Friday, February 13, 2009

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]


  1. Great blog, Carl. I have it bookmarked now!

  2. I have heard that Front Range soils are generally too low in acid and I have access to a lot of coffee grounds (french presses!). I was wondering if I need to compost the grounds, or if I can add it to the soil directly. And should I be worried about adding too much when the soil is notoriously high pH?

  3. It is considered best practice to make compost to use as a soil amendment rather than place coffee grounds, leaves, etc. directly into garden soil. [See the CSU Composting fact sheet at] Don't expect to change the pH of your garden soil much or very quickly with compost made from coffee grounds or anything else. Soils are highly buffered and you pretty much have to deal with the alkaline soils we have rather than trying to change them.

  4. I added your blog to the "blogroll" at the Technology Toolbox blog - both to (hopefully) increase traffic to your site and to show how the "Blogroll" feature can be used to preview snippits of other blogs.

    Anyway, if you'd like to be taken off, for any reason, just let me know.

  5. Hi Carl,

    I see a lot of articles about what to add to the soil, but I don't see much on HOW to actually add it....what sort of tools do you use to break up the soil? We bought a small hand tiller last year that you roll on the ground, but I don't think it really got very deep into the soil. Can you please speak to how to actually till the soil well (not using big machinery). Thank you-- I enjoy your blog!

  6. You note the post gives amendment amounts depending whether you mix in 4 or 8 inches deep. This can be done with tillers, shovels, hoes, cultivators, plows or anything that mixes. Overtilling with engine driven tillers that pulverizes soil too fine and destroys soil structure is detrimental. There are supposed to be lumps! On the other hand, you do need to mix the compost into the soil for it to be effective.

  7. Hello Carl
    Thanks for the great info...I was wondering about not only soil amendments such as compost and manure, but also organic fertilizer. I am currently reading "Gardening When It Counts" by Steve Soloman. He provides a recipe for organic fertilizer that includes seedmeal, blood and meat meal, agricultural lime, gypsum and rock phosphate.
    Is it possible to produce your own fertilizer for organic vegetable gardening? Any downsides or risks to using or handling/storing this type of fertilizer? I have found a source for the seedmeal, blood meal and gypsum but I am having a hard time finding the other ingredients.
    Any comments?


  8. I think of organic amendments as additions meant to improve the physical condition of soils (relieve compaction, increase water holding)as different than fertilization. If your plants need fertiliizer, you can either add a manufactured fertilizer or one from an organic source. The organic source recipe you mention is obviously one meant for acid soils found in the eastern U.S. and elsewhere, not for alkaline western U.S. soils such as those found on the Front Range. We don't lime our soils to make them alkaline because they already are. We don't add gypsum (calcium sulfate) because we already have lots of calcium and indeed sulfates in our soil. As for phosphorus, rock phosphate takes decades to become available to plants in alkaline soils and sources such as steamed bone meal or fish meal are better sources. This is a good example of what I mention in next week's post, to beware of advice written for other soils and climates that is of no use or even counterproductive here. This recipe will make your soil more alkaline than it already is - a bad thing unless you have had your soil tested and know that it needs what is in this recipe. The odds are it does not. For info written for our Front Range soils, see the CSU Garden Note on Organic Fertilizers at

  9. I have really enjoyed reading your blog. We were in a community garden and now just bought a house that has a garden. The vegetables seem to be growing well (from the previous owners) but the soil doesn't seem to be of the best quality. It seems a bit hard and clay like. We were thinking of perhaps planting a cover crop this fall/winter to help enrich and break up the soil. Is this something that you'd recommend? If so, what cover crop would you recommend planting in our situation. (We live in Central Denver if that is helpful).



  10. Possible fall planted cover crops include winter ryegrass alone, rye with Austrian winter pea, or rye with hairy vetch. Pick one of the three. They are all hardy for the Front Range. Plant in mid September (in mid August in the mountains). Turn under a month before planting in the spring. Here is the amount of seed you need for these in ounces per 100 sq ft: rye 6, Austrian pea 6, hairy vetch 3.

  11. Great blog, I have it bookmarked now!