Fertilizing Vegetables During the Growing Season

Soil test will guide you on garden nutrients

Bill Sevier: Ask a Master Gardener  

Saturday, June 18, 2016

Q: I tilled in some organic compost before planting in my vegetable garden. Do my vegetables need more fertilizer now? B.C., Tulsa

A: Most of the rapidly growing vegetables perform best with added fertilizer during the growing season, some more than others. Usually this is done in the form of sidedressing, which means applying fertilizer between vegetable rows.

Plants are already growing, but ideally it would have been desirable to obtain a soil test before planting. At that time, any amendments needed as determined by the test could be tilled into the garden. A soil test will guide you to use only the needed nutrients. Excessive amounts may be detrimental to your plant’s health. This is especially true with phosphorus; do not add this unless it has been shown to be needed.

All fertilizers have three numbers on the container which represent the percent concentration of that nutrient. The nutrients are nitrogen, phosphorus and potassium, in that order. Nitrogen, the first number, is needed much more than the others by most plants.

When fertilizing, many people understandably wish to use all organic. This can be done, but requires more planning and understanding of various materials. The nutrients in organics may be released slowly depending on the temperature and microorganism activity. Firstly, most of the composted manures have low levels of nutrients. Cow manure may have numbers of 1-1-1, whereas a typical commercial lawn fertilizer may have numbers such as 32-0-10, which is 32% nitrogen, no phosphorus and 10% potassium.

Some organics, such as blood meal have numbers of 15-1-1, milorganite has 5-3-2 and cottonseed and cottonseed and soybean meals are in the range of 7-2-2, much higher than manures.

A chart of recommendations for sidedressing for both vegetables and ornamentals is available on the Master Gardener website, tulsamastergardeners.org, in the vegetable section. Specific amounts of both manufactured and organic fertilizers are listed for each vegetable and common ornamental.

The amounts of the fertilizer recommended are widely different and the chart should be consulted for this information. The times and frequency of fertilization for some of the common vegetables are:

·       Asparagus — in spring before growth or after harvest

·       Cabbage, broccoli — three weeks after planting

·       Cucumber, squash, pumpkin — one week after blossoming and three weeks later

·       Onions — 2-4 weeks after planting

·       Peas and beans — after heavy bloom and set of pods

·       Peppers and eggplants — after first fruit sets

·       Potato — when plants are 4-6 inches tall

·       Spinach, kale, turnip and mustard greens — when plants are about 1/3 grown

·       Sweet corn — when plants are 8-10 inches tall and one week after tassels appear

·       Sweet potatoes, watermelons, herbs — none, excessive amounts may reduce yield

·       Beets, carrots, turnips, lettuce — none needed if fertilized at planting

·       Tomato — 1-2 weeks before the first tomato ripens, then 2 weeks and 6 weeks later

·       Blueberries — monthly April through July

·       Brambles (blackberries) — April at start of growth

·       Strawberries — amount depends on type of plant, refer to chart

·       Annual flowers — monthly until frost

·       Daylilies, garden phlox, astilbe, mums — once after blooming

Consider going to the Master Gardener website to print off this useful information.

Garden tips

§  Remain alert for insect damage. Add spider mites to the list. Foliage of most plants becomes pale and speckled; juniper foliage turns a pale yellowish color. Shake a branch over white paper and watch for tiny specks that crawl. Watch for first-generation fall webworm.

§  Some pests can be hand-picked without using a pesticide. Do not spray if predators such as lady beetles are present. Spraying insecticides early in the morning or late in the day will avoid spraying honeybees and other essential pollinators.