Visual Clues: Observing Symptom Location
Identifying nutrient deficiencies in corn starts with visual observation, paying close attention to which leaves are affected first. Essential nutrients are categorized by their mobility within the plant, which dictates where symptoms first appear. Mobile nutrients, such as nitrogen (N), phosphorus (P), and potassium (K), can be moved from older leaves to newer growth when supplies are limited, causing symptoms to manifest on the lower, older leaves. Conversely, immobile nutrients, including zinc (Zn), sulfur (S), and iron (Fe), cannot be easily relocated, so their deficiencies appear on the upper, newer leaves first.
Symptoms on Older (Lower) Leaves
- Nitrogen (N) Deficiency: This is one of the most common deficiencies and is visually striking. Symptoms start on the oldest leaves, beginning as a pale green or yellowish coloration that starts at the leaf tip. This chlorosis then progresses down the midrib of the leaf in a distinct V-shaped pattern, a phenomenon known as "firing". The entire plant may appear light green if the deficiency is severe.
- Phosphorus (P) Deficiency: Early-season P deficiency often causes a purple or reddish-purple discoloration on the margins of the lower leaves, especially when young plants are exposed to cool temperatures. Plants are often stunted and have poor root development. This purpling is caused by the accumulation of anthocyanin pigments and may be a temporary condition that disappears as soil temperatures warm.
- Potassium (K) Deficiency: A telltale sign of K deficiency is the scorching or yellowing of the leaf margins and tips, which eventually turns brown and necrotic. This occurs primarily on older leaves. Severe cases can lead to weak stalks, increased lodging, and poor grain fill.
- Magnesium (Mg) Deficiency: This deficiency presents as interveinal chlorosis on the lower leaves, where yellow or white streaking appears between the veins, while the veins themselves remain green. The symptoms can sometimes be described as a "beaded" streaking. In severe cases, the leaf margins and tips may become necrotic or turn reddish-purple.
Symptoms on Younger (Upper) Leaves
- Zinc (Zn) Deficiency: Zn deficiency appears as interveinal chlorosis on the upper leaves, forming broad white to pale-yellow stripes or bands that run parallel to the midrib. These bands are most pronounced on the lower part of the leaf near the stalk. Plants are often stunted with shortened internodes. Severe cases can result in entirely white or pale-yellow new leaves, an effect sometimes called "white bud".
- Sulfur (S) Deficiency: Unlike the bottom-up yellowing of N deficiency, S deficiency causes a general, uniform pale green to yellowish coloring of the youngest leaves. In some cases, yellow striping can appear on the upper leaves, sometimes confused with an early-stage nitrogen issue, but the location on the plant differentiates the two.
- Iron (Fe) Deficiency: Iron deficiency, while less common in corn, causes distinct interveinal chlorosis on the youngest leaves, with the veins remaining a dark green. The entire leaf can become pale yellow or nearly white in severe cases.
- Manganese (Mn) Deficiency: Similar to iron, Mn deficiency causes interveinal chlorosis on new leaves, with yellowing between the veins and the veins remaining green. The tissue between the veins may develop pale yellow or white streak lesions, which can appear mottled.
Comparison of Key Corn Nutrient Deficiencies
| Nutrient | Leaf Location | Key Symptom | Additional Notes |
|---|---|---|---|
| Nitrogen (N) | Older/Lower | V-shaped yellowing (firing) along the midrib from the tip | Overall pale green appearance; stunted growth |
| Phosphorus (P) | Older/Lower | Purple or reddish-purple margins and tips | Most common in cool soils and young plants; stunted growth |
| Potassium (K) | Older/Lower | Yellowing and scorching/browning of leaf edges and tips | Weak stalks, lodging, poor grain fill |
| Magnesium (Mg) | Older/Lower | Interveinal yellow or white streaking between green veins | Can be confused with Mn deficiency, but appears on older leaves |
| Zinc (Zn) | Younger/Upper | Broad white to yellow stripes between the leaf veins | Stunted plants with shortened internodes ("rosetting") |
| Sulfur (S) | Younger/Upper | Uniform, general pale green or yellowing of the entire new leaf | Stunted growth; can look like N deficiency, but location is key |
Beyond Visuals: Confirming the Cause
Visual diagnosis, while helpful, can sometimes be misleading due to other stressors like drought, disease, or herbicide injury. Therefore, it is crucial to confirm a suspected deficiency with diagnostic testing. A combination of soil and tissue testing provides the most accurate and reliable information.
- Soil Testing: A soil test provides information on the nutrient levels available in your field and the soil's pH. Soil pH is particularly important as it can significantly impact nutrient availability. For instance, high pH (alkaline) soils can reduce the availability of zinc, while acidic soils can cause magnesium to leach out.
- Plant Tissue Testing: This involves sending a sample of corn plant tissue to a lab for analysis. This test confirms whether the plant is successfully absorbing the nutrients it needs. It's often recommended to collect samples from both healthy and symptomatic plants in the field for comparison.
Corrective Actions and Integrated Management
Once a deficiency is confirmed, a tailored approach to corrective action is needed. Depending on the nutrient and severity, options range from immediate foliar applications to longer-term soil amendments.
- For mobile nutrients like N and K, an in-season application may correct the issue, but yield might be reduced if the deficiency was prolonged.
- For immobile nutrients like Zn and Mn, soil-applied fertilizers are most effective when applied at or before planting. Foliar applications can provide a short-term fix but are often less effective at significantly increasing yield.
- Proper soil management, including maintaining an optimal soil pH (around 6.8 for corn), is foundational to ensuring nutrient availability and preventing deficiencies.
By carefully observing your crop, performing timely testing, and implementing targeted management strategies, you can effectively address nutrient deficiencies and protect your corn yield potential. For deeper agronomic insights, see this detailed resource from SDSU Extension.
Conclusion
Diagnosing a corn nutrient deficiency requires a sharp eye and a systematic approach. By first identifying whether symptoms appear on older or younger leaves, you can narrow down the potential culprits to mobile or immobile nutrients, respectively. Following visual diagnosis with reliable soil and tissue testing is the most effective way to confirm the issue and develop an appropriate management plan. Proactive monitoring and targeted intervention, informed by scientific analysis, are essential for maintaining crop health and maximizing yield potential.
