A nutrient management primer for gardeners, part one

Posted Nov. 11, 2011, at 4:38 p.m.

Monitoring your garden’s nutrient levels

Monitoring your garden’s nutrient levels with soil tests and then following recommendations on the test report are the foundation of garden success. Consider, for example, the Stockton Springs garden of Ladonna Bruce and Stuart Hall. I visited their small garden in August and was amazed by the number of plants squeezed into each small bed. Tomato plants grew shoulder to shoulder, their lower branches removed to minimize leaf blight; corn plants erupted from a ground-covering blanket of winter squash vines; six-inch-long cucumbers dangled from leafy vines covering a wire trellis. In every small bed, the close spacing of plants acted as a living mulch, covering the soil, keeping it cool and moist. Ladonna and Stuart garden intensively and break all the rules related to spacing. By season’s end, they had harvested over 900 pounds of produce from their small garden and when I asked Ladonna to explain this phenomenal success, she gave all the credit to soil nutrient management, explaining that she had followed recommendations from a recent soil test, adding lime to decrease the soil’s acidity. She also added rock phosphate and greensand, mineral sources for two essential nutrients, phosphorus and potassium, respectively. She is convinced that the increase in vegetable crop yields this past year was due to the addition of these essential nutrients.

November brings the first hard freeze, the kind that forms ice needles on the bright red and yellow leaves of high-bush blueberries and the still-green foliage of raspberries, that rimes the tawny pappus of goldenrod and aster seed heads. In the woods at the edge of the garden, the tamaracks are golden.

I go to the vegetable garden at the end of my early morning romp with the dogs and notice that the weight of ice has flattened the blades of oats sown as cover crop in mid-September, ending six weeks of nutrient mining. I’m reminded that we still need to collect samples of garden soil for analysis. There is still time, the soil is not yet frozen.

Late autumn is a particularly 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 the website at http://anlab.umesci.maine.edu/default.htm (see FAQs). If you tell the Analytical Lab that your garden is organic, they will respond with organic soil amendment recommendations.

Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S) are all considered major plant nutrients — macronutrients — not because they are more important than other essential elements but because they are used by plants in relatively larger amounts. Essential minor elements — micronutrients — include boron (B) and zinc (Zn), the two most likely to be needed as soil supplements, as well as molybdenum (Mo), copper (Cu), chlorine (Cl), iron (Fe) and manganese (Mn). All essential elements, major and minor, are needed by plants for healthy growth.

Nitrogen — the “N” in the three major element group, N-P-K, printed on commercial fertilizer bags — is one element that the gardener must supply every year. Nitrate nitrogen, the form of nitrogen that plant roots absorb, is quickly leached below the root zone of most plants after it is applied. Composted manure, a rich source of nitrogen in organic slow-release form, added to garden soil in late autumn, will be available for conversion to usable nitrogen by soil bacteria as soon as soil temperatures rise in spring.

Phosphorus is the major element, other than nitrogen, likely to be deficient in the soil.

Sources of phosphorus for organic gardeners include composted stable manures, vermicompost (worm castings) and rock phosphate. Rock phosphate requires acidic soils for phosphorus release and should not be used in soils with a pH above 7.

Hardwood ashes were among the earliest sources of potassium for building soil fertility.

Depending on the type of wood burned, the ashes contain between 3 and 7 percent potassium. Because overapplication of wood ashes can decrease soil acidity (raise the pH, making the soil more alkaline), annual applications should never exceed 20 pounds per 1,000 square feet. Never use wood ashes on blueberries and other acid-loving plants.

Greensand, powdered potassium silicate, contains up to 5 percent potassium, but only a small portion of the potassium in greensand is released each year. The rate of release depends on soil conditions; increases in temperature, moisture, acidity and organic matter all elevate the release rate. If the soil test shows an immediate need for potassium, it may be recommended that you apply wood ashes to correct the immediate deficiency and greensand as a more long-term potassium source. Greensand is also valued for its micronutrient content.

For a new garden, it is a good idea to add both rock phosphate and greensand in the first year, both at the rate of 10 pounds per 100 square feet.

Next week: more on soil nutrient amendments, including sources of calcium, magnesium, sulfur and micronutrients.

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