Improving urban garden soils

In nature soils have a well defined top soil rich in microorganisms, plant roots and organic matter. Urban (landscape) soils differ significantly from these native soils. The top soil and other layers are scrambled, organic content is low, and air and water movement is reduced. How did this happen?

During urban construction, thousands of years of soil development is destroyed in moments. The top soil is scraped off, the soil is severely compacted by heavy equipment and structure destroyed. Structure refers to how the various particles of sand, silt and clay fit together, creating pore spaces of various sizes.

Compacted, unamended urban soils typically have a massive structure with no defined top soil, little organic matter, and few large pore spaces. Large pores are where oxygen enters soils for plant root growth. These soils also have few small pores for storing water for roots.

What can gardeners do with their urban soils?
1. Avoid compaction by not walking or running heavy equipment over your garden soil.

2. Improve aeration and drainage through timely cultivation but do not overwork the soil.

3. Most importantly, feed the microorganisms important for plant root growth and function. Their food is organic matter that you can supply by adding compost and other organic soil amendments. Regular additions can boost the typical 1 or 2 percent organic matter Colorado urban soil to the desired 5 percent over time.

Over-amending is a common problem. Some gardeners try to fix their soil by adding large quantities of amendment in a single season. This can create one or more of the following problems:

• High salts
• High nitrogen
• Temporary low nitrogen as microorganisms take it from the soil to break down the organic matter.
• Holding too much water
• High ammonia (burns roots and leaves)

Add a little organic amendment every year until a soil test shows the soil has reached 5 percent. At that point cut back or don’t add any more for a few years. The recommended annual rate is a 1.5 inch depth of plant-based compost (low salt) mixed into soil 4 inches deep, or a 3 inch depth of plant-based compost cultivated in 8 inches deep. That's 4 and 8 cubic yards of compost per 1000 square feet of garden area respectively. Adding more can cause problems such as:

• High salts
• Excessive nitrogen, phosphorus or potassium
• Ground water contamination
• Iron or other micronutrient imbalances

With annual vegetable gardens, take advantage of the opportunity to add a little organic amendment every year. A good rule of thumb without a soil test is to add the recommended rate for 3 years before cutting back by one third in the fourth and following years. A soil test every 5 years or so will tell you how you are doing.

With perennial fruit plantings there is one, up-front opportunity to amend the soil. Don’t overamend thinking that it is your only chance. Overamending will likely create the undesirable problems mentioned above.

Nurture your garden soil and it will nurture the food plants that feed you.

[Soil photo credit, Carl Wilson]