Just about this time every year, many gardeners face the sudden yellowing of the leaves on many of their plants—especially azaleas, rhododendrons, and roses.
There is usually no indication of insect or disease damage, so the immediate reaction is that something is lacking in the soil, even though a regular fertilization program has been followed.
This assumption is correct.
Big Three Plant Foods
We hear a great deal about the “big three” plant foods: nitrogen, phosphorus, and potassium.
Sometimes, this yellowing results from a deficiency of one of these, but in many more instances, the missing element is iron.
Iron is one of those unsung elements —only a tiny bit of it is needed by the plant, but when it is lacking, it can cause as much trouble as a deficiency of one of the major plant foods.
Most soil experts agree that there is a fair share of iron in almost all soils. But even though it is present in the soil, it can still be a problem if it is unavailable to the plant.
Lack of Iron on Plants
Also, we have some plants that are more susceptible to a lack of iron than others, and there are times during the growing season when plants use more iron than at other times. So, the problem grows a little complex.
If we explore the role of iron in the plant and soil, we can get a clearer picture of how to spot a deficiency in the leaves and some hints on the best way to correct the problem.
In the functions of plant growth, iron is classified as a catalyst. What does this mean?
A catalyst is a material needed only in small quantities to start a process—like the tiny spark that sets off a fire.
Essentials For Making Chlorophyll
Even though we have wood, tinder, and the right conditions for a fire, we must have the spark to set it off.
So it is in the leaves of a plant. We can have light, nitrogen, air, and water—all the essentials for making chlorophyll—but the process won’t start or keep working without iron.
As we mentioned, iron may be in all soils, but yet not available to the plant. All nutrients, including iron, must be soluble in water before the plant can absorb them.
We all know, too, that there are some complex chemical processes that go on in the soil.
Plant Food
We apply a plant food in one form and it is changed chemically to another that is available to the plant.
Most often, we apply a plant food in an insoluble form and it is changed to a soluble form by the action of bacteria or other chemicals already in the soil.
Only when it changes form can the plant absorb it. On the other hand, we sometimes find the reverse to be true—that is, a soluble form of an element changed into one that is insoluble and not available to plants. This is often the case with iron.
In soils that are alkaline (a high pH), the element iron is combined with other elements to form a mixture of elements that the plant cannot use.
Acid-Loving Plants
Hence, even though there is iron in the soil, the plant will show signs of deficiency, and a new supply of iron will have to be added for good growth.
It also seems to be a peculiar characteristic of many plants to demand more iron during the hot summer than during the spring and fall.
That is why the deficiency shows up most prominently in July and August. One group of plants appears to have a greater need for iron than most.
The so-called “acid-loving” plants include rhododendrons, azaleas, camellias, gardenias, and citrus fruits. Iron should be added to the soil around these plants at regular intervals.
Yellowing Of The Foliage
Although many diseases and nutrient deficiencies result in a yellowing of foliage, iron deficiency is fairly easy to spot.
The yellowing of the foliage starts with the youngest leaves on the plants— remember, iron is used in making the green part of the leaf.
The yellowing also starts with the area between the veins—green veins with yellow between—gradually taking in the entire leaf.
In time, of course, the entire plant could be affected, and all leaves were yellow. Early application of iron, though, will correct it before too much of the plant is affected.
Correcting Iron Deficiency
This now leads us to corrective measures. One of plants’ most effective means of correcting iron deficiency is iron chelates (pronounced keyplates).
This compound holds iron in a soluble form—which the plants can use—and prevents the iron from combining with other elements. Therefore, the iron is not wasted in the soil.
Since iron deficiency is most apparent among those plants that use more iron—the “acid-lovers”—most iron chelates are formulated for acid soils.
Non-Selective Killer
Just this year, though, Geigy Agricultural Chemicals of Ardsley, N.Y., have introduced a new form of iron chelates for alkaline soils, recognizing that many plants other than “acid-lovers” show iron chlorosis during the summer months.
Both of these products are sold under the name Sequestrene, but on the package, you touch the plant to wrap it, so I must use a foliage application.
Another material I have used successfully is Du Pont’s Animate. This really does a job when put on the foliage.
This is what they call a nonselective killer that will eradicate all living plants, and that is why it is also used on drives and walks where it is necessary to completely clear the area.
Poison Ivy Killer
Last summer, I was faced with a problem at our vacation cottage near the seashore. Poison ivy seems to abound in the sandy soil near the ocean, and this spot was no exception.
But I didn’t want to kill anything else. I had seen the results of a test made at Valley Forge by Amchem Products, Inc. (formerly the American Chemical Paint Co.), Ambler, Pa.
They have a selective poison ivy killer called ACP Poison Ivy Killer containing amizol. At Valley Forge, the object was to kill the poison ivy and still leave the stand of flowering dogwoods.
The ACP killer was sprayed on the entire area, resulting in an almost complete destruction of poison ivy with no damage to the dogwood.
I used this same material at our summer cottage with similar results. There were some precautions, however, which I observed.
This material should not be allowed to get on the lawn—it will bleach the grass almost white (in time, though, it will regain its natural color).
Also, keep it off roses—it will cause the leaves to drop in addition to poison ivy. This material will also kill scrub oak, so use care to keep the oaks (which was the case on the shore).
Poison ivy does not have to be a summer menace—it can be killed with the right materials.
44659 by William L. Meachem