The following article is courtesy of tropical plant and soil expert Lynn Griffith.
Growers and fertilizer sales reps frequently use leaf analysis or plant tissue analysis to diagnose plant nutritional symptoms or to determine the effectiveness of a fertilization program. A media analysis is sort of like a snapshot of the nutrient status at any given time. A leaf analysis is more akin to a video, showing longer term nutrient absorption status. It shows what nutrients are actually being absorbed by the plant, and in what quantities and ratios.
In some ways, leaf analysis is more useful than media analysis in diagnosing plant problems. Sure, media and soil testing are useful in determining pH, soluble salts, and major and minor nutrient levels, and these data are helpful. However, what is in the media is not necessarily getting absorbed by the plant, for a variety of reasons. Tissue testing can be more revealing diagnostically, especially to diagnose a plant symptom.
Nutrient contents are not uniform throughout the plant, however. They are different in roots, stems, leaves and flowers. Nutrient levels are also different between old and new leaves. Even within a single leaf itself, the tip of the leaf can have different nutrient content than the base of the leaf.
With all of this variability, how can leaf analysis be at all helpful ? Most agricultural laboratories use the most recent fully matured leaf as their benchmark. This is the newest leaf on the branch that has fully sized up and hardened. The desired nutrient ranges are pegged to that leaf. If you sample soft, new tender tissue, the very youngest leaves on the plant, the analysis will generally come back lower in calcium and possibly higher in nitrogen than the true status of the plant.
If you sample older leaves, the levels of magnesium, potassium and nitrogen will likely be lower than they are in reality, and boron and other trace elements may be higher. Also, sampling old leaves may tell you what the nutritional status was like six months or a year ago, and will not necessarily be indicative of current nutrient status.
Therefore, for almost all ornamental plants, you should sample the most recent, fully matured leaves for plant tissue testing, often the fourth or fifth leaf back from the growing tip. Experienced laboratories will have rating standards for various types of ornamental plants. Understand that nutrient ranges can vary a lot by plant species. For example, a leaf nitrogen reading of 3.5% will be in the high range for an oak tree, but in the low range for a chrysanthemum. The lab must use the correct rating standards to interpret the analysis correctly in order to maximize crop response.
This is a problem I encounter frequently. Labs may be relatively new to the business. They may primarily work with agricultural crops as opposed to ornamentals. The labs sometimes lack proper leaf analysis rating standards, especially for less commonly grown ornamental varieties. Some labs constantly attempt to apply generic rating standards, such as “average woody ornamentals” or “average bedding plant” tissue values.
This approach is worthless in my view, and may in fact be harmful. Ornamental plants come from many different plant families from many different environments and parts of the world. Their individual nutrient requirements vary quite widely, as do the nutrient levels they need in their leaves. An improper interpretation may say a value is normal, when for a particular plant it may be low or high or even toxic. To use a media testing analogy, a soil test might say a pH of 5.3 is low. However, it wouldn’t be low if you’re growing an azalea or a blue hydrangea. It would be high.
If you are using a lab that utilizes only general leaf analysis rating standards, try to find a better lab. Or, option B is send the tissue analysis to me and let me rate the values for you. I have leaf analysis rating standards for hundreds of ornamental crops, even some rather unusual ones. If I don’t have standards for your exact variety, I likely have them for similar plants in the same family that are likely adaptable to the variety in question.
I have a couple of other sampling tips. Collect a large handful of most recent fully matured leaves for your lab test. It can sometimes be helpful to compare symptomatic and normal leaves from plants in the same block. However, if you do this, make sure the leaves that you sample are the same age on the plants. It is ok if your leaf sample dries out before or during shipment. The nutrient levels won’t change. However, if the sample begins to rot or decompose, the sample is worthless and you need to collect a new sample. Paper bags are preferred over plastic bags for tissue samples unless you can get the sample to the lab quickly.
Tropical Plant and Soil Expert
“Get the Soil Right”