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May 2007

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Colorado soil offers challenges to new gardeners

By Steven Olson

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At the Larimer County Extension office, if someone calls for advice with a problem related to landscape plant health, the chances that they have a soil problem are 80 percent.

That's understandable, since soil is the foundation of nearly everything. But often gardeners try to solve the problems using gardening knowledge for other regions. Since Colorado has a lot of immigrants from other states, that knowledge is frequently applicable to where there are from and not to where they live now.

Here is an example: Someone from Massachusetts moves here and plans to start a garden. Massachusetts, a New England state, has acidic soils. So the first thing the mythical Massachusetts gardener does is look for a bag of lime to make his soil less acidic.

While this might be a good idea for Massachusetts, it is a mistake here. The soils in northern Colorado are not acidic.

Thaddeus Gourd, Adams County Extension agent, sees this all the time. "It's one of the biggest mistakes people make when they try gardening," Gourd said. "You don't want to put that (lime) on your soil out here. It's a de-acidifier. Out here you need all the acid you can get from the soil because it's alkaline."

People bring their gardening habits with them from other parts of the country and use them automatically without thinking about where they are.

What are the worst ones? Ernie Marx, Larimer County Extension agent, answered that question unhesitatingly: People add too much composted manure to a garden, which brings a lot of salts into the soil, and it's a very difficult problem to correct.

"That is far and away the number one soil management problem I see here in gardens," Marx said.

It is no secret that northern Colorado soil is about 1 or 2 percent organic matter. That's low. It's a function of the skimpy rainfall on the Front Range. To paraphrase Marx, not a lot of things grow here in abundance because there isn't a lot of moisture, therefore over time, not a lot of things die and get incorporated into the soil. According to Marx, building soil organic matter up to 5 to 7 percent is a good goal.

Tom Haynie, co-owner of Creekside Garden Center in Fort Collins, said would-be gardeners first need to amend their soil with organic matter. This will improve drainage, which is needed here, but it's a slow process, vulnerable to gardeners who try to speed it up.

If making a new garden, put 1 to 2 inches of organic matter on top and mix in to a depth of 4 to 6 inches, Marx advised.

Use compost rather than fresh manure, Gourd said. Fresh manure takes longer to break down into a form where the plants can take it up and use it. It could also burn plants or raise health issues.

"Soils need to breathe, and adding organic matter allows that to happen," Gourd said.

Seven percent organic matter is something to aim for, but some gardeners try to get there all in one shot. "What they will do," Gourd said, "is get out their little rototillers and they'll till stuff in until they work up a sweat. Then they'll quit. When they have problems, they'll swear they were tilling stuff in about 6 inches, which is wrong. Small rototillers won't go down that far."

In contrast, too much tillage could affect worm populations, and worms are one of the best organisms to help turn the soil, Gourd cautioned.

Some new gardeners also reason if 7 percent organic matter is good, 14 percent must be twice as good. It isn't. Not only is it not needed, it causes that salt problem.

So how much is too much? In a 3-foot-by-5-foot raised bed, Gourd recommends about 2 inches of compost tilled in a year. Gourd and Marx both recommend testing the soil after three years of amending to monitor salt levels. There is a soil-testing lab at Colorado State University, and there are a number of private labs, too. The Larimer County Extension office can provide a list of labs and procedures for submitting samples.

The second and third problems relate to the heaviness of the soil. Because there is so little organic matter in it, it compacts when stepped on. When it gets muddy, Marx said, the structure of the soil compacts more easily. This is where the word "gypsum" comes in and the second mistake happens.

Gypsum is often marketed as a tool to loosen soils that are "tight" or don't drain well. In fact, gypsum is only useful on soils that are high in sodium, and even then you need to have a method for draining the soil. Sodium is not a problem in most Larimer County soils, and adding gypsum will do nothing to alleviate soil compaction.

"It doesn't work," Marx said. "If you have problems with drainage or compaction, your best bet is to add organic matter and maybe cut back on your rototilling."

The heaviness of the soil also causes mistake number three. Some gardeners try to improve the drainage of the soil by adding sand, which under all-too frequent conditions makes a cement-like texture. Outside of the obvious problem of turning the garden into a primitive sidewalk, the sand issue touches on a larger problem: stratification.

"Plants like the same texture of soil," Gourd said. If the soil is compacted a foot or so down, plants will tend to follow the path of least resistance and spread out. "You have to avoid making an artificial stratification as much as you can," he said.

Soil structure is determined by how individual soil granules clump or bind together. "We call it good soil tilth--the workability of the soil," Gourd said. "Good tilth allows you to prepare a seed bed."

"A good soil will smell earthy," he added. "If the soil smells sour, you probably have a drainage issue."

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