Fredricka Bogardus
Colorado Master Gardener

As gardeners, we really don’t grow plants; that is the soil’s job. Tilth, the soil’s ability to support plant growth, can be improved over time by amending soil wisely.

A soil test provides data-based insight into what steps gardeners can take to help the garden thrive. Three important metrics for characterizing tilth are pH, texture and organic material in the soil.

pH is a measure of hydrogen ions present in the soil, with values on a logarithmic scale ranging from 0 to 14. Values below 7 are acidic and values above 7 are alkaline, though soil pH values between 6.6 and 7.3 are generally considered neutral.

Most plants thrive in soils with pH values from 6-7.2. Colorado soils can be alkaline. If your garden soil is alkaline, it is critical that you select plants that will tolerate your high pH. It is difficult to lower pH in soil with amendments. Acidic soils are more easily mitigated using lime, but that need would be unusual in the mountain region.

Soil texture refers to the relative proportion of sand, silt and clay particles in soil. Texture influences water retention and drainage properties of the soil, and the prevalence of pore space, needed for root health, in the soil structure.

• Sand particles are the largest; water drains quickly and pore spaces between the particles are abundant.

• Silt particles are smaller; drainage, and air availability will be moderate.

• Clay particles are the smallest and densely packed; drainage is poor and gas exchange is limited.

Ironically, there is one solution that works for both excessive water drainage and retention. The addition of organic matter to the soil will slow drainage in excessively sandy soil, and increase drainage in heavy clay soils. Watering practices can also help mitigate both of these issues.

Organic material amendment is perhaps the best approach to optimizing tilth.

Soil organic matter is made up of decomposing plants and animals, as well as living soil fauna including microbes; it is crucial for soil health, fertility and structure. Optimum levels are 5% to 10% of soil if you are growing edible crops. Native plants are adapted to soil organic matter levels below 3%.

Organic amendments include composts, manures, fine wood chips, peat moss, coir, leaf molds and even coffee grounds. They can be incorporated into soil or applied as a top dressing. If you use manure, it is important to confirm that is well-age. if your soil is alkaline, you may want to avoid manure as it is frequently high in sodium and can further raise soil pH.

As the planting season grows near consider, getting a soil test by contacting agsci.colostate.edu/soiltestinglab/ or 970-491-5061.

If you have drainage or retention issues, optimize watering Colorado State University Plant Talk No. 1621 (planttalk.colostate.edu/topics/soils-amendments-composting/1621-watering-colorado-soils/).

The in-person help desk is closed for the season. Submit gardening questions online at ask.extension.org or call 719-520-7684. Questions will be routed to El Paso County Master Gardeners and answered remotely and by phone.