There are lots of examples of people trying to use plant leaf characteristics to identify nutrient deficiencies in the soil. This seems to make a lot of sense. If the soil is lacking a specific nutrient, it should show up in the plant and it seems to follow that by examining the physical characteristics of the leaf you should be able to identify the nutrient deficiency.
How reliable is this method of diagnosis? Can you really identify a nutrient deficiency in the soil by looking at plant leaves?
Identify Plant Nutrient Deficiencies
The image above is just one of the memes used to show people how to use leaf characteristics to identify nutrient deficiencies in the soil. This seems simple enough. Examine the leaf for abnormalities, and then match these to the nutrient that is responsible for the symptom.
Unfortunately this does not work. Plant symptoms are a tool for predicting possible nutrient deficiencies, but in most cases it is not a reliable one.
What Causes the Symptoms
As plants grow, leaves need certain nutrients to grow properly. If even one is missing the leaf will not grow properly and the symptoms you see will be specific for the missing nutrient, but the reverse is not true. You can’t know which nutrient is missing by seeing the symptom.
In addition to nutrients, the leaf characteristics are also affected by environmental conditions, diseases, insect damage, and exposure to pesticides. Any of these can be the cause of the abnormality you see without any nutrient deficiency being present.
The pH of the soil can also affect the plants ability to absorb certain nutrients. At higher and lower pH some nutrients become stuck to soil, preventing the plant from accessing them. You can have plenty of iron in the soil, but at higher pH some plants can’t extract it from the soil. In this case, the leaf will show an iron deficiency but the soil has lots of iron.
Misuse of Nutrient Deficiency Memes
The image above is found quite frequently on social media sites like Pinterest and Facebook. The text provided with the image almost always suggests that it is easy to identify a deficiency by looking at the chart.
If you visit the original site of this mage (ref 1), and read the text in full you get a very different story. It clearly says “Sometimes an unhealthy plant is suffering from a nutrient deficiency or even too much of any one nutrient. Plant nutrient deficiencies often manifest as foliage discoloration or distortion. The following chart outlines some possible problems. Unfortunately many problems have similar symptoms and sometimes it is a combination of problems.”
“Be sure you eliminate the obvious before you kill your plants with kindness.
- Check first for signs of insects or disease.
- Foliage discoloration and stunted plants can easily be caused by soil that is too wet and drains poorly or soil that is too compacted for good root growth.
- Extreme cold or heat will slow plant growth and affect flowering and fruit set.
- Too much fertilizer can result in salt injury. Your plants may look scorched or they may wilt, even when the soil is wet.”
The information provided is quite accurate. Unfortunately people take the image, post it on social media and use it to misinform the general public. This original site is not the cause of this myth, but the misuse of the information continues to support this myth.
Example: Phosphorus or Nitrogen Deficiency
Assume the plant is showing a classic phosphorus deficiency. This could be a phosphorus deficiency, or it might be a nitrogen deficiency.
A lack of nitrogen reduces the plants ability to absorb phosphorus which then shows up as a phosphorus deficiency. Adding more phosphorus to the soil will not solve the problem since this is a nitrogen deficiency. Adding nitrogen will make the symptom go away.
Example: Chlorosis Indicates iron Deficiency
This is a common example found in a lot of gardening advice. A plant is reported to have chlorosis, a yellowing of the leaf, and the diagnosis is usually iron deficiency.
In this example, two major mistakes are made.
Firstly, Iron does not cause chlorosis of the leaf. It actually causes interveinal chlorosis which is the yellowing of the spaces between the veins of the leaf. as shown in the image. The veins remain green.
The second mistake is assuming that there is only one cause for this symptom. As explained in the post, Chloriosis In Plants – Is it Iron Deficiency, numerous conditions can cause interveinal chlorosis.
- manganese deficiency
- a high soil pH
- zinc deficiency
- herbicide damage
- wet soil conditions
- compacted soil
- trunk-girdling roots
- plant competition
- high organic content in soil
- high salts
- high levels of phosphorus, copper, zinc or manganese
- iron deficiency
The bottom line is that interveinal chlorosis tells you there is a problem but it does not indicate the reason for the problem.
The University of Arizona makes some important points about nutrient deficiencies (ref 2).
- Many nutrient deficiencies may look similar.
- It is important to know what a plant species looks like when it is healthy in order to recognize symptoms of distress, for example some plants were bred to have variegated patterns in the leaves when they are healthy.
- Many micronutrients are used by plants to process other nutrients or work together with other nutrients, so a deficiency of one may look like another (for instance, molybdenum is required by legumes to complete the nitrogen fixation process).
- If another problem like water stress, disease, or insect pressure, occurs simultaneously with a nutrient deficiency, or if two nutrients are deficient simultaneously, the typical symptoms may not occur.
How Do You Detect a Deficiency?
The only reliable way to detect a soil deficiency is to do a soil test.
- Identifying Plant Nutrient Deficiencies; http://www.permablitz.net/articles/identifying-plant-nutrient-deficiencies/
- Guide to Symptoms of Plant Nutrient Deficiencies; https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1106.pdf
- Photo Source for chlorosis: Scot Nelson