Soils 101: Guest Blog by Nick Rhodehamel

Don’t tell a soil scientist about the “dirt” in your garden, and don’t call the dust of Mars “soil”, as NASA press briefings generally do. The soil scientist’s outrage may be feigned, but the point will be that soil is much more than something you sweep from your kitchen floor or wash from your hands, and it’s not simply eroded planetary crust.

In a very real sense, soil is alive and evolves—biologically, chemically and physically. It is always changing; it is always growing. It is essential for all terrestrial plant life, and without the food, feed, fiber, and, of course, oxygen that we derive from plants, we and the rest of the animals could not live.

Soil is the home of a vast array of life forms that function in concert with each other and contribute to the functionality and health of soil. In addition to plants, there are a plethora of animals—mostly invertebrates, various arthropods and worms—and there are fungi and bacteria. The number of bacterial cells alone estimated to inhabit the Earth’s soils is staggering—2.6 x 1029. If you’ve forgotten scientific notation, that’s the number 26 followed by 28 zeros. In terms of cellular mass (fixed organic carbon), this is estimated to be about equal the mass of all plants on Earth. All these creatures add to soil and change it through interactions with each other cycling nutrients through complex food webs and directly with soil in chemical and physical ways.

Soil is formed over time through interactions among the parent material (planetary crust, rock), climate, topography (surface shape and features), and biological activity. Soil is a mixture of four different elements—minerals, organic matter, water, and air; minerals and organic matter are solid and comprise about half of a soil’s volume.

Soil has texture. Texture (in part) controls how water moves in soil, affects chemical reactivity and nutrient availability, and plays a role in the potential for erosion of a soil. Texture is determined by the relative proportions of three soil components—sand (2–0.05 mm), silt (0.05–0.002 mm), and clay (<0.002 mm). Sands, silts, and clays are as varied as the parent material from which they come.

Three basic soil types exist: sands, clays and loams. Sandy soils have more than 70% sand and do not retain water or nutrients well; clay soils have 25 to 35% clay and will retain nutrients but drain poorly. Loam soils have the good features of both sands and clays (and also contain silt). The ideal garden (or agricultural) soil is a loam composed of roughly equal amounts of sand, silt and clay. You can determine the texture of your soil and, by inference, its characteristics through the USDA Soil Texture Calculator (see http://soils.usda.gov/technical/aids/investigations/texture/) or by feel (see http://soils.usda.gov/education/resources/lessons/texture/).

Soil has structure. Soil structure is the arrangement and size of the solid particles of the soil and the pore spaces between them. Soil structure too is influenced by the parent material from which the soil developed, and physical–chemical and biological processes are involved in its formation.

Positively charged ions (cations) such as calcium, magnesium, and aluminum initiate binding soil particles together (“flocculation”) into aggregates. Clay particles (platelets) have lots of negative charges and begin to aggregate into microscopic clumps called “floccules” as cations collect between platelet surfaces and bind them together.

Burrowing worms, plant roots, and fungal hyphae create spaces between aggregates; their secretions and exudates further bind those aggregates together. Fungi and other microorganisms also deposit storage materials in the soil that act as organic glues. These along with plants, animals, and their waste and decay products comprise the organic material in soil. Fully decomposed organic matter is called humus. Humus also contributes to binding soil particles together and provides nutrients for plants and the other soil organisms that are essential for healthy soil.

Given sufficient time, most soils will develop stratified layers or “horizons”. Horizons result from chemical weathering, break down of organic matter, and the movement and deposition of humus, mineral particles, and other chemical substances from the upper layers of a soil to the lower layers as water moves through the soil profile. In agricultural fields, the layer familiar to most people makes up the “top soil”, which is brought to the surface and mixed with upper soil layers by plowing. Most soils have a distinct profile or sequence of horizontal layers that can be seen in road cuts or washed out gullies.

Soil health is fundamental to the productivity of your garden. The basis of the idea of soil health is that soil is a living, complex and dynamic environment that does more than hold up your plants (the soil scientist’s refrain). A healthy soil has the capacity to infiltrate water and cycle nutrients to feed growing plants. The more diverse its fauna and flora, the more fully functioning and productive the soil is. There are lots of relatively simple things you can do to manage and improve the health of your soil to increase its productivity.

First, the less you do to disturb the soil, the happier it will be. You often need to work or till soil in your garden (distribute fertilizer or work in soil amendments, suppress weeds, whatever), but too much tillage is a bad thing. Tillage disrupts soil structure and soil organisms. Tillage and too much traffic over soil can compact it. In compacted soil, water infiltration is reduced, runoff (and erosion) is increased, and as a result productivity is decreased.

Consider also that soil is populated by plants and organisms that evolved there together. Long ago they adapted efficient and sophisticated ways of working together to produce and cycle nutrients (food webs). In an ideal state, it is a functioning and self-regulating system. But no system is at equilibrium for long, and occasional inputs of fertilizer or pesticide may be necessary to maximize plant growth and health. But excessive inputs can disturb or entirely disrupt food webs, and this reduces the health of your soil and hence its productivity. Before you fertilize, determine whether your garden (or lawn) needs it. There are soil testers on the market for less than $20 that will give you a good guess about your soil’s nutrient status (http://www.burpee.com/gardening-supplies/garden-growers/electronic-soil-tester-prod001200.html); alternatively, your local county ag agent can advise you on where to get your soil tested professionally.

As much as possible, keep your soil covered. Mulch is a good way to do this. Mulch insulates the soil, reducing rapid temperature changes. It protects soil from the effects of raindrops, which breakup soil aggregate structure, and it retains moisture and reduces water loss. Mulch suppresses weed growth (reducing the need for tillage) and provides habitat for members of the soil food webs. Its breakdown products (ultimately humus) enrich the soil. For your plants themselves, mulch reduces freeze–thaw cycles, frost heaving, and the like. Many municipalities convert their green waste to mulch and deliver the finished product for a nominal charge. Garden stores sell it too.

Cover crops (or green manure) have many of the same benefits as mulch, but in addition they actively encourage and support soil microorganisms. In natural environments, many soil organisms exist in a near-starvation state or go through boom–bust cycles that follow the seasons. Live roots growing throughout the year support and maintain stable populations of soil organisms that utilize the sugars secreted by plant roots and return the favor by translocating minerals and water to plants. At the end of the season, the cover crop can be worked into the soil to provide additional biomass that will be broken down and recycled.

Even in the Northeast, hit last weekend by a snow storm, it’s not too late to plant a cover crop. Burpee has several cover crop mixtures tailored to different growing zones.

The key to improving soil health (and the productivity of your garden) is to remember that soil is a living, changing system that you manage with a light hand. Increase your garden soil’s above- and below-ground diversity with mulches and green manures and as many different kinds plants as makes sense. With time, this will create a more fully functioning and resilient soil that will be reflected by the plants in your garden.

This entry was posted on Tuesday, November 1st, 2011 at 3:33 pm and is filed under Original Posts. You can follow any responses to this entry through the RSS 2.0 feed.
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12 Responses to “Soils 101: Guest Blog by Nick Rhodehamel”

  1. Cathy W said:

    Thank you for that wonderful post. Besides all the great info you mentioned, I went out to your links to USDA and found out all kinds of more info including that states have a designated type of dirt, just like they have state flowers. Keep them coming…cw

    • Nick said:

      Dear Cathy,

      Thanks for reading. Yes, indeed, there are state soils. Ah, legislators: I suppose one state got a state soil and then they all had to have one. The only one I know is the Antigo silt loam, state soil of Wisconsin. There is also a song (which I’ve not heard) about this soil composed by soil scientist F.D. Hole, second author of Soil Genesis and Classification, now in its fifth edition. No doubt you can find someone performing the song on YouTube.

  2. Betsy of Chatham said:

    A superb article! Thanks so much for sharing your exceptional knowledge–it is much appreciated.

    • Nick said:

      Dear Betsy,

      Thanks very much for your kind words and taking the time to post.

  3. Elizabeth Kessler said:

    Can soil determine if a Cupressus Blue Ice keeps its blue color or turns green? I planted one in June and it is no longer blue. It’s a native of Oregon. Is their soil akaline or acidic?

    • Nick said:

      Dear Elizabeth,

      I think the tree you’re talking about is Cupressus arizonica var. glabra ‘Blue Ice’. It’s actually native to the southwestern USA. This particular cultivar was found as a sport in a garden in New Zealand in 1960. I very much doubt that soil acidity–alkalinity has any effect on the color of a healthy tree. It should be grown in full sun and tolerates dry soil, salt, and heat well. The tree does not like compacted or heavy, poorly draining soil. Nor does it like highly alkaline soil. Standing water can kill it. If the tree appears healthy, I’d be inclined to leave it alone and see if the blue color comes back with a new flush of growth in spring. It may well be that the greening is seasonal color variation and is characteristic of the tree in fall.

  4. Steve McNew said:

    Wonderful edition on soils! Will this be part of a series, I hope? Any thoughts on steaming beds, as is sometimes done at a heritage farm in our area? Thank you!

    • Nick said:

      Dear Steve,

      Thanks for your kind words. The series idea is a good one; I’ll think about it. I don’t know much about soil steaming. I know it has been used as a substitute for soil fumigation with methyl bromide to kill soil-borne pathogens such as Fusarium species. Methyl bromide is pretty toxic. There is a relatively new approach to kill weed seeds with steam applied to the soil in narrow bands (effective between rows). This has been used in organic vegetable operations that otherwise, because no herbicides are used, would require lots of weeding, mostly hand weeding. Some of the drawbacks are that steaming uses lots of energy and is more effective in particular soil types (better in sands than loams). It is certainly also going to kill resident beneficial soil organisms as well as weed seeds or pathogens. In some cases, it’s probably the way to go, though. Off the top of my head, it could be useful in something like salad greens cultivation where the crop cycle is very short.

  5. Maxine Brink said:

    Hooray for your guest blogger! Tomatoes and Soil are worth reading and further consideration. Keep him as a contributor.

    • Nick said:

      Dear Maxine,

      I’m happy that you liked both articles. Please keep reading and posting.

  6. Anita said:

    Great article thanks

    • Nick said:

      Thanks very much, Anita. Glad you liked it and thanks for taking the time to post.

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