When you ask a Master Gardener a question about almost anything, quite often the answer will be, “It depends.” It’s not that we don’t know the answer, but rather that we need more information to answer correctly. This is especially true when it comes to a question about fertilizing vegetables, such as, “It’s mid-season. Should I fertilize my tomato, sweet corn and bean plants?”

To answer this question, we need to know what you did before planting. If you incorporated a three or four-inch layer of compost, then likely you don’t need to fertilize now. A healthy addition of compost pre-planting will provide most of the nutrients the plants will need.

If you fertilized the soil before planting, we’ll want to know what you applied and how much. Organic fertilizers are slow release and will last longer than chemical fertilizers. The latter release their nutrients quickly and may require replenishing.

We also will want to know the nutrient values of the fertilizer you used. Vegetable plants require 16 essential nutrients, the most important being nitrogen (N), phosphorus (P) and potassium (K). On a fertilizer package, the NPK content is expressed in that order, with each value representing a percentage by weight. For example, a 10-10-10 fertilizer (a “balanced” fertilizer) is 10 percent nitrogen, 10 percent phosphorus and 10 percent potassium by weight.

Master Gardeners welcome this kind of question because it means the person seeking advice is not just going to the garden store and purchasing fertilizer at random. At this time of year, some gardeners are prone to applying fertilizer “just in case,” an approach that can produce negative results.

Vegetable plants have differing nutrient requirements so applying the same fertilizer to all is not a good idea. What’s good for your tomatoes is not necessarily good for your corn. Tomatoes seldom need additional nitrogen during the growing season. Feeding with nitrogen will just encourage them to produce excess foliage.

The only nutrient I might feed my tomato plants during the growing season is calcium, which helps prevent blossom-end rot. However, I don’t add calcium unless I see that tomatoes are actually developing the problem. I sacrifice a few tomatoes by waiting, but that’s preferable to adding unneeded calcium to the soil.

Sweet corn, on the other hand, requires copious amounts of nitrogen during the growing season to ensure that the ears develop fully. I never feed bean plants with nitrogen as they create, or “fix,” nitrogen in the soil rather than consume it.

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Before adding fertilizer, always ask yourself a simple question: Why I am doing this? If you can’t answer the question, then don’t add the fertilizer.

Free Guided Tree Walk: Join the UC Master Gardeners of Napa County on aguided tree walk on Monday, Aug. 7, from 10 a.m. to noon or from 6 p.m. to 8 p.m. On this fun, informational walk through Fuller Park in downtown Napa, you will learn about the park’s history and get acquainted with 41 different trees. Wear comfortable shoes. Water and restrooms are available. All are handicap accessible. The book “Trees to Know in Napa Valley” will be offered for $15 each, cash or check only. Walks begin promptly. Fuller Park is at 560 Jefferson Street in Napa. Online registration or call 707-253-4221. Walk-ins are welcome, but you can be assured of receiving a complimentary map if you register at least 48 hours in advance.

Do you want to become a UC Master Gardener of Napa County? To apply, you must attend an information meeting. The first meeting is on Saturday, July 29, from noon to 1:30 p.m., at the UC Cooperative Extension, 1710 Soscol Ave., Napa. For other meeting dates, location and times, or to learn more about the program and volunteer commitment, visit the UC Master Gardener of Napa County website.

UC Master Gardeners of Napa County ( http://ucanr.edu/ucmgnapa/) answer gardening questions on Monday, Wednesday and Friday, 9 a.m. to noon, at the UC Cooperative Extension office, 1710 Soscol Ave., Suite 4, Napa, 707-253-4143.