New vegetable garden soils

So you've dug up soil for a new vegetable garden and are less than pleased. Heavy clays, low organic matter content and mixed top and subsoil layers are typical of urban Front Range soils around homes. What are your options?

Just like a human baby can’t do everything that an adult can do, a new garden soil can’t be expected to grow what a long-time garden with carefully cared for soil can. And don’t think that tossing a handful of fertilizer on these tough-to-garden soils will solve your problems. Poor physical soil conditions are the issue, not fertility.

Foot-long ‘Imperator’ carrots are likely not in your immediate future. Perhaps golf ball like ‘Thumbelina’ carrots are a more achievable objective. Better yet, rather than thinking about carrots that require loose soils for root development, consider fibrous rooted, pioneering crops such as lettuce, spinach and kale.

Another thought is to work on soil building your first year. Apply 2 inches of organic compost in the spring and mix in to the top 4 to 6 inches of soil. Plant a summer cover crop such as buckwheat (photo right), till it under in the fall and immediately plant a winter cover crop such as winter (annual) ryegrass. Let the roots of these crops do the work of penetrating these tough soils. Through their burial, add fresh organic matter to feed beneficial soil microbes and build your soil. Ideas on cover crops for Colorado conditions can be found in the CSU Garden Note #244, Cover Crops and Green Manure Crops.

Raised beds with hard sides (wood) or soft sides (soil beds photo left) are another option. Moving or importing desirable soil to construct these beds increases the depth of useable soil and jump-starts your efforts at building a good garden soil. Note that they too will likely require soil building measures such as compost additions and planting cover crops.

A final thought is to avoid overtilling your soil. Beating it up with a tiller is not the answer to building a good soil. Have a good reason to till (turning under a cover crop or mixing in compost for example) and don’t till frequently. Also till at moderate moisture content – not too dry and not too wet. Care for your garden soil and it will care for you.

How do you manage your garden soil? Comments welcome.

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]