What do soil tests say about the soils in Eagle Heights and University Houses Gardens?
The most recent tests done on our soils were performed by UW-Madison’s Soil and Plant Analysis Laboratory in November 2009. We tested soils from several “average” plots (plots that we think have been fertilized at least a bit over the years), as well as samples from plots that have been either fertilized well or not (so far as we know) fertilized at all in recent years. We also tested soil from the forested area south of the Eagle Heights garden shed, soil from the blueberry plot near the entrance to Eagle Heights (which has been deliberately acidified), and the soil-like compost from the community compost pile at Eagle Heights. Results for selected tests are shown in the table below, together with average values for Eagle Heights soil from 1993 (the last time extensive soil tests were conducted). See the previous section for an explanation of why we performed the tests shown here and not other ones.
Some comments on these test results are provided after the table. If you are looking at this table for information about what to do in your own plot with respect to fertilization, keep in mind that all of these soil properties vary naturally from place to place, and that any two adjacent plots in our gardens may have very different histories. To get the soil from your own plot tested, you can submit a sample to the UW Soil and Plant Analysis Laboratory (go to http://uwlab.soils.wisc.edu/madison/ and look for the section titled “Lawn and Garden”). This service is not free but is not very expensive for a single sample. If you do have your soil tested, you could do other gardeners a favor by sending the results of your soil tests to the garden registrar. With your permission, she or he may share them with the rest of the gardens.
| Sample description | Organic matter % | pH | Phosphorus (P) parts per million (ppm) | Potassium (K) parts per million (ppm) |
|---|---|---|---|---|
| Average Eagle Heights soil, 1993 | 4.6 | 7.5 | 190 | 215 |
| Average plot in the 200s at Eagle Heights, 2009 | 8.0 | 7.3 | 209 | 95 |
| Plot in the 600s at Eagle Heights, 2009 (believed to be unfertilized for years) | 6.0 | 7.2 | 176 | 86 |
| Plot in the 700s at Eagle Heights, 2009 (known to be well fertilized for years) | 8.0 | 7.3 | 215 | 130 |
| Plot in the 1100s at Eagle Heights, 2009 (known to be well fertilized for years) | 9.2 | 7.2 | 199 | 116 |
| Plot in the 1300s at Eagle Heights, 2009 (believed to be unfertilized for years) | 5.1 | 7.1 | 142 | 84 |
| Soil from forest south of Eagle Heights garden shed, 2009 | 3.5 | 6.9 | 56 | 90 |
| Soil-like material from the community compost pile at Eagle Heights, 2009 | 6.9 | 7.4 | 104 | 109 |
| Plot in the C row at University Houses, 2009 | 5.5 | 7.3 | 185 | 150 |
| Soil from blueberry planting near Eagle Heights entrance | 6.5 | 3.1 | 146 | 109 |
Except for the blueberry planting (which is much too acidic for most plants, and perhaps even a little too acidic for blueberries), all of the soil tests shown in the above table indicate soils with pH and organic matter suitable for growing crops. The soil from the forest near Eagle Heights has a lower organic matter content than soil from nearby garden plots that probably used to be forested as well. Forest soils naturally have relatively low organic matter content, and most gardeners have added leaves, compost, and other materials to their plots over the years such that their soils’ organic matter levels are higher than they would have been before gardening began here. Soil tests in 1993 showed very high phosphorus and potassium levels throughout the gardens. This presumably reflected the fact that for many years, all of the garden plots at both Eagle Heights and University Houses were tilled annually using large machinery and manure from UW’s dairy barns was applied heavily during the process. Manure contains large amounts of phosphorus and potassium relative to nitrogen, and unlike nitrogen, both elements (but particularly phosphorus) can be held in the soil for long periods. As a result, even though this manuring stopped almost two decades ago, gardeners today are still “spending” the extra phosphorus and should be able to do so for years or even decades to come. In 1993, levels of potassium were also still very high, but they have declined since then such that potassium fertilization would make sense in most plots.
For gardeners who are interested, two soil samples (one from a plot that has been fertilized occasionally and lightly over the past five years, one of which has been fertilized annually and heavily over the past five years) were also analyzed in fall of 2009 for their micronutrient content and their levels of exchangeable cations. These results are too complex to explain in depth here, but the results of the tests are shown in the following table. Besides pointing out that levels of most nutrients are adequate in both plots for most crops, one of the main take-home messages from these results is that fertilization with nitrogen, phosphorus, and potassium-rich fertilizers (the main purpose of the fertilization done to one of these plots) has not substantially altered the levels of micronutrients relative to those found in the less intensively fertilized plot. To do that, fertilization with special micronutrient-rich fertilizers would probably be required.
| Average plot in the 200s at Eagle Heights, 2009 | Plot in the 1100s at Eagle Heights, 2009 (known to be well fertilized for years) | |
|---|---|---|
| P % (10,000 ppm = 1 %) | 0.12 | 0.14 | K % (10,000 ppm = 1 %) | 0.34 | 0.34 | Ca % (10,000 ppm = 1 %) | 1.73 | 2.09 | Mg % (10,000 ppm = 1 %) | 0.63 | 0.63 | S % (10,000 ppm = 1 %) | 0.06 | 0.07 | Zn ppm | 89 | 86 | B ppm | 11 | 13 | Mn ppm | 1006 | 789 | Fe ppm | 13,591 | 12,466 | Cu ppm | 12 | 14 | Al ppm | 16,081 | 14,282 | Na ppm | 180 | 156 | Exchangeable Ca | 3013 | 3628 | Exchangeable Mg | 620 | 542 | Exchangeable K | 382 | 625 | Exchangeable Na | 38 | 32 |