Ask Miss Jean!
Jean Lovell, long-time Resource Central volunteer and former master gardener, tackles your gardening questions!
Submit your question(s) for Miss Jean to: GardenInfo@ResourceCentral.org
Q: Amending my soil feels daunting. What should I consider when thinking about it?
A: The soil in much of Colorado’s Front Range is clay and alkaline. Soil amendments are materials added to soil (compared to mulch which is applied on top of the soil) to improve its composition and fertility and thereby enhance plant growth. On all soils, amendments improve soil structure as well as feed and provide homes for organisms and microorganisms. On clay soils, soil amendments, increase porosity and permeability and improve aeration, drainage, and rooting depth. On sandy soils, soil amendments increase the water and nutrient holding capacity.
Last month we talked about soil testing. This month we are discussing amending your soil. Here’s an important caveat, however: native plants or well-adapted, non-native plants which make up much of our Garden In A Box kits, tend to thrive in these local soil conditions without very much conditioning. Please consider the nature of the plants you intend to grow when considering how much soil conditioning, if any, is needed.
Before we get into conditioning the soil let’s talk a bit more about soil.
The best garden soils are about half solids (sand, silt, clay, plus organic material) and half pores or spaces (filled equally with air and water). Much of Colorado’s soil is clay; it also tends to be slightly alkaline and low in organic matter which makes it less porous. Walking on the soil, leaving it exposed to the elements, or working on it when it’s too wet all degrade its composition.
Fertile soil is the foundation of a good garden. In addition to physically supporting plants, soil and the biological life that lives there provide storage for plant nutrients. Soil changes constantly as plants utilize the available nutrients and rains deplete them. Some aspects of soil fertility:
Tilth – the physical condition of the soil related to its suitability for plant growth (presence or absence of clumps, water channels, and aeration).
Water holding capacity – ideally well-drained but holding adequate water. Clay soil tends to hold onto water for a longer duration while sandy soils drain water quickly.
Nutrient holding capacity – ability to retain needed minerals. Clay soils typically have higher ratings, but do have other drawbacks.
Organic matter, ideally 4-5% – important to biological activity and affects the other three aspects too.
SOIL CONDITIONING or SOIL AMENDING
Soil conditioning addresses all of the above aspects of soil fertility. It includes regular (or annual) applications of organic material, cover crops when appropriate, providing channels for water drainage, avoiding walking on garden soil, and applying gravel or mulch to cover bare soil.
A soil test (see February’s Newsletter/blog post) will include recommendations for correcting problematic properties including nutrients, texture, water retention, pH (acidity or alkalinity). Soil amendments are frequently part of those recommendations.
There are two types of soil amendments: organic and inorganic. Organic amendments come from something that was once alive; they include sphagnum peat, grass clippings, dead leaves, compost, manure, biosolids, and sawdust. Organic amendments increase pore space which allows for better oxygenation for root growth and makes it physically easier for roots to penetrate the soil; improve water infiltration; provide homes for earthworms and microorganisms; and some even provide plant nutrients. Inorganic are either mined or man-made; they include vermiculite, perlite, and sand.
THINGS TO CONSIDER WHEN DECIDING ON AMENDMENTS:
– To work properly amendments must be thoroughly mixed into the soil.
– Don’t use uncomposted wood products; they interfere with soil conditioning. (what are these)
– Don’t add sand to clay soil – leads to concrete-like soil structure.
– Expanded Shale, an inorganic fertilizer mined in Golden, CO; can improve alkaline clay-based soil – adds aeration and space for roots and microorganisms; also a pest (rodent) deterrent and attracts beneficial organisms such as earthworms.
– On sandy soil, you might consider some worm castings.
– For quick and long-lasting results, use a combination of materials.
– Manure, while it is readily available, use it with caution. It’s often high in salts, which can build up in soil and become toxic to plants. Unaged manure can be harmful to plants – so always seek aged manure which will have little to no foul smell. Certainly, don’t use fresh manure on food producing plants.
– Plant-based composts are low in salts, which is preferred.
– Sphagnum Peat vs. Mountain Peat – sphagnum peat is an excellent amendment and semi-renewable; mountain peat is not as effective and is renewable only over hundreds of years, if ever.
– There is much controversy over biosolids; CSU advises avoiding their use in vegetable gardens.
– Compost Tea – Biologically active compost steeped in water. All of the benefits of compost plus it can be sprayed on the leaves to suppress foliar diseases. This method also increases nutrient availability.
HOW TO APPLY SOLID AMENDMENTS
– Mix it thoroughly into the soil to a depth of about 6-8 inches
– In a new garden, use a shovel to thoroughly mix it with the soil
– In an established garden, rock a garden fork or pitchfork back and forth to work amendment into the soil with as little damage to plant roots as possible
Join us next month as we dive into the topic of fertilizing.