By Carl Wilson, Horticulturist with Denver Cooperative Extension
Colorado soils are very different than the soils found on both coasts and in the Midwest. Improving the physical drainage, aeration and water holding characteristics of Colorado soils is more important than fertilizing as detailed in the message on "Soil Preparation." When you do fertilize, there are some points to keep in mind.
The predominant reaction of Front Range soils is alkaline or basic. Some mountain soils are acid in nature more like soils in the Eastern U.S. A soil test can confirm how basic or acidic your soil is - a test known as pH [pronounced "Pea, H"].
With eastern, acid soils, epsom salts, magnesium sulfate, can be added to promote healthier growth of plants such as tomatoes. Due to the acid nature of these soils, magnesium is unavailable to plants and eastern gardeners benefit from fertilizing with epsom salts. In alkaline Colorado soils, magnesium is highly available to plants and adding epsom salts is a waste of time and effort .
Wood ashes are alkaline in reaction and contain salts high in potassium. In eastern acid soils lacking potassium or "potash," disposing of wood ashes in the garden is an excellent fertilization practice. For our Colorado soils that are both alkaline and contain large amounts of potassium, wood ashes are a useless addition.
What are useful fertilizers to add under alkaline Colorado soil conditions? Nitrogen is generally in short supply and because plants use it in large amounts, frequent nitrogen fertilizer additions are beneficial.
Our alkaline soil reaction makes iron and other micronutrients unavailable to plants. Iron deficiency of plants shows up as yellowing leaves with skeleton-like green leaf veins. Selecting native or adapted plants that grow in soils of an alkaline nature is one way to avoid iron problems. Another is to add a fertilizer containing iron on lawns and to selected plants.
The alkaline nature of most Colorado soils calls for applying specific kinds of fertilizers. Know what they are so you don't waste money and time on fertilizers useless to our soil conditions. More specifics on dealing with iron deficiencies and other fertilization concerns can be obtained from a nursery or garden center, a Cooperative Extension office, or botanic gardens.
Photograph courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010