Providing plants with the right concentrations—not too little, not too much—of all essential nutrients is vital. Underfeeding can lead to suboptimal fertilization and nutrient deficiencies, while overfeeding can cause nutrient toxicities. In this blog post, we’ll discuss why nutrient toxicities occur, their specific symptoms and how growers can avoid them.
Causes of nutrient toxicities
Overfertilization is one of the main reasons nutrient toxicities occur.
There is a misconception that more is better when it comes to fertilizing plants, but nothing could be further from the truth. Plants require a unique balance of macro and micronutrients, and providing too much can be just as detrimental as providing too little.
When too much of a nutrient is applied, nutrient antagonism can occur. Also known as nutrient “lockup” or “lockout,” this is when one nutrient prevents another nutrient from being absorbed. For instance, high phosphorus levels can prevent plants from absorbing micronutrients like iron and zinc, leading to phosphorus toxicity and potentially iron and zinc deficiencies.
Another cause of nutrient toxicity is a high substrate pH. A high pH often locks out certain nutrients, such as iron, causing the plant to absorb more of the available nutrients. When this happens, there’s a risk for both nutrient toxicity and deficiency, which is why it’s important to keep the substrate pH between 5.5 and 6.2, the range in which all essential nutrients are available for absorption (Figure 1).
Figure 1. Nutrient availability based on pH range.
Any nutrient can be toxic for plants. While macronutrients constitute the bulk of the nutrient composition of plant tissue, micronutrients like boron, which compose a tiny fraction of the plant (Figure 2), can do just as much harm if they exceed the amount the plant needs.
Symptoms of nutrient toxicities
One of the first signs of nutrient toxicity is burned leaf tips. When excess nutrients accumulate, plants attempt to distribute them throughout their tissues. They can’t move these molecules any further than the tips, so this is where they accumulate first.
Other symptoms of nutrient toxicity resemble those of nutrient deficiencies. Stunted growth, off-colored leaves—often white, yellow or purple—and abnormally shaped leaves, stems and roots can all signal nutrient toxicities. Certain plant parts, including the root system, may even break down. Specific symptoms of nutrient toxicity vary by the nutrient involved (Figure 3).
Essential nutrient
Symptoms (Some of these symptoms can be detected only through tissue analysis.)
Nitrogen
Turns leaves dark green, curls tips down, induces “claw leaves” and causes stem stiffening.
Phosphorus
Prevents absorption of other nutrients, results in small internodes, burns leaf tips and causes interveinal chlorosis.
Potassium
Blocks the absorption of other nutrients and turns leaves—primarily older ones—pale.
Calcium
Prevents the absorption of other nutrients—mostly potassium, iron, manganese and magnesium—and wilts leaves. |
Sulfur
Delays growth, wilts and dries out leaf tips, and turns foliage dusky green.
Magnesium
Turns foliage stark green and stunts growth. |
Manganese
Forms brown spots on leaves, gives young leaves dark orange to brown mottling and causes leaves to fall off.
Boron
Develops necrotic spots on tips and edges of leaves, yellows leaf tips and causes necrosis of leaf margins.
Copper
Ages or decays the plant rapidly, enlarges the roots and causes severe leaf chlorosis.
Iron
Turns leaves bronze, inhibits the uptake of phosphorus and causes interveinal chlorosis that starts at the back of the leaf near the petiole.
Molybdenum
Blocks iron absorption, potentially resulting in signs of iron deficiency.
Zinc
Wilts older leaves and causes iron deficiency.
Figure 3. Summary of early symptoms caused by the toxicity of each nutrient.
Nutrient burn may appear gradually if it is only a slight overdose. But it can also progress rapidly over a few days. If the plant experiences crop-wide yellowing and wilting overnight, with leaf curl and a lack of turgidity, it is probably a chronic overdose. If this happens during the veg phase, start over because recovery may be impossible or take longer than restarting.
How to avoid nutrient toxicities
Knowing the visual cues of nutrient toxicities can help growers take swift action to address the problem and avoid the worst-case scenario of crop failure. But the best way to prevent nutrient toxicity is to avoid overfertilizing and keep the substrate pH in the ideal range. Maintaining a balanced nutrient program, conducting regular tissue analyses, and closely monitoring the substrate pH are all preventive measures that can help avoid both nutrient toxicities and deficiencies, ensuring healthier plants that can deliver maximum quality and yields.
Emerald Harvest Team
Craig
Thank u so much,Very helpful