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Index : Soils and Fertilization
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- 7. Summer Depleted Plants Need Fertilization - Top
- The heat of summer watering of plants on a more frequent basis. Plants need this extra water because they are actively growing. But new growth also requires plenty of nutrients. Frequent irrigation, combined with summer rains, washes vital nutrients such as nitrogen and potassium out of the top layers of soil and away from our plants. As a result, plants can quickly become nutrient starved, making it necessary to fertilize to replenish the supply.
The nutrient need by plants in the largest quantities, and unfortunately the one most depleted by irrigation and rainfall is nitrogen. It is responsible for the promotion of leafy growth and a major component of green chlorophyll. Plants lacking in nitrogen turn yellow and stop growing.
In many cases, nitrogen deficiency symptoms can be subtle. Older leaves first turn a pale yellowish-green. This early onset may not be noticeably. But the symptoms rapidly progress. Older leaves turn yellow, and the yellowing proceeds upward until the entire plant looks pale and anemic. In addition, little or no new growth will develop, and if it does, leaves will be smaller and stunted.
The quickest way to correct a nitrogen deficiency is to fertilize with a water soluble plant food. Products like Miracle-Gro and Peters fertilizer come in powder or liquid form that can be diluted in water and poured around the base of nitrogen-starved plants. Don't bother wetting the leaves with the fertilizer. The tiny amounts of nitrogen absorbed through the leaves is negligible.
Water soluble plant food can be applied on a weekly basis until the plant begins greening up. Since these fertilizers are very mild, they rarely burn plants. Label directions will tell you how much to apply based on the type of plant (flower, vegetable, tree or shrub) and it's size. Another advantage of soluble fertilizers is that they contain a number of other essential plant nutrients.
In the case of container plants with well-drained soils, a better way to add nutrients is by using timed-release fertilizers such as; Osmocote, Vigoro, Peters or Dynamite. These fertilizer beads that have been coated with polymers to slow the nutrient release. They feed over a longer period of time than water soluble products. In cooler climates timed-release products last for 3 months or more. Here, with our summer heat and constant watering, expect timed-release fertilizers to last only half the time indicated on the product label.
Another type of fertilizer to provide a quick supply of nitrogen are soluble granular fertilizers such as Ammonium sulfate (21-0-0). When used in small amounts, it can also provid a "quick-fix" for plants suffering late Summer nitrogen deficiencies. For small plants, one or two tablespoons is enough. For medium to large shrubs, three feet or more in height and spread, will require one-half cup of ammonium sulfate. For trees, use one cup per inch of trunk diameter. Measure the trunk diameter one foot above the ground.
Complete fertilizers containing nitrogen, phosphorous and potassium may be substituted for ammonium sulfate on flowering and fruiting plants are not preforming up to snuff. If organic fertilizers are used, expect a slower greening effect. Organic matter must be broken down by soil microbes before they are available to the plant, and that can take several weeks.
In addition to nitrogen, iron deficiencies are also common at this time of year. Young leaves that turn yellow, with contrasting green veins is the best indication of an iron deficiency. Most non-desert plants are susceptible, but citrus, rose, pyracantha, nandina, privet and photinia are some the plants most frequently effected. Iron deficiencies can also develop on over-watered summer lawns.
A deficiency of iron can be remedied with applications of water soluble iron, called "chelated iron". The most commonly available forms of chelated iron are; Sequestrene 138, Ferrione 138, Ferriplus, and Sequestar.
Finally, to help prevent continued nutrient deficiencies, cut back on watering when summer rains occur and as the temperatures begin to cool down later this month.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
- Updated: September 1, 2002
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