying learning as much as I can about soil health and the complex biogeochemistry that goes on down below our feet. Above you can see the fantastic results we are getting using black eyed peas (Vigna unguiculata) as a warm season cover crop in Massachusetts (peas purchased for $.99 a pound at the local supermarket). After this cover treatment, the 40 square foot garden bed went into a fall greens mix of spinach, arugula, and lettuce.

Not only was the cover a fantastic weed suppressor, it also helped retain soil moisture, sequester nutrients, and build soil aggregation.

Below you will see the aggregates in action (dark, sticky, brown clods of dirt on the plant roots):


Sorghum soil aggregates. We had a few sorghum mixed in with the black eyed peas.

A question that has come up for me while learning more about cover crops is, do the edible forest garden plants in our garden build, support or act on soil in similar ways, and is there soil aggregation happening on the perennial roots too?

Most of the information out there about soil health and soil aggregates is limited to and focused on annual, conventional crop production. As a forest garden, food forest grower, there is very little information regarding "soil building" with perennial plants. One could assume that a diverse perennial garden acts more like a wild forest ecosystem (that it generally produces its own healthy balanced soil)... but it is a far leap to assume that my garden is acting like a wild forest!

Without putting a lot of intention towards it, what Eric and I have achieved at 'Paradise Lot' seems to resemble what Christine Jones describes as the way to build soil:
"The essential first step to rebuilding topsoil is to maximize photosynthetic capacity. A permanent cover of perennial plants provides an ongoing source of soluble carbon for the soil ecosystem, buffers soil temperatures, inhibits weeds, reduces erosion, improves porosity, enhances aggregate stability and water infiltration, slows evaporation."

Jones continues, "The soluble carbon exuded into the rhizosphere by perennial ground cover plants [grasses/forbes] and/or transported deep into soil by mycorrhizal fungi, provides energy for the vast array of microbes and soil invertebrates that produce sticky substances enabling soil particles to be glued together into lumps (aggregates). When soil is well aggregated, the spaces (pores) between the aggregates allow the soil to breathe, as well as absorb moisture quickly when it rains... Well-structured soils retain the moisture necessary for microbial activity, nutrient cycling and vigorous plant growth and are less prone to erosion."
Unfortunately where Jones and many other soil advisors are lacking is how these ideas relate to a "woody" permanent cover of perennial plants (which is what our garden is).

At a recent soil health event, I asked Christine Jones about the role of trees in building soil, particularly their role as soil aggregators... her response was something like (at least in semi-arid Australia), "Trees don't aggregate soil, but if you want to find out you can dig one up and look for aggregates."

I haven't dug up any of our trees, and I don't plan to, but what I do know is that our perennial polyculture of multipurpose plants garden has become extremely fertile, and that rich humus soil is now down to a depth of 12 inches.

So how did we build such good soils? This recent picture might be a first step in finding out:


See those big black clumps of soil... Low and behold you are looking at a picture of fantastic soil aggregation on the roots of a woody gooseberry bush.

How good are woody plants at building soil with root aggregation? The jury is still out... Look for more on this topic in future posts.

Photos by Eric Toensmeier and others.

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