A nutrient management primer for gardeners, part two

Posted Nov. 18, 2011, at 2:34 p.m.

The organic gardener’s goal is to provide plants with sustainable levels of essential elements needed for healthy growth. We accomplish this goal by adding organic matter to the soil, digging in composted manure and mulching with shredded leaves or compost. Microbial decomposition of the organic matter releases nutrients in a slow manner.

In last week’s column, we looked at sources of nitrogen, phosphorus and potassium, three of the major elements required for healthful plant growth. Use of composted stable manures, rock phosphate and greensand (as a source of potassium) was presented as one sustainable way of meeting plant needs for these three nutrients. In his book, “Four Season Harvest,” Eliot Coleman suggests applying both rock phosphate and greensand directly on the soil in the first year of a new garden plot, adding each at the rate of 10 pounds per 100 square feet. In subsequent years, he suggests lightly sprinkling both products on the green layer of the compost pile (a yearly total of one pound of each mineral per 100 square feet of garden space). Each of the three remaining major nutrients, calcium (Ca), magnesium (Mg) and sulfur (S), can be supplied in a number of ways, depending on your soil’s specific needs as determined by a soil test. For example, if the soil test results indicate a need to reduce the acidity (raise the pH) of your soil and also show a need for both calcium and magnesium, you should use dolomitic lime, which supplies both of these nutrient elements while raising the soil pH. On the other hand, if no magnesium is needed but calcium is deficient, you can use calcitic lime (calcium carbonate).

If there is no need to change the pH, magnesium needs can be met with Sul-Po-Mag, a mined rock often used as a potassium source in commercial fertilizers. Use Sul-Po-Mag only if you need both potassium and magnesium. To supply magnesium without adding potassium, use epsom salts (magnesium sulfate).

What if there is no need to change pH, but calcium is deficient? Your best choice is gypsum (calcium sulfate), which supplies both calcium and sulfur without altering soil acidity.

Gypsum can also improve the structure of clayey soils, binding the clay particles together to create more space for air and drainage. The soil test report makes all of this simple by telling you exactly what you need to add to your soil to overcome nutrient deficiencies and, if needed, adjust the soil pH. It will also tell you how much of each recommended substance to use.

The essential minor elements (micronutrients), including boron (B), zinc (Zn), molybdenum (Mo), copper (Cu), chlorine (Cl), iron (Fe) and manganese (Mn), tend to be present in adequate quantities, often added along with sources of the major elements. For example, granite dust, used to supply potassium, contains a wide range of minor elements.

Greensand, in addition to adding phosphorus, also supplies iron and other micronutrients.

Seaweed, while sustaining soil fertility, hinges on annual soil testing. One approach is to have the same one or two specific beds (or rows) tested every year so that you can develop a profile of how your soil management strategy is working. Also, independently test new beds and any beds where you have intentionally changed soil management. And, send in samples from any areas where plants grew poorly.

Now is a good time to collect soil samples from your garden, since the Analytical Laboratory and Maine Soil Testing Service (University of Maine, Orono) offers fee discounts for samples received between Jan. 1 and March 1. Simply take the samples now and let them air dry until the first of next year. For complete details, including instructions on how to take the samples and where to deliver them, visit their website at (see FAQs).

Soil’s lab launches new soil biology test

New this year at the Analytical Lab is their Soil Biology test, a measure of the microbial activity and thus the nutrient delivery potential in your soil. Since the test is based on rewetting dried soil, it can be conducted on soil samples taken now but not submitted until after January.

According to the Analytical Lab’s Director, Bruce Hoskins, when soil dries the native microbes go into a resting state but can remain viable for years in dry storage. When the soil is rewet in the lab, there is a large flush of microbial respiration, a carbon dioxide “burst” that when measured reflects both soil microbial mass and soil biological health.

The new test provides recommendations on how to improve soil organic matter content as well as useful information for many situations, such as crop rotations and making the transition to organic gardening.

The cost is an extra $10 when requested with any standard soil test. Without doubt, money well spent.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to Include name, address and telephone number.

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