Soil A Primer

Do you know dirt about soil? Here's a three-step primer

It's brown. It's underfoot. And it's the key to a good garden: good soil. If you understand its structure, biology and chemistry, you can give plants a good start.
It's brown. It's underfoot. And it's the key to a good garden: good soil. If you understand its structure, biology and chemistry, you can give plants a good start. To foster a healthy community of biological life in the soil, till it less, compost dead plants and turn them back into the soil.
Gardeners obsess over that stuff in the ground. Year after year, we throw potions and powders at it to break it apart and build it up. But most people don't know dirt about soil.That's a big problem.
"There are so many earnest people doing so many earnestly bad things to their soil," says Jean Reeder, consultant and retired soil scientist. "They mean well, but don't know they're harming it."
Achieving real understanding of soil can be daunting. You don't have to ask many questions before you're buried in hydrogen potential, electrical conductivity and hydro phobicity. It's enough to make your brain beg for simplicity.
So we went to the top minds in what's underfoot. Here's how they broke it
Structure: Soil is a building
Here's your first geeky soil term: tilth.
It refers to the suitability of soil to support plant growth, and it's a combination of texture, structure, microorganisms and fertility. Soils with good tilth take many forms.
"You're shooting for a sense of balance that promotes healthy plants. The classic rule of thumb is that an ideal soil is 50 percent solids and 50 percent pore space. Of the pore space, half is filled with air, half with water," said Reeder. "The solids are the minerals (sand, silt, and clay), plus organic matter, which in most Colorado soils is usually less than 3 percent."
Where gardeners go wrong: doing things that decrease pore space (walking on wet soil, for example); and overdoing organic amendments and fertilizers.
Second geeky term: texture. That's how much sand, silt, or clay your soil has. Structure is how those ingredients group together and create pore spaces of various size. All three, clumped with organic matter, form aggregates.
Different soils have different types of aggregates, leading to different types of structure. Topsoils often have a granular structure that allows water to drain rapidly. Soils with high clay content usually have a blocky, chunky structure where water drains moderately well.
But compacted soils, or soils with very high clay content, usually have a plate-y structure. So water moves very slowly through those plates. Think of the differences between a bucket of pea gravel and a bucket of pennies.
"Water and air move differently in soil," said Reeder. "Water coats particles and fill small pore spaces. Air resides in the big pore areas. This is why clay soil holds more water than air; it has really small pore spaces. Sandy soils are the opposite: With mostly large pores, it holds more air, but less water."
When different soil textures come in contact with one another, the results work against plant success.
Third geeky term: Textural interface — which is what it's called when this happens in soil. If one soil type is layered against another, a boundary is formed. The differences in pore sizes mean that water won't move from one soil into another until the first soil is completely saturated. This is commonly called a "bathtub effect." You get it when you're planting trees or building raised beds
and fail to blend the edges of the hole you've dug, and the soil you're putting into it, together.
(Scurfing, or mixing, the two edges ensures that water will drain from one type of soil into the other).
Fourth geeky term: Compaction. It's the No. 1 problem in urban soils, compressing pore space and limiting oxygen.
"The worst thing to do is walk on, drive on, or work wet ground," Reeder says. "When you do that, you measurably increase compaction."
The best time to deal with compaction is before landscaping goes in, by adding organic matter. But if the landscape is already installed, you can help by making paths to walk on, aerating the lawn, orplanting cover crops in vegetable gardens. In perennial beds, a blanket of mulch prevents compaction when you're weeding and working.
And a final, non-geeky term: caution.
Be sparing when adding organic matter and fertilizer to gardens, Reeder said. Too much of a good thing ceases to be helpful. Many commercial composts, especially the animal-manure based ones,are very salty and contain excessive levels of plant-available nutrients. That's harmful to plants and the creatures that live there.
Biology: Soil is a zoo
Within the soil is a teeming community of bacteria, fungi and soil fauna such as protozoa and nematodes, says Mary Stromberger, associate professor of soil microbiology in the Department of Soil and Crop Sciences at Colorado State University.
Bacteria are only the simplest life forms in soil. Extremely tiny bacteria break down dead plant material. Then they excrete a sticky waste that holds
soil particles together. Called "biological glue," this gummy material is vital to keeping soil together and helping roots grow.
"As decomposers go after carbon, they excrete unused nutrients like nitrogen and phosphorus, turning them into a form the plant can take up. This is known as mineralization," Stromberger explained.Some bacteria even live in symbiosis with the plant, colonizing roots and converting the very nutrients those roots need. Gardeners planting peas or beans, for example, often inoculate their seed
with the bacteria Rhizobium. This gives the plant a head start.
Fungi in the soil often work to help plants, too. Some wrap hyphae — long strands of cells — around soil particles to hold them together. They also decompose plant litter. And some, such as mycorrhizae, take up residence on plant roots. There, they pull carbon from the plant and in return,
make phosphorus, nitrogen, and micronutrients to feed the plant. They even specialize:ectomycorrhizae grow along the surface of tree roots, while arbuscular mycorrhizae grow within the roots of grasses, vegetables, and shrubs.
Healthy communities need predators, Stromberger said. Soil life has its share.
Protozoa and nematodes swim through the water in the soil to feed on bacterial or fungal spores, keeping these populations in check. The fungus Trichoderma wraps its hyphae around other fungi, then releases a digestive enzyme that dissolves them so the Trichoderma can feast. (Before you feel sorry for the victim, it's often pest fungi, such as Phytopthora, that are on the dinner plate).
Such benefits from soil microorganisms are what Stromberger focuses on in her research. She points to antibiotics, streptomycin, and cancer-fighting drugs that have their origins in soil biology. But her driving focus is how to harness the beneficial side effects of our tiny soil neighbors. "I'm interested in soil's role in food security, sustaining it in the face of drought."
So: that handful of grayish brown stuff from your yard is really a zoo. What can you do to conserve its life forms?
Stromberger: "To build a healthy community, you need good organic matter. That's lots of food for those species. Then you need good structure of the soil. Don't disturb it. Tilling rips up fungal hyphae, and many can't survive that."
Vegetable gardeners need to add organic material, she acknowledged, but they should be cautious and limit the number of times soil is tilled. Leave as much surface residue as you can, and your soil will feed the plants that feed you.
Chemistry: Soil is a symphony
If soil is a symphony, pH is its tympani. It speaks loudly. Muting its effect, or at least helping it not overtake the rest of the ensemble, takes long and careful orchestration. That's why we turn to Thomas Borch, CSU associate professor of environmental soil chemistry.
Recently named one of the world's top 15 brightest minds in this area, Borch is passionate about soil conservation, and explained why understanding pH is key to understanding the fate of pesticides, nutrients and contaminants in the ground. "So much is influenced by pH, and any change to that can potentially change the availability — or toxicity — of a substance," he said.
These two letters express a measure of the acidity or alkalinity of a soil. The scale is 1 to 14, and 7 is neutral. Most Colorado soils are slightly alkaline (above 7). And that's a condition Borch said is difficult to change because of our soil's high amounts of calcium carbonate."When you add something like sulfur (to an alkaline substance) in water, it releases acid. But calcium carbonate, dissolved in water, is a base, (so) together they neutralize each other," or cancel each other out. Meaning: You're wasting your time and money.
In fact, Borch said, "We have so much calcium carbonate, whenever you add a little acid, it neutralizes it; add more, it neutralizes that, and so forth. This is called buffering." And it's why you're never going to get a yard full of soil that acid-loving plants adore.
What you can do is this: Add organic material. It gradually lowers pH over the long term — we're talking a decade here — plus it benefits many other things. "Take good compost and till it in, then add a layer each year and turn that in if you really want to change pH."
Because here's the real deal with soil: We've broken it down into structure, biology and chemistry, but all three of these properties interact. And organic matter helps them all.
"Organic matter increases the water holding capacity of the soil; you'll have water available to plants over a longer time. It increases the aggregation of soil, so you increase oxygen. It increases nutrient retention, so they're less likely to leach away. It limits erosion and stimulates a healthy microbial community," Borch explains. It does more: It actually prevents pollution. Leaching and runoff of nutrients, pesticides, and contaminants is where Borch focuses his research. "What really controls the fate of pesticides, nutrients, and contaminants in the ground is retention, binding it to soil minerals or soil organic matter," he said.
Good soil structure and biology keep nutrients and pesticides from flowing right out of the soil. If your soil is healthy, it helps the good stuff bind to soil, meaning a Colorado cloudburst won't wash it away into groundwater, where it does your plants no good and may do harm. They limit how much damage can do.
Borch is passionate that organic management of lawns and landscapes is vital to keeping soil healthy. Over applications of chemical fertilizers, by contrast, are harmful in a variety of ways.
Phosphate, a negatively charged ion, binds to soil minerals such as iron oxides and remains there — unless rainfall or irrigation causes runoff of soil. Then it flows into the stormwater system. Even worse, nitrogen is highly water soluble. It leaches right down into groundwater.  Overapplication of either chemical has ripple effects throughout the environment, making it even more important to apply at the right time and in the right amount.
Borch says it's better and safer to use organic fertilizer that is slowly released through the action of microbes. "It's much easier to control than synthetic soil amendments. You don't' have to be afraid of contamination by runoff or leaching. But if you want to use synthetic, follow the directions on
the bag for when, how much, and don't over-apply it."