Tomato Chlorosis in a High Tunnel
In a 30- by 96-ft high tunnel, tomato plants are grown in raised beds with drip irrigation. The soil test shows pH 7.8, Mehlich 3 phosphorus 95 ppm, potassium 210 ppm, calcium 3,200 ppm, magnesium 320 ppm, and DTPA iron 4.0 ppm. Organic matter is 2.8%. Irrigation water has 180 ppm bicarbonate and a pH of 8.1. The second tomato crop is showing interveinal chlorosis on the newest leaves, poor fruit set, and roots that appear healthy but have brown staining. The grower has applied 10-10-10 fertilizer and foliar iron sulfate twice, with no lasting improvement. Night temperatures have dropped to 50 degrees F, and the tunnel is not vented well during the day. He plans to add more phosphorus and lime.
1. What is the most likely diagnosis? Explain how the soil, water, and plant symptoms support it.
Read the worked answer
The pattern points to iron deficiency induced by high pH and bicarbonate rather than a simple soil iron shortage. Interveinal chlorosis on the newest leaves is characteristic of iron, which is not readily moved from old to new tissue. The soil pH is 7.8, calcium is high, and irrigation water contains 180 ppm bicarbonate at pH 8.1; these conditions reduce iron availability even when DTPA iron is present. The very high Mehlich 3 phosphorus can also interfere with iron uptake and transport. Foliar iron sulfate gave no lasting response because iron is poorly translocated and the underlying pH and bicarbonate problem remained. The cool nights and poor ventilation add stress. Adding lime would raise pH further, and more phosphorus is not needed at 95 ppm.
2. Why did the foliar iron sulfate applications fail, and why would adding lime and phosphorus be counterproductive?
Read the worked answer
Iron sulfate is not chelated, so it can be oxidized and inactivated on the leaf surface, and any iron that enters the leaf is largely immobile. It may green the treated leaves temporarily but does not correct new growth or the root-zone availability problem. The high-pH, high-bicarbonate irrigation water continues to tie up iron in the root zone. Lime would raise pH toward or above 8, worsening iron, manganese, and zinc availability. Additional phosphorus is unnecessary because the soil already tests very high, and high phosphorus can antagonize iron uptake. The grower should stop liming and adding phosphorus and instead address water alkalinity, root-zone pH, and iron delivery.
3. Recommend a corrective plan for this crop and the next planting.
Read the worked answer
First, stop lime and phosphorus applications. Test the irrigation water and acidify it to about pH 5.5 to 6.0 with an appropriate acid, monitoring safely, to neutralize bicarbonate. Use a chelated iron product, preferably Fe-EDDHA, through fertigation or as a soil application, and consider a chelated foliar product only as a supplement. Improve ventilation and avoid cold stress during fruit set. For the next crop, reduce soil pH slowly with elemental sulfur if the soil test supports it, maintain phosphorus at a moderate rather than excessive level, and use a fertilizer program based on soil and water tests. Choose a tomato cultivar with better iron efficiency if available. Recheck pH, bicarbonate, and tissue iron after the changes rather than adding more fertilizer blindly.