Tomato Greenhouse: Yellow Leaves and a Fertilizer Decision
Mr. Ali manages 600 tomato plants in a plastic greenhouse in Trinidad. Over three weeks, the lower leaves have turned yellow between the veins, the upper leaves are pale green, and flowering has dropped by about 40%. Two weeks ago he applied 50 kg of ammonium nitrate to the 0.1-hectare bed. A soil test now shows pH 5.1, low exchangeable magnesium, high nitrate, and adequate phosphorus. At midday the greenhouse reaches 36 °C with 55% relative humidity, and the ventilation fans run only from 09:00 to 15:00. Mr. Ali plans to double the ammonium nitrate next week because he believes yellow leaves always mean nitrogen deficiency. He asks whether this will restore the crop before the next harvest in 21 days. The cost of another 50 kg bag would use most of his remaining input budget.
1. Which factor is most likely limiting photosynthesis, and why would doubling ammonium nitrate probably not solve the problem? Use the leaf symptoms and soil data.
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The key clue is the pattern of yellowing: lower leaves are yellow between the veins, while the veins remain greener. This is classic interveinal chlorosis, which points to magnesium deficiency rather than nitrogen deficiency. Magnesium is the central atom of chlorophyll, so a shortage reduces light absorption and therefore the light-dependent reactions of photosynthesis.
The soil data support this: exchangeable magnesium is low, while nitrate is already high. At pH 5.1, magnesium is less available to roots, and a high concentration of ammonium ions can compete with magnesium uptake. Adding more ammonium nitrate would therefore intensify the imbalance, not correct it. It could also raise soil salinity and further depress water uptake.
The 36 °C midday temperature and limited ventilation add a second constraint. If stomata close to reduce water loss, carbon dioxide entry falls and photosynthesis slows even if chlorophyll were sufficient. So the crop is limited mainly by magnesium supply and heat-related stomatal closure, not by nitrogen. Doubling the fertilizer would waste money and may worsen the deficiency.
2. Recommend two low-cost changes Mr. Ali could make in the next 21 days and predict their likely effect on photosynthesis and yield. Justify each recommendation.
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Recommendation 1: Apply a foliar magnesium spray, such as magnesium sulphate, at the label rate. Foliar feeding bypasses the low-pH soil and the competition from nitrate, delivering magnesium directly to leaves. Chlorophyll synthesis can begin within days in deficient tissue, so light absorption and photosynthetic rate should improve. The effect on final yield is limited because flowering has already dropped, but it may reduce further leaf loss and improve the size of fruit that is already set.
Recommendation 2: Improve ventilation and reduce midday heat load rather than adding nitrogen. Running fans through the hottest period, opening vents wider, or using a temporary shade cloth from about 11:00 to 15:00 would lower leaf temperature. If stomata remain open longer, carbon dioxide uptake continues and photosynthesis is less restricted. The cost is low, but shade must not be so heavy that it reduces light too much.
A lime application could raise pH over months, but it will not act within 21 days and could confuse the immediate comparison. The judgement is to treat the magnesium deficiency directly and manage heat, while stopping the planned ammonium nitrate increase.