Yellow young leaves and black blossom ends in a lime-rich tomato field
A tomato grower on sandy loam irrigated with bicarbonate-rich tube-well water applies a 19:19:19 NPK fertiliser every week. Soil pH is 7.9 and organic carbon 0.3%. A DTPA extract gives iron 1.2 mg/kg (deficient below about 4.5), with adequate nitrogen, phosphorus, potassium, calcium (800 mg/kg) and magnesium. On the upper third of the plants, the youngest leaves are yellow between the veins while the veins stay green, and leaf margins are not scorched; the oldest leaves remain dark green. Fruits on the third truss show a sunken, leathery black patch at the blossom end while the rest of the fruit looks normal. The grower's remedy is to spray more NPK and to add gypsum, believing that 'the yellowing is a nitrogen shortage and the black patch is a calcium shortage in the soil'.
1. Diagnose the yellowing using the principle of phloem mobility of nutrients. Explain why the pattern rules out nitrogen, and why extra NPK will make it worse rather than better.
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The position of the symptom is the diagnostic clue. Nitrogen, potassium and magnesium are phloem-mobile: when supply is short, the plant exports them from older leaves to growing points, so deficiency appears first in the oldest leaves as general pale colouring, with magnesium giving interveinal chlorosis there. Iron, manganese, zinc and calcium are phloem-immobile, so a shortage shows first in the youngest leaves, because the plant cannot redistribute them from older tissue. Here the youngest leaves are interveinal yellow with green veins, the oldest are dark green, and the DTPA iron of 1.2 mg/kg is below the 4.5 mg/kg threshold, with pH 7.9 and bicarbonate-rich water - the classic lime-induced iron chlorosis picture. Nitrogen shortage would have paled the old leaves and stunted the whole plant.
More 19:19:19 makes things worse: the extra nitrogen drives new growth that dilutes the little iron available, phosphate can precipitate iron in the root zone and in the plant, and the treatment does nothing to the high pH that keeps iron insoluble as oxides and hydroxides.
2. The soil calcium is 800 mg/kg, yet fruits show blossom-end rot. Explain how this is possible, and evaluate whether gypsum will correct it.
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Calcium moves almost entirely in the xylem, driven by transpiration, and it is phloem-immobile, so it cannot be redirected to tissues that transpire little. Tomato fruits transpire very little compared with leaves, so their calcium supply depends on a steady, moderate xylem flow into the fruit. Any factor that reduces that flow - fluctuating soil moisture between drought and heavy irrigation, high salinity, high evaporative demand, root damage, or excess potassium, magnesium or ammonium competing for uptake - produces a localised calcium deficiency at the blossom end even when soil calcium is plentiful. The result is the collapsed, leathery black patch, and it is a distribution problem, not a soil stock problem.
Gypsum adds calcium and slightly lowers pH, but it does not fix delivery to the fruit, and it raises soil salinity, which can make the flow problem worse. Nor does foliar calcium reliably reach the fruit. The effective measures are steady drip irrigation with mulching, avoiding excess potassium and magnesium, and moderating nitrogen so that vegetative growth does not outrun calcium supply to the trusses.
3. Give a corrective plan for both problems, ranked, with the reasoning behind each choice, and state how you would judge whether it has worked.
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First, treat the iron problem at its chemical cause. Ferrous sulphate applied to soil at pH 7.9 oxidises to insoluble ferric hydroxide and is wasted; the appropriate material is an EDDHA-chelated iron, which stays stable up to about pH 9, applied by fertigation at a modest rate. Alongside it, acidify the irrigation water to roughly pH 5.5-6.0 with nitric or phosphoric acid to counteract the bicarbonates, and add compost or manure to improve buffering and micronutrient availability. A foliar spray of ferrous sulphate with a surfactant gives quick greening of existing young leaves while the root-zone correction takes effect, but it is a stop-gap.
Second, address the calcium delivery problem: schedule irrigation so moisture is steady, mulch to reduce evaporation swings, and stop excessive potassium and magnesium applications. Do not add more gypsum.
Judge success on new growth, not old: leaves that are already chlorotic will not green fully, so watch the next two trusses for symptom-free young leaves, confirm with a tissue test (iron in the tens of mg/kg), and record the proportion of fruit with blossom-end rot at the following harvest.