Many plants growing in our region of California suffer from the chronic occurrence of leaf yellowing, or chlorosis. Chlorosis is the result of the loss of the green pigment, chlorophyll, which is essential to the process of photosynthesis. In effect, chlorotic plants suffer from a food shortage, which has consequences that range from unsightliness and low vigor to death.
Chlorosis often is a symptom of a nutritional deficiency, for which there may be many causes. The most common cause in our region is insufficient uptake of iron, although sometimes manganese or zinc is the culprit. The discussion and recommendations that follow focus on iron, but manganese and zinc deficiencies can be treated similarly.
Plants require tiny amounts of iron, and it is one of the most abundant elements in soil, so iron deficiency usually is the result of a plant’s inability to extract the element despite its abundance. Plants can take up only the iron that has dissolved in the soil water, and this poses two big problems. One is that the solubility of iron in soil water is very low. The other is that iron solubility is strongly affected by soil pH: for each unit increase in pH, the soluble iron concentration decreases 100-fold to 1000-fold.
The alkaline soils (those with a pH greater than 7) that are common in the Central Valley would not have enough soluble iron, manganese, or zinc to meet the needs of any plants if it weren’t for the existence of chelates. These are soluble organic compounds whose interactions with metals such as iron increase their solubility and their availability to plants. Many of the naturally-occurring organic compounds in soils act as chelates. In addition, plants that are adapted to alkaline soils produce root exudates that act as chelates. In our landscapes on alkaline soils, chronic chlorosis usually occurs in plants that are adapted to more acidic soils. Landscape trees that are particularly susceptible to pH-induced chlorosis include Liquidambar species (sweetgum), Liriodendron tulipifera (tulip tree), Magnolia grandiflora (southern magnolia), and Acer saccharinum (silver maple). These plants don’t produce many organic chelates and, therefore, cannot absorb adequate amounts of iron, manganese, or zinc from high-pH soils. The characteristic symptom of the ensuing deficiency is yellowing of leaf tissue between the veins (termed interveinal chlorosis), especially on developing or recently-expanded leaves (Fig. 1, Fig. 2).
The most effective remedy may be to lower the soil pH by adding elemental sulfur. In warm, moist soil, bacteria can convert sulfur to sulfuric acid within a couple of months. Unfortunately these reactions proceed slowly, if at all, when sulfur is not incorporated into the soil, so it can be difficult to acidify soil in established landscapes. We recommend using a water jet to create 2-foot-deep holes on a five-foot spacing under the canopy of mature trees, and filling the holes with a mixture of 25% ammonium sulfate and 75% granular soil sulfur. Even with this method of sulfur treatment, it can take up to two growing seasons before an effect color is noticeable.
Calcareous soils — those that contain calcium carbonate (limestone) — are almost impossible to acidify. No decrease in pH occurs until all of the calcium carbonate has reacted with the sulfuric acid. For some calcareous soils, a pound of sulfur per square foot of soil may be needed just to neutralize the calcium carbonate. Fortunately, these soils are relatively uncommon in the Sacramento Valley and the Bay Area. They typically are found where annual rainfall is less than 10 inches or, in some cases, where topsoil has been removed or has eroded away. The seat-of-the-pants test for the presence of free calcium carbonate in soil is to add hydrochloric acid and see if it fizzes. A better approach, if a soil is thought to be calcareous, is to have it analyzed by a soil testing lab. Correcting chlorosis of plants in calcareous soils is not a sure thing. There have been reports that use of chelated fertilizers (described below) is effective when applied every year, but this practice is not always successful. Calcareous soils also introduce other problems for plants, particularly phosphorus deficiency.
Another way to treat chlorosis is to add the fertilizers in a chelated form. Several types are available, but products containing the chelate EDDHA (ethylenediaminedi-o-hydroxyphenylacetic acid) are most effective in alkaline soils. Several products provide this form of chelated iron, among them Sequestar and Sprint 138. Both products contain 6% iron by weight. They should be broadcast on the soil surface under the tree canopy in the spring at rates of 1/2 to 1 pound per tree, then watered into the soil. Mixing the product with sand or dry soil prior to broadcasting helps to achieve uniform distribution. This practice should be repeated annually.