Gardening : Why Plant Leaves Turn Yellow
“Why did the leaves on my . . . (you fill in the blank) turn yellow?” That’s the question nurserymen, professional gardeners, consultants and, yes, even garden writers hear almost daily. On occasion, they even ask themselves.
It’s not an easy one to answer. “There are 1,000 reasons why leaves might turn yellow on a plant,” said William Darlington at Soil & Plant Laboratory in Santa Ana, which does tissue and soil analysis for growers and gardeners alike. “It’s a very nondescript symptom.”
But some reasons are more likely than others.
Not enough nitrogen, the primary plant nutrient, is one. Your clue: If the whole plant is pale in color and the leaves turning yellow are the older ones, at the base of the plant, it probably just needs fertilizing.
For quick results look for fertilizers that have some of their nitrogen in the nitrate form, shown on the label. This form of nitrogen becomes immediately available to plants; others take a couple of weeks, or longer.
Salt in our water and soil can be an indirect cause of yellowing because it inhibits the uptake of nitrogen, but thanks to all the rain, salts in the soil shouldn’t be much of a problem this year.
If the leaves turning yellow are near the top of the plant, however, on the new growth, it is probably a deficiency of some other element, such as iron, zinc in citrus, or manganese.
This is called chlorosis and most gardeners know to look for interveinal chlorosis, where the leaves turn yellow but the veins stay green, as a sign that the plant needs iron or another minor element.
“After nitrogen, iron is by far the most limiting element in alkaline soils like ours,” Darlington said.
Garn Wallace at Wallace Laboratories in El Segundo also runs tissue and soil tests for professional and home gardeners and he is convinced that chlorosis is the the culprit in a majority of cases, but adds some interesting qualifiers.
He points out that iron is actually plentiful in California soils. Perhaps 5% of our soil is iron, by weight. But it is unavailable to the roots because of alkalinity of western soils, caused by too much lime.
To counteract the lime, he believes every gardener should be adding gypsum to the soil, whenever the soil is turned or prepared for planting.
He also believes that a number of heavy metals, some found in soil amendments or conditioners that contain sewage sludge, interferewith the uptake of iron, and that cold, heavy, waterlogged soils, do the same.
He expects to see a lot of yellowing of the chlorosis kind after our wet winter this year.
Solutions? He suggests adding gypsum and organic matter when preparing a soil for planting.
Gypsum has many benefits, reducing salinity, subduing heavy metals, binding organic matter to the soil particles and the like. Wallace suggests adding about 50 pounds to every 1000 square feet.
Homemade compost is the best organic soil amendment, but if you don’t have any, add mushroom compost, peat moss or other organic matter so it becomes about 10% to 20% of the soil volume. If you are cultivating to a depth of a foot, that would be about a one- to two-inch layer.
Where there are permanent plantings, such as trees and shrubs, and you do not want to turn or disturb the soil, scatter chelated iron over the surface. Wallace favors a product called Sequestrine 138 Fe, because it gets to work faster. He calls it a “designer chelate,” because its formula was developed years ago at UCLA.
Sequestrine 138 Fe, also sold as Sprint 138 Fe, carries a designer price tag as well, selling for about $13 for one-pound canister. It is also hard to find. One source is Burkard’s Nursery in Pasadena. It is also available by mail order from Complete Green, 365 Coral Circle, El Segundo, Calif. 90245-4640.
Wallace also suggests “vertical mulching” on permanent plantings. This is done by digging holes near the effected plants and then filling the holes with improved soil, mixing in amendments such as homemade compost or mushroom compost.
He has dug a hole as wide as two feet across, filled it and then found it stuffed with roots a few years later, all able to take up nutrients needed by the plant.
He suggests digging four or five holes, two or three feet deep and a foot wide, spaced evenly around the tree out near what’s called the “drip line,” at the edge of the tree’s or shrub’s canopy. You could even use a posthole digger to make them. Fill them with improved soil.
Soil sulfur, sold at nurseries, can also help trees and shrubs. It will acidify the soil, making iron more soluble and usable. Sprinkle it on the surface and water it in.
If you think the problem is serious, or want to fine tune your solutions, get a soil test or leaf tissue analysis done by a professional lab. They are usually listed under Soil Testing in the classified section of the phone book. A soil test costs around $65, and tissue analysis, $50-65.
You might find that you actually have one of the other 1,000 reasons leaves turn yellow.
