Plant Answers

Yellowing of Plants

Texas, South Texas in particular, is different and we are proud of it! However, different is not always good especially if you are talking about the different soil pH of South Texas. The term pH refers to how acidic or alkaline the soil is. It is a calculated scale which goes from 0 to 13 with a reading of 7.0 being neutral, above 7.0 indicating alkalinity and below 7.0 being acid.

So what does pH mean to you as a gardener? It may be the reason certain plants look like death warmed over since pH affects the availability of nutrients, the physical properties of soil and the activity of vital soil organisms. In other words, it affects how well plants grow!

A pH of 6.5 is ideal. I am certain that somewhere in Texas this perfect pH of 6.5 exists. I am just as certain that such a perfect pH area is NOT often found in South Texas. South Texas soils have an average pH of 7.5 with many soils over 8.0. Groundwater and parental soil materials of an area determine the soil pH.

If the soil pH is too high or alkaline, essential elements such as phosphorus, iron, zinc and copper become unavailable for plant uptake even though these elements may be present in the soil. Unavailability of mineral elements is caused by physical encapsulation by soil calcium or chemical change of added nutrients. It is similar to giving a child a jar of candy with the lid on too tight ? ? he can see it, the candy is there, but unaccessible.

Iron is needed for chlorophyll development. Chlorophyll makes plants green so lack of iron and consequently lack of chlorophyll causes yellowing of plant foliage. Everyone has been alarmed by yellowing foliage of okra, black-eyed peas, gardenias, roses, strawberries, blackberry bushes, pyracantha, peach, grape and lawn grasses ? ? the problem is lack of available iron causing iron chlorosis.

Chlorosis is most common during early spring when plants are growing vigorously. This irregularity is characterized by streaks of green through a predominately yellow leaf. In severe cases, brown margins or spots will develop but this will occur after the leaf has been yellow for a period of time.

Zinc is another minor element such as iron which, if limited, can manifest itself in major ways on plant growth and production. Zinc is used by the plant to manufacture growth hormones and cellular constituents. Lack of zinc is most noticeable on pecan trees causing small, wrinkled leaves and lack of nut production. The Texas Cooperative Extension Pecan Spray Schedule recommends several zinc applications during each season. Applications of zinc MUST be made with a foliar spray of zinc sulfate or NZN since soil applications are not effective. Soil alkalinity makes the zinc unavailable before the pecan tree can reap the benefits of a soil application.

Vegetable crops which are extremely sensitive to alkalinity (soil pH above 7.0) are asparagus, mustard, pepper, pumpkin and sweet potato. Crops which can prosper in a mildly alkaline (up to pH 7.5) soil include beans, beets, Brussels sprouts, carrots, cauliflower, eggplant, Irish potato, lettuce, okra, sweet corn, tomato, and watermelon. The champion producers in extremely alkaline soils include broccoli, cabbage, cantaloupe, cucumber, onion, parsley, peas, radish and spinach. The three ranges listed indicate tolerances of vegetables crops. However, all vegetables listed have an optimum production of pH of 6.5.

Ornamental plants vary as well in their sensitivity to alkalinity. Some infamous alkaline?sensitive plants include azaleas, gardenias and hydrangeas. To avoid perpetual problems with yellowing foliage of plants BE SURE to plant only Extension recommended, tried?and?proven plant types. Lists of recommended ornamentals are available at local county Extension agent's offices or at:

http://www.plantanswers.com/resources.htm#landscaping
Now that we know some of the problems caused by alkalinity, the next question is what is the solution. There is not an easy, enduring solution. South Texas soils are buffered or resistant to change. Adding to this dilemma is the fact that our water is also alkaline so every time we water we contribute to the problem.

The alkalinity problem can be treated or temporarily altered. The temporary alteration involves elimination of the culprit causing the problem ? the soil. If extremely acid?loving plants such as azaleas or gardenias are to be grown, an artificial growing media should be used. Standard potting soil is not acid enough and will not maintain an acid condition over a long period of time with the alkalinity bombardment of water and soil leachings. To insure an acid?enduring growing media, mix two?thirds Canadian sphagnum peat and one?third WASHED sand, not regular sand since it contains lime and weed seed. Excavate a suitable size hole, line the hole with a porous, plastic material to exclude earthworms which will infiltrate the mix with alkaline debris and fill with the sphagnum peat sand mix. DO NOT INCORPORATE ANY OF THE NATIVE SOIL WITH THE MIX! This may sound like a lot of trouble but it prevents a lot of ugliness later. Many rose growers follow a similar plan of action to insure vigorous plant growth.

What about using iron sulfate sometimes referred to as copperas? Iron sulfate is a better source of iron than it is acidifier and is sometimes recommended as a "cure" for iron chlorosis. However, in an alkaline soil, the iron of iron sulfate (copperas) rapidly becomes unavailable for plant use so an iron chelate product will furnish a more lasting remedy. Iron chelates are expensive so gardeners should make a "synthetic chelate" in their compost pile. Simply add 7 pounds of iron sulfate (copperas) to 400 pounds of organic matter. You can add more if you want but this is just a guide. Particles of iron will adhere to the surface of the compost material (leaves, grass clippings, etc.) and will be released for plant use as the compost decomposes while being used as a mulch around plants or when incorporated into the soil. Iron sulfate can also be applied as a foliar spray if a spreader (two teaspoons of liquid detergent per gallon) is used.

If you want to try a synthetic chelating material for trees and shrubs, punch several 4?6 inch holes in the ground near the drip line (outer edge of tree canopy) of the patient and put four tablespoons of iron chelate (Sequestren 138 or Ferri-Plus (Helena Chemicals) into each hole in early spring or now and late summer. Treatment more often may be necessary in severe cases.

Organic matter such as leaves and compost release organic acids as decomposition occurs so it also helps alleviate a soil's high alkalinity problem. Hopefully, all of those folks who ask if oak or pecan leaves are too acid to use in a garden will now know the answer -- the more acid the better in South Texas! Lime should NEVER be used unless extreme acidity is confirmed by soil testing. Such a situation is about as rare as hen's teeth in South Texas but can occur in “blow” sands.

Unless you grow plants which are adapted to this area's limey soil conditions, expect the yellowing symptoms of iron chlorosis. Remedying the situation is an acid test of anyone's growing perseverance.