Roughing up soil is the best way to make it healthy. Here’s a primer on amendments for this area.
BY CAROL O’MEARA
Roughing up soil is the best way to get it in shape for both plants and beneficial microbes. Here are the best soil amendments for this area’s growing season.
Every gardener dreams of a verdant yard bursting with riotous blooms. So when spring finally warms the ground, we head outside and eagerly plunge a spade into the earth, where we run smack-dab into the concrete reality of Colorado soil. To put it mildly, it’s not a dream come true.
Heavy clay clings to shovel blades, pushing back attempts to dig deeper. Chipping away, gardeners labor to grub out the smallest of holes in which to tuck precious plants. Out of frustration, we employ pickaxes, consider backhoes, and in the end, realize the soil needs some tweaking before we can realize our dreams.
Organic amendments like compost, sphagnum peat and aged manure are great amendments for Colorado soils, as they create air pockets between the dense clay particles that are essential to vigorous root growth. When adding amendments, spread 2 inches of organic material on the soil and work it in with a shovel or rototiller it in to a depth of 4 to 6 inches. Remember, it takes years to properly improve clay soil, so amend soil for vegetables and flowers yearly, for trees and shrubs at planting.
Not all amendments are created equal, so here’s a brief primer on amendments to get your soil in shape. But the best first step is to get a soil test so you’ll know exactly what your dirt digs.
For rich, crumbly, healthy soil, a long-term approach is crucial. In annual or vegetable beds, applications of plant-based compost over several years will build up organic matter, which is the most important material you can add to soil. Organic matter is usually low in Colorado soils—less than 2 percent. Ideal soil contains 5 to 6 percent.
You can make compost yourself or purchase it in bags or bulk. Several compost sources are available, such as mushroom- or manure-based, but for low-salt, high-performance compost, stick with plant-based compost. Because it’s a source of organic matter, compost encourages the growth and well-being of soil microbes, creating a community that keeps soil rich and plants healthy.
Though compost contains some nutrients, it isn’t fertilizer. What nutrients it has are released slowly over time, so plants will need additional nutrient sources.
Traditional gardeners consider manure to be the ideal amendment. Cheap and readily available, manure builds up soil organic matter and adds nutrients, depending on its age. Composted manures often have small amounts of nutrients, while fresh manure can be high in ammonia. During composting, ammonia gas is lost from the manure, lowering nitrogen levels. But composted manure can be high in salt, so if your soil is already salty don’t apply manure.
Be cautious about applying fresh manure to kitchen gardens, because of the risk of E. coli contamination. For gardens with edible crops, manure should be aged six months or longer before application.
Sphagnum peat is a good soil amendment, particularly for sandy soils, as it helps retain water and nutrients and its low pH is useful in our alkaline soils.
Look for sphagnum peat sustainably harvested from Canadian bogs, which can re vegetate and grow back relatively quickly if they’re not over harvested. Avoid Colorado mountain peat mined from high-altitude wetlands; these sources take hundreds of years to rejuvenate, if they ever do.
Bio-solids, or composted sewage sludge, add slow-release nutrients and organic matter to soil. Containing 5 to 6 percent nitrogen, bio-solids benefit the soil, but use this amendment in moderation as some brands are extremely high in salts.
Check the bag or contact the distributor for an analysis; if the bio-solids have more than 10 ds/m (a measure of electrical conductivity), avoid them. Choose only Grade 1 bio-solids, which means the product doesn’t contain excessive heavy metals and the pathogens have been killed.
Also, never use bio-solids on root crops because of exposure to the plants’ edible parts. Make yearly applications of this amendment only if you routinely test soil for salt. Bio-solids are available from some communities or sewage-treatment plants, or in bags at garden supply stores.
Perlite and Vermiculite
Because of their expense, reserve per-lite and vermiculite for bettering soil in small plots. Both are common in potting soils and planter mixes, yet they perform very differently. Vermiculite is heat-expanded silica that holds water well and increases pore space by physically opening up spaces between clay particles, enabling roots to expand. Per-lite is heat-expanded volcanic rock that doesn’t retain water, so it only improves pore space.
Lime, or calcium carbonate, raises soil pH. Most soils in our area are already slightly to very alkaline, so before adding to garden woes by increasing the pH, get a soil test to determine if your soil needs more alkalinity. Chances are it doesn’t.
Gardeners hoping to improve their soil by adding sand should brush up on ancient history first. For millennia, humans have combined clay, sand, organic matter and water to make bricks. Although wonderful for building adobe homes or pyramids, sand isn’t good for soils that grow plants. When mixed with clay, the sand particles’ pore spaces fill with clay, creating a dense soil that roots have difficulty expanding into.
Touted as a clay-busting calcium-rich amendment, gypsum, or calcium sulfate, is not a good amendment for this area. In soil, the calcium separates from sulfate over time, then bonds to clay particles while the sulfate leaches slowly away. Because Colorado soil is naturally high in calcium, adding more of it is a waste of money.
How ’bout Lasagna?
Popularly known as lasagna gardening, sheet composting is a cold-composting method in which raw materials are layered onto soil or into a trench and left to gently rot. Layers of newspaper, brown plant material and green manure or green plant material are set on top of each other in “lasagna-like” fashion.
Popularized by gardening gurus in other states, this method doesn’t impress many local experts due to our semiarid alkaline soils. “Lasagna gardening works great in areas that have a lot more moisture than we do,” says retired soil scientist Dr. Jean Reeder. “Can we do it and be successful here in Colorado? Yes, but you have to keep it moist throughout the winter, and never let it dry out.
“Chop up the brown pieces—things like leaves, cardboard, dead plants—into very small pieces, then make sure you add enough nitrogen in the form of green plants or grass. Keep an eye on your lasagna, making sure it stays moist, and you’ll probably get it to work.
“But don’t think you can start this in spring and then plant straight into the sheet compost. Your plants won’t grow as well in it because the composting process ties up the nitrogen, which will make the plants a lot smaller and less productive than those planted in amended soil.”