Chlorosis on loquat & how to fertilize trees.
Dillon’s got two mature loquat trees, planted 20 yards apart, but one’s got pale leaves with dark green veins, most likely a sign of interveinal chlorosis caused by a deficiency of the micronutrient iron. But it could be something else, so I’d recommend a soil test that includes a test for micronutrients. Our soil pH in Central Texas is on the alkaline side, which inhibits micronutrient availability. And that means that just adding fertilizer may not actually correct the issue.
Also, not only may the soil and growing conditions change just enough to affect plant health over a much smaller area than we would expect, slight variations in the genetics of the two plants, as well as in their overall health prior to planting may cause problems that take years to manifest visibly.
Now, how should we fertilize trees? We reached out to certified arborist April Rose, Urban Forest Health Coordinator for the City of Austin, to get her take. First, April reminded us that native plants are well adapted to Central Texas soils and generally don’t need fertilizer.
But she also pointed out that all plants can benefit from practices that improve soil conditions, such as top dressing with compost and applying mulch. When we nourish the soil, we nourish the plants.
April also notes that yellowing or chlorotic leaves may be caused by problems that can’t be helped with fertilizer, so it’s important to rule out other sources of plant stress before applying anything. A Certified Arborist can help determine the cause of symptoms, and because there may be many factors involved, treatment without diagnosis is not recommended.
If a soil or foliar test determines that one or more nutrients is lacking, fertilization can restore the tree’s health, but sometimes the required nutrients are already present in the soil, just not available, and adding additional nutrients could lead to toxicity and cause more problems than it solves. Also, excess fertilizer may wash away into streams, creeks, and lakes, which can cause problems for other plants and for wildlife.
If the ultimate recommendation is to fertilize, the best way to apply fertilizer will depend on site conditions and other environmental factors. Also, you want to ensure that the entire root zone receives treatment, so generally speaking, the more soil volume you can affect, the better for the tree, especially for large, older trees.
April explains two ways to fertilizer. The simplest method—and often the cheapest—is to broadcast material over the surface of the soil and let gravity and water carry it to downward into the rootzone. Powders, granules, or pellets can be spread by hand with scoops or shovels, or a wide range of machines can distribute material evenly over a wide area.
In addition to placement, timing is also important: for trees planted in lawns, adding a concentrated chemical to grass during hot weather can scorch leaf surfaces.
Compost, if it’s well-aged and of good quality, can be spread up to ½” thick any time of year with little risk of damage to plants. Surface applications of anything, even compost, should always be watered in.
This method may not work on lots with steep grades or in severely compacted soils, where a surface application may wash away before it can be absorbed. This not only wastes money but can kill aquatic wildlife. Another disadvantage of surface application is that the nutrients may be taken up by turf grass roots before they reach the root zone of trees.
In these cases, deep-root fertilization may be needed. This method requires specialized equipment to inject a solution directly into the soil, so generally, professional arborists or landscape companies should be hired to do the work.
Although it is more expensive, there are several advantages of deep-root applications. The liquid is injected below grass roots and there is little to no runoff. Also, the solution used can be customized to include humates or mycorrhizae, nutrients are available to the plant more quickly, and they are not as likely to burn plant tissue.
Another benefit is that the injection needle can puncture layers of compacted soil, which both alleviates compacted soil, and allows water, air, and nutrients to reach the roots zone of compacted soil more effectively than a surface application. A Certified Arborist can help you determine if your trees need treatment and if so, which methods are best suited to your landscape.