Our friend, the earthworm! Photo by H. Casselman, Wikimedia Commons
(Click here to listen to an audio version of this blog!)
Something important to appreciate about soil is that it is alive. Soil is not itself a living organism, but it is so chock full of such astoundingly large numbers of living organisms that their activities generate a dynamic, living system.
According to the USDA-Natural Resources Conservation Services, a single square yard of soil is likely to have 500 to 200,000 individual arthropods ranging from very small (and very numerous) collembola and mites to very large (but much less numerous) beetles and spiders, etc.). This square yard of soil may also contain 50 to 300 earthworms which because of their large body sizes can dominate the total animal biomass in a soil. A teaspoon of soil may also contain several hundred nematodes and between 100 million and 1 billion bacteria. A soil will also have abundant fungi that, because of their long hyphae (sometimes many meters long!), may actually have a greater biomass than the much more numerous bacteria!
It is reasonable to say then, employing a powerful and very accurate metaphor, that soil is alive, and since it is alive (in this metaphorical way), it is possible to kill it. Dead soil is the focus of this blog! But first, I need to talk about some of my recent Colorado experiences.
One of the things that Deborah and wanted for our new house was a place to grow some vegetables and flowers. The previous owners did not have an area that was defined as a garden, and most of their flower beds were occupied by water hungry marigolds, some extremely thorny roses and some plastic fir trees (literally … fake Christmas trees stuck into the ground!). Several of these planting areas also had some hardy, possibly native plants that survived the two months of our “no watering” policy. We’ll figure out what they are eventually! Even if they are “weeds,” I respect their tenacity and will probably leave them!
Anyway, to get the square footage we needed to put in a Spring Garden (tomatoes, peppers and so on), and to plant some of the prairie flowers we want to decorate our yard, we decided that we needed to build some raised garden beds and have some soil brought in to fill them. Our son, Joe, and his wife Marlee were visiting for a couple of weeks, so we recruited both of them into the planning and actualization of this project.
Photo by D. Sillman
Joe designed and built four, three foot by six foot, cedar planting boxes, and we placed them in the gravel area which we had previously made on the northwest side of the house. We then ordered three cubic yards of a soil mixture advertised to be ideal for raised bed gardens. It was a mix of top soil (two-thirds by volume) and compost.
We had been advised that the planting soil would need to be covered with a tarp or else it would blow away, and when the truck from the garden center arrived to deliver the soil, I saw that the covering tarp was a very reasonable precaution. The soil was powdery, completely lacking any moisture or structure. There were no clumps, clods, or adhering pieces or parts. It’s consistency was somewhere between talcum powder and bread flour. When it poured out of the dump truck or when we shoveled it from the pile into a wheel-barrow, it sent up clouds of a fine, brown dust that irritated our eyes and throats and clogged up our noses.
Dry soil. Photo by D. Sillman
Fortunately, we all had masks that we wore when we made our grocery and wine store runs, so we put them on to handle this alien material. We didn’t want to irritate and inflame our airways with silica and compost dust. In many ways that seemed at least acutely worse than getting exposed to Covid19!
Once the raised beds were filled, we added water to try to rejuvenate the soil. The added water, though, very surprisingly puddled up on the soil surface and would not percolate into the soil layer at all! Only the top half centimeter or so of soil under the ponded water showed any moisture! The added water ran in surface sheets down into the lowest section of each raised bed and dribbled out through the gaps in the cedar boards. This hydrophobic, powdery material was the strangest soil I have ever encountered!
I went to my computer and Googled “extremely dry soil” and came up with some important pieces of information. Many of the online posts about dry soils were from either from the desert southwest of the United States or from Australia, but all of them told the same story. When soils extensively dry out, not only do their arthropods and other invertebrates dry out and die or migrate out of the system, but the soil bacteria and fungi also become inactive or die. Without these vital microorganisms the soil separates (the sands, silts and clays of the mineral soil) no longer adhere to each other to form complex, labyrinthian structures that are full of tiny air and water pores. Further, without these small, aggregated soil pieces, the larger channels which form when a multitude of these smaller structures randomly tumble together to form a loosely packed soil system also will not form. These larger channels are the vital “gravitational” water channels through which water percolates down into the very deep layers of a living soil.
These gravitational channels were obviously missing in this dry, powdery soil. Further, the smaller pores within the soil aggregates were also missing although less visibly so. These micro-channels are the “capillary channels” of a soil which hold water in the soil against the downward pull of gravity and also against the outward pull of evaporation. These capillary channels generate most of a soil’s water holding capacity. Without these, a soil will not be able to retain or deliver water to support the growth of plants!
To make matters worse (or, at least, much more water repellant) the organic materials in the soil had also extensively dried. This caused them to adhere to each other in very thin layers and generated an oily, hydrophobic coating on the surfaces of the sands, silts and clay particles of the soil.
Water pooling on soil surface. Photo by D. Sillman
Water could not pass down through large gravitational water channels, could not be absorbed into the capillary channels of the soil aggregations, and could not cross the oily coating of the baked-on organic matter that coated all of the mineral materials! The structure of this purchased soil was not at all conducive to plant maintenance or growth!
Our raised garden bed soil, then, was dead. There were no active bacteria or fungi to hold the sands, silts and clays together in complex aggregates, and there were no arthropods or earthworms crawling or burrowing about the system to dig large channels or seed in fresh fungal spores or bacterial inoculations. Our dead soil desperately needed a reclamation and resuscitation program!
The key to soil rejuvenation is the slow, continuous addition of water. Bacteria and fungi from the air or from dry propagules in the dead soil itself, will begin to colonize the soil and grow and start the slow process of aggregation and formation of structural complexity. We watered each raised bed every afternoon. For a week or two the water remained pooled on the surface and didn’t seem to be getting down into the soil profile. We used metal rods to poke water channels into the soil and watched the added water flow into and fill up each channel. Eventually, the added water began to pool less and less. The surface percolation allowed the water to seep down an inch or two into the soil and then several inches. We also used a hoe to turn and mix the surface materials to try to facilitate the wetting and awakening process.
Soil after two months of remediation. Photo by D. Sillman
After a month of wetting and poking and hoeing the soil in the raised beds, the wetting profile had expanded almost a foot down into the beds. The bottom layer of soil, though, still remained completely dry even after an extensive soaking. We hope that continued wetting and poking and turning will allow the added water to rejuvenate the soil all the way through the raised garden bed. Winter’s freezing and thawing cycles and maybe even some colonizing arthropods or earthworms may also get these beds ready for our spring planted tomatoes and hot chili peppers!
The soil we bought from the nursery had been sitting out through the hot Colorado summer exposed to very dry air and very high daytime temperatures. It had been dried and baked out in the dump piles of the garden center into a lifeless state. These soils, though, are not unusual or something that exists outside of nature. Here in Northern Colorado we are surrounded by soils just like this! The harshly baked, exposed soils on the surface of the disturbed grasslands all around us are bereft of significant microbial activity, and their scanty percentages of organic matter have been cooked into oily coatings on their mineral fractions. Wind carries away many tons of this dried , surface soil each year, and rainwater falling on these dead soils runs off in surface sheets because it is unable to percolate into the water repellant surface. This water flow erodes away even more tons of soil and also can gather quite suddenly into flash floods that wreak even more havoc throughout their watersheds.
Photo by A Eminov, Flickr
Watching the powdery soil pouring out of the delivery truck in clouds of dust, then, is a focused picture of the soil problems in the disturbed soil ecosystems of the arid sections of the southern Great Plains! The soil’s erodibility by wind and water, its tendency to repel and shed water and its inability to retain moisture and, thus, sustain plant growth are all driving aspects of the process of desertification and the overall decline in the productivity and sustainability of these abused soils.