When growers think their plants might have a nutrient deficiency, there are several ways to approach the problem — first by taking an account of the symptoms and the growing environment and then possibly testing to confirm the cause of the symptoms. Here’s advice from Cornell’s Neil Mattson on ways to properly diagnose a nutrient deficiency.
The location on plants where symptoms occur can be evidence of what the deficiency is. Broadly, symptom location can be grouped into two areas: old growth and new growth.
1. Symptoms On New Growth
Iron deficiency. Uniform yellowing between veins on upper leaves or new growth.
Manganese deficiency. Yellowing between veins on upper leaves or new growth, which may look similar to iron deficiency. As symptoms advance, scattered necrotic spots may form between the veins.
Boron deficiency. The very newest growth (the shoot meristem) can become aborted and disfigured, causing branching below that growing point and further distorted growth and abortion of lateral meristems.
Calcium deficiency. Youngest leaves will have marginal (leaf edge) chlorosis and necrosis and can be distorted or strap-like.
2. Symptoms On Old Growth
Nitrogen deficiency. Uniform yellowing of lower leaves. The whole leaf surface is affected, not just leaf edges.
Phosphorus deficiency. Purpling on lower leaves or dramatically smaller and greener lower leaves.
Potassium deficiency. Chlorotic and necrotic leaf edges of lower leaves. As symptoms advance, scattered necrotic spots may form across the leaf but especially near margins.
Magnesium deficiency. Yellowing between veins, often begins near the leaf margins.
3. Symptoms On Plant Roots
Discolored roots. Brown or black roots or roots peeling away from the rest of the root system can indicate disease, waterlogged soil and roots that have died and sloughed off. That can be the primary cause for certain nutrient deficiencies to develop, such as iron, boron or calcium deficiencies.
Boron deficiency. Short and stubby roots, but thicker than normal.
Calcium deficiency. Roots are shorter and thicker, but noticeably black. Black root rot (thielaviopsis) can have lookalike symptoms.
Phosphorus deficiency. Fewer roots than a healthy root system, but longer roots overall.
4. Observe The Growing Environment
Cold temperatures. Temperatures below 55˚F can make it difficult for roots to absorb phosphorus and can lead to phosphorus deficiency. In this case, warmer temperatures are required to make phosphorus available to the plant to correct the deficiency.
Cold, cloudy, humid and waterlogged conditions. Calcium and boron need actively transpiring plants to be drawn up by the roots. Under cold, cloudy, humid or overcast conditions with poor airflow, or when roots are waterlogged, you might see a calcium or boron deficiency.
Phytotoxicity. Look for uniformity of symptoms. Phytotoxicity can occur where pesticide may have sat on the leaf surface for too long, leading to tissue damage. With phototoxicity, you tend to see a pattern of symptoms, all leaves of a certain age showing leaf yellowing or spotting across the whole crop, for example. With a nutrient deficiency, you might see more variable symptoms on plants that were sitting wet the longest or dried out too fast.
5. Diagnostic Tools To Use
pH and EC testing. Check the substrate pH and EC. High pH can be a symptom of micronutrients becoming immobile, namely iron or manganese. Low pH can mean problems with calcium or magnesium availability, and can also make phosphorus very soluble, which can lead it to leach quickly. Low EC can indicate not enough fertilizer is being applied. High EC indicates excessive fertility or high salts in the irrigation water.
Tissue sample testing at a commercial lab. Some nutrient disorders look similar (e.g., iron and manganese deficiency). Labs can help you pinpoint the exact cause of the symptoms of nutrient deficiency. Commercial labs have recommendations on how to send samples, but they’re usually looking for a handful of affected leaf samples and a handful of leaves of a similar age that are unaffected as a comparison.
Substrate sample testing at a commercial lab. Substrate sample testing allows you to be more proactive in diagnosing nutrient disorders and to catch potential nutrient deficiencies, even before they show up in the plant. It can tell you about the integration of fertilizer practices with watering practices that the plant has experienced. This can help you diagnose whether the cause of a nutrient deficiency is from lack of supply of that nutrient or another factor such as improper pH, which can reduce nutrient solubility.