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Missouri Environment & Garden


David Trinklein
University of Missouri
Plant Science & Technology
(573) 882-9631

Wet Spring Adds Importance to Side-Dressing

David Trinklein
University of Missouri
(573) 882-9631

Published: June 12, 2019

person hand fertilizing under a tomato

The term "side-dressing" is not widely used outside of the gardening world. I strongly suspect that nine out of ten people randomly stopped in a public setting would have no idea what it means. To gardeners, however, the term is quite familiar and refers to the practice of placing fertilizer in a band along-side rows of plants, or around the perimeter of individual plants in an attempt to boost lagging soil fertility.

The extremely wet spring we have experienced in Missouri has reduced the fertility level of garden soils due to nutrient leaching. Additionally, our garden plants remove nutrients from the soil as they grow. There are several ways to counter this reduction in soil nutrients and to maintain better plant growth throughout the coming summer. Side-dressing is one of the solutions to the problem and can give garden plants a needed boost in growth and productivity.

One of the fertilizer elements that quickly is leached from the soil but also is very critical for good plant growth is nitrogen. Side-dressing with a nitrogen fertilizer or a complete fertilizer high in nitrogen usually is beneficial about four to six weeks after planting. Compounds such as ammonium nitrate, urea, ammonium sulfate and calcium nitrate contain high levels of nitrogen and are often used for the purpose of side-dressing. If these fertilizers are not available, a complete fertilizer high in nitrogen can be used. Beware of lawn fertilizers, however. Most are high in nitrogen but may be "weed and feed" in formulation. The herbicides contained by the latter should not be applied to garden plants.

As mentioned above, side-dressings are normally applied close to the soil surface. The soluble forms of nitrogen found in compounds such as ammonium nitrate or urea move into the root zone easily as a result of rainfall or irrigation. If mulch has been applied, pull it back and apply the side-dressing beneath it along the row. Push the mulch back into its original place after the fertilizer has been applied. Ammonium nitrate and similar fertilizers are applied at a rate of about one pound per 100 feet of row (or 100 square feet of bed area). Side-dressings should be placed at least six inches away from the main plant stem to avoid burning. A strip along each side of the row is considered ideal. If the planting has not been mulched, incorporate the fertilizer application lightly into the soil with a garden hoe or rake. Garden fertilizers lower in nitrogen (e.g. 12-12-12) should be applied at about three pounds of fertilizer per 100 feet of row or square footage of bed.

Another method to avoid nutritional problems caused by leaching involves the use of slow (or timed) release fertilizers. As their name implies, slow release fertilizers release the nutrients they contain over time, making it possible to fertilize only once during the growing season.

The most popular slow release fertilizers used today release their nutrients by osmotic action in resin-coated types, or by bacterial action in organic types. Although the initial cost of some of these slow-release materials might be higher, the need for extra labor is eliminated. Slow-release fertilizers vary greatly in analysis, rate of release and use. Some are formulated to be effective for several weeks to months; others last for an entire growing season. Read and follow label directions carefully when using slow-release fertilizers. Using excessive amounts can lead to soluble salts injury, since there is no way to leach slow-release fertilizers from the soil.

Blood meal and cottonseed meal are among organic slow release choices that are good source of nitrogen for side-dressing. The nitrogen content for blood meal is about 12 percent while that of cottonseed meal is about seven percent. Therefore, one would side-dress with blood meal at the rate of about two pounds per 100 feet of row, while cottonseed meal should be applied at about three pounds per 100 feet of row (or 100 square feet of bed area).

Plants are good indicators of their nutritional status. Therefore, watch how plants grow to gauge their need for additional fertilizer. Response to nitrogen sources normally occur within about one week, given soil moisture is adequate. Thus, applications can easily be made at the time growth seems to be slowing. Applying excessive fertilizer (especially nitrogen) can be counterproductive. For example, tomato should not be over-fertilized with nitrogen early in the life of the plant. The result will be lush vegetative growth and poor fruit set. Additionally, blossom-end rot problems may increase later. Instead, wait until plants begin setting fruits before applying additional nitrogen.

The following table lists general recommendations for the timing of side-dress applications to popular garden vegetables:

Asparagus Before new growth begins in spring or after harvest
Cabbage, cauliflower, broccoli Three weeks after transplanting
Carrots, beets, turnips, parsnips and lettuce Side-dressing normally not needed if soil is fertilized adequately before planting
Cucumber, cantaloupe, pumpkin One week after blooming begins; repeat three weeks later
Onions (mature) Two to four weeks after planting
Peas, beans After heavy blooming and pod set
Peppers, eggplants After first fruits sets
Potato (Irish) When plants are 4—6 inches tall
Rhubarb When plants are 2—10 inches tall
Spinach, kale, mustard and greens When plants are about one-third grown
Sweet corn When plants are 8 to 10 inches tall; again one week after tasseling
Sweet potatoes, watermelons, herbs Side-dressing not recommended. Excessive amounts of nitrogen will reduce yields or lower quality, or both.
Tomato When plants begin to set fruits; repeat every two to three weeks

Table credit: Gregg Eyestone, Kansas State University Extension

Avoid applying excess nitrogen to flowering annuals, especially those that are not flowering. The result is likely to be lush vegetative growth and poor, delayed flowering. However, many species of flowering annuals (e.g., petunia) benefit from side-dressing with nitrogen every six to eight weeks during the growing season. The same compounds recommended for vegetables can be used on flowering annuals.

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