Two chlorosis patterns in one tomato field on calcareous soil

Patch A - magnesium deficiency. Magnesium is the central atom of chlorophyll and is mobile in the phloem. When supply is short, the plant withdraws magnesium from older leaves and redirects it to growing points, so symptoms appear first and most severely on older, lower leaves. Magnesium is also needed for phloem loading of sucrose, so starch accumulates and interveinal tissue becomes chlorotic and then necrotic, while veins stay green. The exchangeable magnesium of 0.6 cmol(p+)/kg is low, and the soil is not being given any magnesium source; DAP and muriate of potash supply none.

Patch B - iron deficiency. Iron is needed for chlorophyll synthesis but is immobile in the plant, because it is bound in forms that cannot be retranslocated. New leaves therefore cannot be supplied from older tissue, and the youngest leaves become chlorotic with veins remaining sharply green; growth is stunted because iron is also needed for electron transport and for the enzymes of chlorophyll biosynthesis.

Why both occur together. The two deficiencies have different causes - an absolute shortage of exchangeable magnesium, and iron that is present in the soil but unavailable because of high pH and calcium carbonate. Patches reflect local variation in calcium carbonate content, drainage and compaction. Adding urea addresses neither: nitrogen is not the limiting factor, and ammonium nitrogen can even aggravate magnesium shortage by competing for uptake.

Chemistry of the soil. At pH 8.3 with 12% free calcium carbonate, iron is present almost entirely as insoluble ferric oxides and hydroxides; the concentration of Fe3+ in soil solution falls by roughly a thousandfold for each unit rise in pH in this range. Plant roots can mobilise iron by extruding protons and reducing Fe3+ to Fe2+ at the root surface, but the bicarbonate in the irrigation water buffers the rhizosphere and neutralises that acidification, so the strategy fails. Bicarbonate taken up by the plant also moves into leaves, where it raises apoplastic pH and inactivates iron, so even iron that reaches the leaf may not be used - the reason symptoms appear on the youngest leaves first.

Why the mixture fails.

• A general mixture supplies small amounts of several elements, so the dose of any one is below the amount needed to correct a real deficiency.
• In a tank mix, phosphate, copper and zinc can precipitate iron, and ferrous sulphate oxidises rapidly in bicarbonate-rich water.
• Spraying a mixture obscures the diagnosis: if the response is partial, the farmer cannot tell which element helped.
• Soil-applied iron in a calcareous soil is rapidly fixed; the material must be chelated with EDDHA (stable at high pH) or applied as a foliar spray.

The correct approach is to correct the specific deficiencies identified, with a separate and correctly acidified iron spray.

For the standing crop (fast correction).

• Spray 0.5% ferrous sulphate with a little citric acid (to keep iron reduced and available) on the young foliage, two or three times at 10-14 day intervals, in the evening, with a surfactant; wet the underside of leaves.
• Spray 1% magnesium sulphate on the older foliage of patch A, repeated after a week, since foliar magnesium is absorbed and redistributed to some extent.
• Stop adding urea to the affected patches; irrigate less heavily and avoid waterlogging, which worsens both problems.

Why foliar feeding is faster. A foliar spray bypasses the soil entirely: it avoids precipitation of iron at pH 8.3, avoids bicarbonate buffering at the root surface, and delivers the element directly to the tissue that cannot be supplied from older leaves. Uptake through the leaf cuticle and stomata begins within hours to days, whereas soil-applied iron must dissolve, move to the root, be reduced and taken up - steps that are all blocked in a calcareous soil.

For the next season (long-term).

• Use a soil-applied iron chelate such as Fe-EDDHA, banded near the root zone rather than broadcast, because it stays soluble at high pH.
• Apply magnesium sulphate to the soil with the basal dose, and increase organic matter (compost, green manure) to improve structure and nutrient-holding capacity.
• Use acidifying amendments such as elemental sulphur, oxidised by soil microbes to sulphuric acid, and prefer ammonium sulphate or ammonium nitrate over urea for part of the nitrogen.
• Obtain a properly stratified soil sample from each patch and retest DTPA-iron and exchangeable magnesium; use tissue analysis (youngest fully expanded leaf for iron, and a recently matured leaf for magnesium) to confirm the diagnosis before investing in amendments.