How do you know when your trees have a nutritional deficiency?
The first step in determining if a nutrient deficiency exists is to identify the tree. Many trees have nutrient deficiencies that are commonly associated with them. For example, in the Midwest, pin oak trees growing in high pH soils often develop iron chlorosis. In these scenarios, iron exists in the soil in sufficient quantity to support healthy tree growth, but the alkaline pH ties it up and prevents it from going into the soil solution. Again, there can be many causes of odd-looking tree leaves and stems, so one reasonable place to start is with the known causes.
A good next step is soil sampling. Start by taking a representative sample from the tree that you suspect has a nutrient deficiency. What is “representative”? In the case of a soil sample for a tree, remove a trowel full of soil from nine or 10 sites at various levels of rooting. Since most will be in the upper 24 inches of soil, some should be taken at a 4-inch depth, some at a 12-inch depth, some at 18 inches and some at 24. As well, some samples should be a few feet away from the trunk, some under the drip line and some twice the drip line distance. Overall, the samples should be random in depth and distance from the trunk, and represent the active root zone of the tree. The extracted soil should be mixed together in a plastic (not metal) bucket and taken to a reputable soil-testing laboratory. As best as you can, remove small pieces of roots and rocks before submitting the sample.
Though the desirable amount of nutrients varies from species to species, the general ranges to consider for most trees are:
(N) Nitrate nitrogen – 5 to 20 ppm
(P) Phosphorus – 25 to 50 ppm
(K) Potassium – 100 to 150 ppm
(Mg) Magnesium – 200 to 400 ppm
(Ca) Calcium – 300 to 800 ppm
(S) Sulfur – 10 to 20 ppm
(Zn) Zinc – 2 to 4 ppm
(Fe) Iron – 50 to 150 ppm
(Cu) Copper – 3 to 8 ppm
(B) Boron – 0.5 to 1 ppm
Desirable pH range – 5 to 7
Lastly, there are unusual visual symptoms of leaves, needles, stems and flowers. Gauging the presence of a deficiency based on the way that a plant looks is tricky at best. However, if you are experienced in caring for a particular species, then you should have a pretty good idea of how the overall appearance of the tree should look. Your previous encounters with healthy specimens then can be compared with the one that you suspect to be deficient.
The impact of specific nutrients
There are three groups of nutrients required by plants for healthy growth: the macro-nutrients of nitrogen, phosphorous and potassium, the secondary nutrients of Ca, Mg and S, and the micro-nutrients of Fe, B, Cu and Zn. Some symptoms of nutrient deficiencies for each are listed here:
Broadleaf trees: Leaves are uniformly yellowish-green; this color is more pronounced in older leaves. The leaves are small and thin, have high fall color, and drop early. Shoots are short and smaller in diameter than usual. Shoots may be reddish or reddish-brown.
Conifers: Needles are yellowish, short and close together. Older plants exhibit poor needle retention. Lower crowns may yellow, while upper crowns stay green.
Broadleaf trees: Leaves are green to dark green. Veins, petioles, and lower surfaces may become reddish, dull bronze, or purplish. Foliage may be sparse, slightly smaller than normal, and distorted. Leaves drop early. Shoots are normal in length unless the deficiency is severe, but they may be small in diameter.
Conifers: Needles turn purple in young seedlings, starting at the tips of lower needles and progressing inward and upward. Few or no secondary needles may appear. Needles die, starting in the lower regions and spreading upward through the tree. Buds may set early or seedlings remain dormant longer than usual. Older trees take on a dull blue or gray-green color.
Broad leaf trees: Leaves exhibit marginal and interveinal chlorosis (yellowing), followed by scorching that moves inward between the main veins to the entire leaf. Older leaves are affected first. Leaves may crinkle and roll upward. Shoot tips die back late in the season. Shoots from lateral buds result in zigzag growth that is short and bushy.
Conifers: Older foliage takes on a dark bluegreen color that progresses to yellow and reddish- brown; finally, necrosis (death) occurs at needle tips. Needle retention is poor; needles are often stunted. Seedlings have short, thick, abundant buds; frost injury is frequent.
Broadleaf trees: Leaves become chlorotic and/or necrotic; young leaves are small and distorted with tips hooked back. Shoots are stunted with terminal dieback. Roots are usually affected first, with dieback of root tips severely reducing growth.
Conifers: Primary needles are usually normal, but secondary needles may be stunted or killed. Terminals are stunted and needles may hook at tips. Symptoms are most severe in the youngest foliage in the upper crown.
Broadleaf trees: Leaves are thin, brittle, and drop early. Older leaves may show interveinal and marginal chlorosis, reddening of older leaves, with interveinal necrosis late in the season followed by shedding of leaves. Shoot growth is not reduced until deficiency is severe.
Conifers: Needle tips are orange-yellow and sometimes red. Primary needles remain blue-green in young seedlings, but in older plants, older needles and the lower crown show symptoms first. In affected needles, the transition to green may be sharp.
Broadleaf trees: Leaves are entirely pale yellow-green in both young and old plants; they are small on some species and exhibit other symptoms associated with nitrogen deficiency. Shoots are stunted.
Conifers: Symptoms similar to those associated with nitrogen deficiency, needle tips may be yellow, red, or mottled, particularly on older needles. Necrosis may follow. Needle retention
Broadleaf trees: Young leaves are yellow with contrasting narrow green veins; older basal leaves remain darker green. Exposed leaves are bleached and eventually will exhibit apical or marginal scorch. Leaves may be small; symptoms will be severe in cold, wet springs. Shoot length is usually normal, but diameter will be small; twig dieback and defoliation will occur when the deficiency is severe.
Conifers: New growth will be very stunted and chlorotic. Older needles and the lower crown will remain green. In seedlings, cotyledons remain green.
Broadleaf trees: Leaves are occasionally bronzed or scorched. Young leaves are affected first. Leaves are small, thick, brittle and sometimes distorted. Shoots exhibit rosetting, discoloration, and dieback of new growth, which becomes zigzag, short, brushy, thick, and stiff.
Conifers: Shoot tips are bent and the meristematic tissue of the main leader may split. Necrotic blotches are visible on magnified cross-sections of buds and cause the death of terminal and some lateral buds. Plants may be more like shrubs than trees.
Broadleaf trees: Leaves are uniformly yellow, sometimes mottled with necrotic spots. Leaves are small (littleleaf ), very narrow and pointed; older leaves drop. Shoots of small diameter have tufts (rosettes) of leaves at their tips, which may die back.
Conifers: Branches and needles are extremely stunted; foliage yellows. Trees lose all but their first- or second-year needles; terminals die back.