Fertilizing plants can be a tricky proposition. It's not easy to figure out how much or what kind of food would best benefit every plant in our gardens. Is it necessary to have a degree in soil science to have a successful garden? Or can a basic understanding of what is necessary for plant growth be all that is necessary to eliminate the confusion that one feels when confronted by all the choices out there?
Walking through the fertilizer section of even a moderately-sized garden center is somewhat akin to strolling through the cereal aisles in a supermarket. The amount of choices can be overwhelming. Is all this necessary?
Sixteen chemical elements are known to be important to a plant's growth and survival. Of these, 13 are derived from the soil. This is in the gardener's control and by applying fertilizer, we manipulate the amounts of the mineral nutrients available to our plants. In order to know which are needed, we need to know what we have on hand in our soil.
Soil tests tell us much about what is present and what is missing in our soils. Tests are ideally performed every few years in vegetable gardens and can be less frequent in the rest of the landscape. Without a soil test we have no idea what is there. Forget the little testing kits you see in the stores. This is a job for the pros. Contact the Southern Oregon Research & Extension Center at 776-7371 for information on soil testing.
Plant nutrition is complicated by the wide variation in the needs of plants for the amount and types of nutrients necessary for their growth. Annual vegetables, flowers and lawns are all heavy feeders, but not of the same nutrients. While many forest trees attain great size, their fertilizer needs are not high. If this is all starting to sound confusing, it is!
Fortunately for gardeners, much of the work of determining individual requirements of different plants has been done for us. Much study has gone into developing specific formulas and ratios of nutrients for plants commonly grown in the garden. Most vegetables thrive when given a fertilizer that contains a balanced ratio of the major nutrients. That's exactly what you'll find in a fertilizer that is labelled for use by vegetables. Lawns need mostly nitrogen to maintain green, leafy growth and that's what is contained in food formulated for them. It's as simple as matching the picture on the bag with the plants you have at home.
If we determine the amount of fertilizer to apply to the soil from a soil test and the type of fertilizer to use from the manufacturer's recommendation, what's left? The gardener must determine when to fertilize for the best results.
Most landscape plants derive benefit from having food available to them when growth commences in the late winter or early spring. Plants that have a single, short burst of growth, like needled evergreens, should be fed lightly and only once a season. In fact, a second application of fertilizer may burn the needles. Shrubs that grow and bloom over long periods of time like ever-blooming roses and escallonia will benefit from several applications during the growing season.
There is environmental concern regarding runoff from all fertilizers. By applying them at the proper time, in the proper amounts needed by the plants, and only those nutrients that are shown to be lacking in the soil by a soil test, we can greatly decrease the amount of runoff associated with our gardening practices. That's good for the soil, the water, our plants and our wallets.