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Identifying and preventing nutrient deficiencies in plants

Eagle Heights Gardens was established in 1962, making it one of the oldest community gardens in the United States. That’s great, because that means a lot of people have been gardening here for a long time, but it’s also a problem, because for decades many gardeners have grown plants without using fertilizers. As a result, if fertilizers are not used now, the plants tended by today’s gardeners can suffer nutrient deficiencies that lead to poor growth and reduced yield. Fruits and leaves of nutrient-deficient plants may also be more vulnerable to disease, less nutritious and store less well than produce from plants that have everything they need.

Before we go farther, it’s good to emphasize that fertilizers should be used every year even in plots that have been fertilized regularly in the past (see the garden manual and previous Garden Plot columns available under “Gardening Tips” at http://www.eagleheightsgardens.org/ for advice on how to use fertilizers; keep in mind that leaves and compost, while useful in the garden, are NOT fertilizers!). If you garden intensively, you take a large weight of produce out of your plot each year. While most of that weight is water, and much of the rest is carbon, hydrogen, and oxygen (chemical elements that plants get from either air or water), you’re still taking away many kilograms (or pounds) of the 14 essential elements that plants can only get from the soil (see previous section for a list). Some of these 14 elements are abundant in the soil, but others are not and must be replaced regularly. It’s also possible for a nutrient to be present – even abundant -- in the soil but locked away chemically in a form that’s unavailable to plants.

For those who know what to look for, nutrient-deficient plants may display one or more symptoms or visual signs indicating that something’s wrong. In some crops and for some nutrients, these symptoms may be obvious. Corn is one of the easiest crops to “read” this way. In corn, a lack of nitrogen (the most commonly deficient nutrient for all crop plants) can make a plant’s leaves turn yellow, with the yellowness moving in a V-pattern down the center of older leaves. Potassium deficiency in corn also causes yellowing of older leaves, but the yellowness moves in from the edges of the leaves. A lack of phosphorus in corn causes reddening, again beginning with the oldest leaves. In each case, a severe, prolonged deficiency causes tissue death, once again starting with the oldest (bottommost) leaves on the plant.

Why should the oldest leaves on a plant be affected by nutrient deficiencies and not the younger ones? The answer has to do with the mobility of certain nutrients within plants. Some elements, including nitrogen, phosphorus, and potassium (the three that plants need from the soil in the largest quantities) are “phloem-mobile.” This means that if a part of a plant like a new leaf or a rapidly growing fruit needs one of these elements, the plant can take it from some old tissue and move it through fluid-filled tubes called phloem to the new place.

Other nutrients, including calcium, are not phloem-mobile. As a result, plant parts like tomato fruits that need a lot of calcium can end up without enough of it. In tomato plants, this can result in a condition called “blossom end rot,” in which the bottom of each affected fruit turns black and soft. Blossom end rot is common in Eagle Heights garden plots, in part because of failure to use fertilizers containing calcium (or any fertilizers at all). Related conditions are also common in squash, cucumbers, and watermelons, all of which are fast-growing fruits that need a lot of calcium.

Unfortunately, most nutrient deficiencies are not easy to diagnose visually, and some are easily confused with diseases. If you suspect a nutrient deficiency, you can submit some tissue from your plants to the UW Soil and Plant Analysis Laboratory (go to http://uwlab.soils.wisc.edu/madison/ and look for the section titled “Plant analysis… for diagnosis of nutrient problems”; this service is not free but is not very expensive). On the other hand, you could also just try providing your plants with what’s called a “complete” fertilizer – one that provides all of the nutrients plants need. This won’t necessarily fix the problem within the current gardening season (once a plant has visible deficiency symptoms, it’s often too late to help it), but should help to prevent the same problem the following year.

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