What tests are commonly done on soils to determine if they are “good” for gardening?
When you send a soil sample to a lab, there are many, many possible tests you can request. Unless you have a good reason to request a particular test (for example, you are concerned about the possibility of lead contamination from old pipes or paint, which are fortunately not a problem in Eagle Heights or University Houses), there are really only a few very useful things to know about. Descriptions of the most important tests are below, together with a brief description of what your results might tell you.
- Soil pH is a measure of the acidity of a soil. This is possibly the most important single soil test because the pH influences the availability of all the essential elements that plants obtain from the soil. It also affects the activity of soil microorganisms. pH values above 7 represent “basic” or “alkaline” soils, while pH values below 7 represent “acidic” soils. Most plants grow best at a pH of between 6.6 and 6.8, though many species can tolerate soils that are as acidic as pH 6 or as alkaline as pH 8. A few plants such as blueberries do best in soils that are strongly acidic. If a soil’s pH is at or below 6, adding agricultural lime (calcium carbonate or calcium magnesium carbonate) is a good idea. If the pH is above 7, this is generally not a problem unless you want to grow plants like bluberries.
- Organic Matter influences the moisture-holding capacity of the soil (more organic matter means the soil can hold more water), serves as a supply of nutrients, feeds microorganisms, insects, earthworms, etc., and makes it easier to dig in the soil. The amount of organic matter depends on temperature, rainfall, aeration and other factors. It tends to be a characteristic of a soil type in a given place and is not easily changed. Rototilling and even frequent hand tilling can result in loss of organic matter through oxidation (soil-dwelling organisms eat it up) and erosion. It is possible to increase a soil’s percentage of organic matter, but this requires large additions of crop residue, compost, mulch, or manure (NOT wood chips, sawdust, or other wood products!), and is usually possible only on the scale of a garden. There is no ideal target number of organic matter when testing – generally, more is better, and a soil that has less than 2% organic matter is likely to be less productive and harder to work in than a soil containing 5% organic matter.
- Phosphorus is usually reported as parts per million, or ppm, where 1 ppm is approximately equal to 0.1 pounds of available phosphorus per 1,000 ft2 of vegetable garden (0.05 kg per 100 m2). In general, levels of phosphorus should be about 60-80 ppm, though higher or lower levels may be better for particular plant species, and different methods of soil testing can give very different results for this element. Shallow-rooted plants such as onions, carrots and radishes require higher phosphorus levels than do deep-rooted plants like sweet corn and legumes (beans and peas). This element is needed for energy transfer reactions in plant cells and stimulates root development and flowering in plants.
- Potassium is usually reported as parts per million, or ppm, where 1 ppm is approximately equal to 0.06 pounds of available potassium per 1,000 ft2 of vegetable garden (0.03 kg per 100 m2). In general, levels of potassium should be greater than 180 ppm, though higher or lower levels may be better for particular plant species. is an activator of many plant enzyme systems. It is involved in carbohydrate and protein formation and movement of many other nutrients within the plant. Abundant potassium promotes disease resistance and, in perennials, some degree of winter hardiness. Root crops (radishes, carrots, potatoes) have particularly high requirements for this element. Low potassium levels result in weak stems in many plants. Fertile garden soils contain 250-350 lb. of available potassium per acre.
Given the importance of nitrogen in plant growth (and its presence in many fertilizers), you might think that you would want to have a test done for nitrogen levels in your soil. However, unlike phosphorus and potassium, nitrogen does not accumulate in the soil in forms that are available to plants. If you attempt to build up high levels of these forms (nitrate or ammonia), they are either taken up by soil-dwelling organisms other you’re your target plants, or they leach (wash away) in rainwater, potentially contaminating nearby lakes and streams. While there are several different soil nitrogen tests, these are generally only useful for farmers and researchers. Gardeners trying to decide how much nitrogen fertilizer (or manure, or other nitrogen-rich material) to use should look at recommendations for how much is needed by a particular crop, and apply accordingly each season. Many other tests can be done on soil, and some of these (for example, tests for elements like zinc and boron that plants need in very small amounts) may be useful in some circumstances. However, doing these tests makes getting soil tested more expensive. More importantly, fertilizing with a either an organic fertilizer (manure, bone and blood meal, etc.) or a “complete” synthetic fertilizer (that is, one that contains nutrients other than just nitrogen, phosphorus, and potassium) is likely to provide these elements in sufficient quantities such that you don’t need to test for them.