Just like when our plants are lacking in necessary nutrients, excess levels of any nutrient in your soil may sometimes cause symptoms of toxicity in your plant. Different nutrient toxicities are expressed in the plant in different ways. But there is an additional layer of concern when it comes to plants having too much of one or more nutrient.
Sometimes an overabundance of one nutrient will actively prevent a plant from being able to take up enough of one or more other nutrients. This will lead to symptoms of deficiency of those other nutrients rather than symptoms of having too much of the former.
Put in a different way, excess nutrients may cause deficiencies of other nutrients; so, you may have symptoms of toxicity that occur on top of symptoms of a deficiency. That’s why it’s important to rule out other possible reasons for your plants symptoms before you start adding additional nutrients because you may do more harm than good.
Let’s start with the big three nutrients: nitrogen, phosphorous and potassium.
Too much nitrogen can manifest itself in burnt leaf tips. Nitrogen toxicity may also show itself in very deep-green leaves, plants that seem to mature more slowly, maybe are tall but with weak stems, or that seem like they are needing much more water than usual. Excess nitrogen can also cause a visible deficiency in potassium or iron since high nitrogen can interfere with a plant’s uptake of those nutrients.
Excess phosphorous can disrupt the uptake of iron, manganese, copper and zinc in the plant which could cause the plant to display symptoms of those deficiencies.
Too much potassium available to your plant can interfere with the plant’s uptake of magnesium and calcium. Those three, in particular, work tightly together for fruit formation, so too much or too little of one will almost always cause problems with the other.
What about some other nutrients? It’s unusual for many of the micronutrients to be available in too large quantity to do too much damage, with the exception of a notable few.
Too much magnesium can interrupt the plant’s uptake of calcium and potassium. This will likely only show up as a deficiency in calcium or potassium and no other symptoms.
Excessive amounts of calcium are complicit in visible deficiencies in potassium and magnesium, but excess calcium can also interrupt boron uptake.
It’s not as common to have an excess of boron but when it does happen the older leaves on the plant may look scorched, the leaves will start to turn yellow and die off from the tips and edges slowly moving in toward the veins, and the leaves will start to just drop off the plant.
If you have a zinc toxicity, usually uncommon, the only real symptom will be the leaves turning a much darker green than is normal for the plant.
While having nutrient deficiency is more usually the cause for problems in plants, it’s not always. Have a soil test done before the season starts and keep track of what amendments and fertilizers you have added. I added too much nitrogen one season to my broccoli bed by adding the wrong amendment prior to planting. If I hadn’t made my notes and realized that I had applied the incorrect fertilizer, I may have made the mistake of adding a fertilizer with a higher amount of potassium because the symptoms looked similar. That would have made the problem even worse and likely would have killed those plants. So, notation, notation, notation, in your garden journal.
If you see problems in your plants, also take into consideration other environmental factors, like cold snaps or heat waves, too much water or not enough. All of these environmental events can cause temporary symptoms that mimic many of the symptoms of nutrient deficiencies or toxicities.
Your Friend in the Garden,
