February 1, 2022

Elemental Deficiencies: You Get What You Test For!

The maintenance of plants in the urban landscape is, at times, demanding. The many challenges that imperil trees in the urban forest are numerous and increasing. Plant health care (PHC) requires education, experience and credible resources to make informed decisions on plant interventions. These skills are needed especially for understanding and recognizing nutritional requirements, as well.

Plant diagnostics can be extremely difficult. Trees can’t really tell us what is wrong, so we rely on the expression of symptoms and identification of signs to differentially diagnose the causal agent or agents. All photos courtesy of the author.

Plant diagnostics can be extremely difficult. Trees can’t really tell us what is wrong, so we rely on the expression of symptoms and identification of signs to differentially diagnose the causal agent or agents. Our first and most intuitive diagnostic step is typically through visual associations, when we notice a plant with a less-than-healthy presence. This includes a comparative recognition of appearances considered typical against atypical representations of the plant. Commonly, this results in identifying leaf discoloration, such as yellowing of the live tissue or specific chlorotic patterns, and that is all we get to help determine nutrient deficiencies. And it just isn’t enough.

Often, leaf-tissue analysis detects fertilization problems within an existing program. A PHC technician may be applying supplemental nutrition, but the plant may not be responding as expected, which indicates availability issues or abundance concerns. This allows for adjustments in management strategies. Trees and their growing environment differ even within the same site and cannot be managed collectively; individual cohorts may require specific PHC needs.

Soil testing and leaf-tissue analysis do not assess all the same factors, so care must be taken to choose the correct test when diagnosing nutritional concerns.

The PHC predicament is simply, “Are you capable of making an informed decision with a visual identification regarding nutrient deficiencies?” Simply, the answer is no. Responsible, ethical plant health care requires critical thinking that engages the best resources and the best application of the solution. This requires identifying the appropriate testing methodology for nutrient deficiencies and pH, as well as an understanding of how the law of minimums can apply to plant health and nutrition.

When agricultural scientists centuries ago began to realize mineral elements in a plant were taken up from the soil in which the plants grow, it was a logical step to suggest that chemical analysis of plants could be used as a means of assessing the nutrient supply of the soil. This is just what Justus von Liebig did in the last century in his Law of Restitution. This premise suggests that plant analysis could be used to determine the quantities of nutrients removed from the soil by a crop and, therefore, the amounts needed to maintain the supplying power of the soil.

Von Liebig’s Law of the Minimum-states that yield (growth) is proportional to the amount of the most limiting nutrient, whichever nutrient it may be. From this, it may be inferred that, if the deficient nutrient is supplied, yields (growth) may be improved to the point that some other nutrient is needed in greater quantity than the soil can provide, and the Law of the Minimum would apply in turn to that nutrient. (van der Ploeg, R., Böhm, W. and Kirkham, M., 1999, “On the Origin of the Theory of Mineral Nutrition of Plants and the Law of the Minimum.” Soil Science Society of America Journal, Vol. 63, issue 5, 1055-1062).

Deep root fertilization. The formula for maintaining a healthy, sustainable tree depends upon the specific application of essential elements.

Basically, all plant growth is regulated by its greatest need – or the nutrient in the shortest supply – regardless of the abundance of other nutrients. Some soils have the necessary essential elements, but supplementation may be needed to overcome deficient soils or enhance growth.

Various factors affecting availability include soil type, pH, cation exchange capacity (CEC) and water holding capacity (WHC). Understanding soil properties allows us to better diagnose for a more prescriptive solution to deficiencies often found in urban soils.

Pin oak decline. Leaf-tissue analysis tests all the factors that influence nutrient availability and uptake.

It is estimated that less than 2% of a tree’s materials are derived from essential elements taken from a soil. Failure to obtain all essential items leads to primary deficiencies, secondary symptoms and metabolic dysfunction (White, P. J., & Brown, P. H. [2010]. “Plant nutrition for sustainable development and global health. Annals of botany,” 105(7), 1073–1080).

The formula for maintaining a healthy, sustainable tree depends upon the specific application of essential elements. Essential elements in excess or inadequacies can lead to minor health issues at the least, or devastating failure and even death in worst cases.

Soil and leaf testing

Soil analysis is helpful in formulating and improving a fertilization program, because soil testing measures organic-
matter content, pH and extractable nutrients. Soil analysis is particularly useful when conducted for several consecutive years, because trends can be observed. However, plant-health-care professionals cannot rely on soil analysis alone to formulate a fertilizer program or to diagnose a nutritional issue in the landscape.

Diagnosis of potential nutritional problems should be a common practice. Quantifying nutrients in soils and trees eliminates guesswork when adjusting a fertilizer program. Elemental deficiency or surplus will cause trees to grow poorly or affect them negatively. However, soil tests aren’t the only option for nutrient deficiencies. Soil testing and leaf-tissue analysis do not assess all the same factors, so care must be taken to choose the correct test when diagnosing nutritional concerns.

Leaf-tissue analysis is a very useful tool to detect problems more specifically and adjust fertilizer programs for trees and shrubs, because leaf-nutrient concentrations are the most accurate indicator of any plant nutritional status. Considerable research involving leaf testing has established its reliability as a management tool, but sampling guidelines should be followed precisely to ensure that analytical results are meaningful.

Soil test kit. Soil testing measures organic-matter content, pH and extractable nutrients.

Leaf-tissue analysis tests all the factors that influence nutrient availability and uptake. Tissue analysis shows the relationship of nutrients to each other as well. For example, K (potassium) deficiency may be from a lack of K in the soil or from excessive Ca (calcium), Mg (magnesium) and/or Na (sodium). Similarly, adding N (nitrogen) when K is low may result in K deficiency, since the increased growth caused by N requires more K. There are many nutrient interactions when it comes to prescriptive nutrition programs. Effective PHC requires knowledge of these interactions to prevent compounding problems. This could be considered similar to drug interactions with people and their prescriptions.

Comparatively, leaf analysis is a useful tool to detect problems, because leaf-nutrient concentrations are the most accurate indicator of crop nutritional status. Leaves reflect nutrient accumulation and redistribution throughout the plant, so the deficiency or excess of an element in the soil is often reflected in the leaf.

Soil tests are quite useful for pH, organic-matter content and other qualities. Relative to nutrition, soil tests primarily measure extractable elements present in the sample. However, they don’t measure the total amount of nutrients present, nor the quantity actually available to the tree.

Testing availability

How can you get the appropriate testing? Test kits are available from many sources for leaf analysis and especially so for soil tests. However, test results are only as good as the sampling techniques and protocols used in the process. Each state differs with available locations. Some land-grant colleges continue to offer both evaluations. Each state in the U.S. typically has this wonderful, often untapped resource for many things arboricultural housed within a well-known university.

Soil test kit. Soil testing measures organic-matter content, pH and extractable nutrients.

The Land-Grant College Act of 1862, or Morrill Act, Act of the U.S. Congress (1862), provided grants of land to states to finance the establishment of colleges specializing in agriculture and the mechanic arts. Named for its sponsor, Vermont Congressman Justin Smith Morrill (1810-98), it granted each state 30,000 acres (12,140 hectares) for each of its congressional seats. Funds from the sale of the land were used by some states to establish new schools; other states turned the money over to existing state or private colleges to create schools of agriculture and mechanic arts. While many land-grant schools with extension programs have stopped this service due to rising overhead costs and funding, assistance is still available.

Agents at your local cooperative extension center may be able to provide testing locations and help develop a fertility program for your trees based on your soil- and tissue-analysis results. They also will be happy to answer any questions you have about your fertility program. For example, Purdue University provides many crop services, including diagnostics at the Purdue Plant and Pest Diagnostic Laboratory. However, soil testing is outsourced to private facilities. There are several private labs nationwide that are quite cost effective. Check with local agricultural and horticultural labs for these services.

Conclusion

Finally, a sampling or testing program can be most effective if conducted annually, especially on problem plantings. Leaf-tissue testing is valuable for all elements. Soil testing is most useful for consideration of pH, P (phosphorus), Ca, Mg and Cu (copper) concentrations.

A key point to remember is that visual-identification tools are unreliable, because virtually every deficiency reveals similar expression of symptoms, which is chlorosis. There also are some key points to remember when sampling for tests to get best results. Pesticide spray residues on leaf surfaces affect sample results, so be sure to wash leaves for accurate analysis. Especially avoid sampling recently treated trees.

Interpretation of leaf and soil tests should be used to make fertilizer or supplement decisions. A broad-spectrum or “shotgun” approach is never accurate and is less effective and often not economical as well. Wise use of the results allows optimal health and growth, minimizes environmental impacts and reduces fertilizer waste with inaccurate applications.

Lindsey Purcell is an ISA Board Certified Master Arborist (BCMA), an American Society of Consulting Arborists (ASCA) registered consulting arborist (RCA) and an urban-forestry specialist in the Department of Forestry and Natural Resources at Purdue University in West Lafayette, Indiana.

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