The dirt on soil amendments

Soil amendments can improve water availability to plants, keeping more water in the root zone where plants can reach the moisture. “That happens through improving the infiltration of water into soil and improving water retention,” says Jessica Davis, head of the Department of Horticulture and Landscape Architecture at Colorado State University.

Davis is speaking of compost, specifically. Following a study the university conducted evaluating topdressing compost on turf, “We were surprised that even topdressing had a big effect on improving water infiltration and reducing run-off,” she says.

Considering water scarcity in many parts of the country, especially out West, the more we can do to enhance soil’s ability to hold on to water, the less dependent we will be on irrigation. “In Colorado, we are always concerned about water,” Davis says. “We’ve had large increases in population in the last decade or so, and so we have more urban water needs and less water available for farming, or even for landscaping.”

Beyond helping soil retain water, soil amendments also create stronger, healthier turf that can stand up more readily to weeds and disease. The key is to choose a quality soil amendment – and to apply it properly. Here’s a deeper look at how to select and use soil amendments, and why these organic add-ins can elevate the service you offer to customers.

Do-good ingredients.

Technically, a soil amendment is any material added to a soil to improve its water retention, permeability, drainage, aeration and structure. “Basically, amendments have organic matter in them, and as they decompose, they become humus in the soil and increase the soil organic matter content,” Davis says. “Organic matter is much more effective at holding water in the soil as opposed to sandy soil, where the water drains too freely.”

The ultimate goal of a soil amendment is to create a better situation for roots. There are a couple different ways to include soil amendments into a lawn care regimen. Compost can be applied as topdressing on to turf, or it can be worked into the soil before planting.

In landscape beds, working in soil amendments before planting can supplement the nutrients and organic matter in soil to assure planting success, says Jim Sellmer, professor of horticulture in the Department of Plant Science at Penn State University. Sellmer prefers compost over most other organic amendments, which can include sphagnum peat, wood chips, grass clippings, straw, biosolids, sawdust and wood ash. Inorganic amendments range from pea gravel, sand, perlite and vermiculite, among others.

First, identify the goal for soil – then choose an amendment. For example, if you want to improve soil quickly, you’ll want an amendment that decomposes rapidly. If you’re looking for long-lasting improvement, select an amendment that decomposes slowly. “Soil amendments take time – they are like a slow-release fertilizer,” Davis says. “They have to decay, release nutrients to plants, and then with time, less fertilizer will be needed.”

Also, Davis says, “Whatever goes into the compost will affect what you get out of it.”

For example, you’ll find macronutrients like nitrogen, phosphorus and potassium in composted manure, along with micronutrients like zinc and manganese. “If you’re dealing with composted cow manure, those animals eat plants and then excrete plant nutrients,” Davis says.

There is no exact recipe for compost. “Feedstock affects characteristics, so each batch can be different,” Sellmer says. “Knowing in advance what the characteristics of the compost to be purchased are is an important piece of information for the contractor.”

Ask for a compost test from the supplier so you know exactly what you’re getting. “If that is not possible, request the right to reserve a pile and do the compost test yourself,” Sellmer says.

“We’ve had large increases in population ... so we have more urban water needs and less water available for farming.” Jessica Davis, horticulture department head at Colorado State University

This is important because compost impacts soil pH. “If the compost is heavily manure-based, wood-based or a mushroom substrate, it can raise the pH,” Sellmer says, adding that there can be a short spike in pH as materials initially break down into the soil.

That said, compost generally reduces soil pH, which is especially helpful in regions where soil tends to be alkaline. “Generally speaking, as organic amendments decompose, they release organic acids and reduce soil pH,” Davis says.

Sellmer advises testing soil pH prior to planting in new soil – and before choosing plants for a landscape bed. A soil test will determine the nutrients available on a site so you know what supplements might be required so the plants will thrive.

“I recommend a preplant or predesign soil test, followed by a soil test about every three years,” he says. “This allows the contractor or client to compare the original test with the present test to see if things have changed in the landscape.”

Soil tests should come with recommendations for whether nutrients are needed. “Soil amendments should be based on soil test report results because they are intended to supplement the nutrients and organic matter in the soil to assure planting success,” Sellmer says.

Sourcing quality amendments.

Even with organic matter and the benefits soil amendments bring to the earth, you can put down too much of a good thing. “It’s important to know that you can over-do it,” Davis says.

According to a Colorado State University Extension report, ‘Choosing Soil Amendments,’ manure-based composts should be applied at a rate of 1 inch incorporated 6 to 8 inches into soil. Plant-based soil amendments with lower salt levels can be used in greater volume, with an average application rate of 2 to 3 inches worked into a 6- to 8-inch soil depth.

With topdressing, you’ll add a thin layer of material on top of turf – about ¼ to ½ inch of topdressing depending on the turf height. Ideally, turf should be aerated before or after topdressing to work compost into the soil.

“It’s important to take a measured approach,” Davis says. “I’d rather see people apply small amounts of compost topdressing annually for several years rather than trying to ‘catch up’ quickly by applying a lot.”

Proper application is critical – “moderation is key,” Davis says. Also, you want to be sure to use quality compost. “That is one of the biggest challenges that contractors need to be careful about: where you source compost, and make sure it is well composted,” Davis says.

If manure is not fully composted, it will release an odor. (If composted right, this is not a problem.) Also, poor-quality compost that is not heated at high temperatures can contain weed seed. You don’t have to guess whether compost was done right if you request a compost analysis. “It is possible to do damage with bad-quality compost,” Davis says.

Providing this analysis to customers can give them peace of mind that the soil amendments you’ll use on their property aren’t “garbage in,” and will make a healthy, lasting impact on soil quality and plant health.