Do you ever see chlorotic or stunted leaves and wonder if it could be a particular nutrient deficiency? Soil nutrient testing can be a good tool to assess levels of macro and micronutrients. We use soil tests to help make decisions about adding fertilizer and organic materials to the soil. But what soil tests don’t always tell you is……how much of the nutrient is available for plant uptake? To get a more accurate picture of nutrients that could be deficient (or excessive), we look to foliar nutrient content analysis.
When we see symptoms of yellowing, either alone or in combination with the expression of other colour pigmentation (e.g. reddish or purplish leaves), we made first need to take a closer look at the plant before we jump to any conclusions about a nutrient deficiency
The first thing I do is turn the leaf over and look for signs of sucking insects or mites or their resulting injury. Note the population of two-spotted spider mites on the underside of this Viburnum leaf. It’s easy to see how some horticulturalists could misdiagnose the leaf discolouration as a nutrient deficiency.
If no pests or diseases are apparent, inspect the stem and crown of the plant looking for signs of unhealthy or necrotic tissue. If possible, examine the soil for moisture level and texture to determine if it is appropriate for supporting plant growth. By taking a closer look at this plant, you can see that the interveinal leaf chlorosis is likely due to the reduced amount of live tissue that would be conducting water and nutrients between the root system and the canopy from a sudden drop in temperatures back in the fall or winter (as is the case for this Cornus kouza above). Its no wonder that the leaves turned yellow once conditions become hot and dry. Nutrients are taken up in their dissolved form in the free water that roots absorb from the soil. Reduced water uptake means reduced nutrient uptake but the killed conductive tissues in the stem further restrict both water and nutrient uptake. But you know better.
Along the same lines, if there is too much water in the root zone, root tips may die from the buildup of toxic compounds and become much more susceptible to the root pathogens that would proliferate under such conditions. When roots succumb to anaerobic conditions and disease, the root system’s function becomes restricted in its ability to take up water and dissolved nutrients that it needs for growth and maintenance. The result is wilting and chlorosis of foliage which leads to branch dieback and eventually plant mortality if left unmanaged. Although nutrient deficiencies can be found in this symptomatic foliage, chasing them with fertilizer is not going to solve the problem. Check the root system of container plants to ensure that the root cortex is white and that the roots snap when broken, as opposed to appearing brown and mushy inside. In field production or in the landscape, you can did up a portion of the fibrous root system, assess the soil for texture and drainage and also look around to see how other plants are fairing, especially those that require well drained soil conditions (like the Taxus in our photo). Soil texture, topography and precipitation data can tell you a lot about root health.
If root and stems tissues look healthy, the next logical step is to take leaf samples for foliar analysis. A good rule of thumb is to take enough symptomatic LEAVES (not twigs) to fill a paper lunch bag. And paper is the way to go (not plastic). Samples left in plastic will begin to degrade, potentially changing the nutritional content and giving false lab results. Where possible, place the leaves out in a thin layer to dry and keep them dry until you submit them for analysis. Try to select leaves from the middle of this year’s growth and avoid the very youngest or oldest leaves. Try to be consistent about where you take them on the tree. Usually 50 maple leaves is a good sample (about 100 grams fresh weight). The smaller the leaf, the more leaves you will need.
If possible, take a comparison “healthy” sample from the same species (or cultivar/variety) from trees that are on a different property. Remember that trees can be experiencing nutrient deficiency symptoms for years before they show any symptoms of stress. So the nutrient levels between a healthy looking Quercus robur and a chlorotic Quercus robur on the same property could be….very very similar. If you can’t find a healthy species growing just off-site, there are “book values” that you can refer to and most labs include them on foliar tests now. I often refer to the Plant Analysis Handbook II by Mills, Jones and Benton.
Ontario labs that conduct plant tissue and soil testing include A&L Labs in London, SGS Labs in Guelph, Stratford Agri-Analysis and University of Guelph Lab Services. Analysis should run about 40.00 per sample and it can take a week or two depending on how many samples the lab has to run. But the information is very valuable and can be used to make long term nutritional recommendations to grow healthier trees and shrubs!
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