Question
QUESTION: I have 3 "natural" ponderosa pines near my property that have yellowed needles.  They have been in this condition for over a year.  Being concerned about the health of the tress I sent samples of the needles to the county extension agent, who in turn sent them to Montana State University for testing.  Their analysis was that the trees were not diseased and they suggested that I get the soil tested.  I did that and was told that the soil had a high iron content and that the sulfur content was almost nonexistent.  They had no suggestions for ferilizers.

Do you have an insight as to what the problem may be and what fertilizers, if any, should be used.

ANSWER: Iron chlorosis is caused by the plant not being able to obtain the iron it needs. Iron is needed for the production of chlorophyll. Therefore, a lack of iron results in a loss of the green color in the leaves. In severe cases, leaf color may change from yellow to white to brown. If uncorrected twigs and eventually branches may die leading to the death of the tree.

Symptoms may appear over the entire tree, on one side only, or be limited to individual branches.

Iron chlorosis occurs when iron in the soil is either deficient or unavailable to the tree. There are three ways to provide iron to the plant: foliar application, soil treatment.

Foliar application. If a rapid response is needed to correct a chlorotic condition, a foliar spray with iron sulfate or iron chelate solution may be applied when the tree is in full leaf. A rate of five pounds iron sulfate in 100 gallons of water (2.5 ounces iron sulfate in three gallons water) is recommended. Soybean flour may be added to the solution to help it adhere to the leaves. Adding a tablespoon of detergent will also help to wet the foliage. Iron chelates are water-soluble forms of iron that remain in the solution once added to the soil or tree. Follow label instructions for determining the proper concentration when applying chelated iron directly to foliage. When applying either iron sulfate or iron chelate, it is best to spray during the evening or during periods of cool weather.

Although a foliar spray produces quick results, the improvement is temporary because iron will not move into the tree beyond the tissue that was sprayed. New growth emerging after the treatment will be chlorotic.

Soil treatment. Lowering the soil pH is a more permanent way of correcting iron chlorosis caused by high pH soils. Unfortunately this is also one of the most difficult treatment methods because it is almost impossible to lower the soil pH significantly to depths of 18 to 24 inches.

A more feasible approach is to add iron to the soil in sufficient quantity to increase the amount available to tree roots. In bare soils, iron sulfate can be applied to the ground under the tree canopy at a rate of one pound per half inch of trunk diameter measured 4?feet above the ground. Trees growing in turf-covered soil should be deep-fed either by liquid injection with a hydraulic sprayer or by iron sulfate placed in holes drilled into the soil.

Drill holes spaced two feet apart and 15 to 18 inches deep should form concentric circles around the tree, beginning two to three feet from the trunk and extending beyond the ends of the branches about three feet.. This treatment remains effective for two to three years. Chelated iron may be used, but be sure to follow label directions for rates. Best results for the current year will occur when treatment is made in early spring just as buds begin to swell.  Also, high pH soils may prevent most iron chelates from working.  For any soil with a pH above 7.2, use an iron chelate with EDDHA.  Such products include Sequestar 6%, Sprint 138 and Millers FerriPlus.

An alternate soil treatment is to mix sulfur and iron sulfate at a 1:1 ratio. The sulfur lowers the pH in a small area and makes the iron more available to the tree. The recommended rate is one pound of iron sulfate to one pound of sulfur per inch of trunk diameter at 4?feet above the ground for trees four inches and under. The recommended rate for trees over four inches is two pounds of iron sulfate and sulfur mixture per inch of trunk diameter at 4?feet above the ground.

Since your soil is low in sulfur i would recommend that you mix the sulfur into the iron sulfate.

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QUESTION: I am confused by your answer.  As I mentioned in my original question I had the soil tested and the analysis revealed a high iron content (13000 mg/kg).  Is there another possible cause for the yellowing of the ponderosa pine needles ?

Answer
I am sorry I miss read your question. I read "soil had  iron content and that the sulfur content was almost nonexistent" Read right through the "high".

Sulfur is a constituent of essential amino acids (cysteine, methionine, and cystine) involved in chlorophyll production and is thus required for protein synthesis, and plant function and structure. It is also a constituent of coenzymes required in protein synthesis. It is contained in the plant hormones thiamine and biotine, both of which are involved in carbohydrate metabolism. S is also involved in some oxidation-reduction reactions. It is less mobile in the plant than N so that deficiency tends to appear first on young leaves.

Replenish S  in the soil by applying N and P fertilizers that contain S (e.g., ammonium sulfate [24% S], single superphosphate [12% S]).

Use about 1 lb per diameter of the tree trunk. ( a 10 inch diameter tree will need 10 lb of fertilizer). Spread it on the ground under the tree's foliage but not with in a foot of the tree trunk. Water the fertilizer in good. I would do this now and again in the spring and then at three month intervals until the color returns. DO NOT use a feed and weed fertilizer combination the herbicide in this will damage the trees. You can use granular sulfur but you will have to ask the nursery how much to use. I would use the Ammonium sulfate.
Sorry about the confusion. I hope this explains it.