Iron chlorosis and phosphorus lock-up in a calcareous lemon orchard
A 2.0 ha lemon orchard, cv. Interdonato, on the Kiti plain has 8-year-old trees and a target yield of 30 t/ha; last season it produced 18 t/ha. Soil analysis (0–30 cm) shows pH 8.3, free CaCO3 46%, organic matter 0.9%, Olsen P 6 mg/kg, exchangeable K 340 mg/kg, exchangeable Mg 60 mg/kg, CEC 18 cmol/kg and EC 1.2 dS/m. Young leaves show interveinal yellowing while older leaves remain green. The farmer plans to broadcast 600 kg/ha of triple superphosphate, spread 25 t/ha of cattle manure (0.6% total N), and incorporate 200 kg/ha of ferrous sulphate into the soil before winter. Irrigation is by drip from a borehole at 4,500 m3/ha per year, with water EC 2.5 dS/m and nitrate concentration 30 mg/L. The field lies inside a Nitrates Vulnerable Zone.
1. Critically assess the three planned amendments using soil chemistry. Why is each likely to fail or cause harm?
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Triple superphosphate, 600 kg/ha, broadcast. In a soil with 46% free lime and pH 8.3, broadcast phosphate is rapidly precipitated as poorly soluble calcium phosphates. Only a small fraction will be available in the year of application, while the unused phosphate adds to the soil phosphorus pool and to leaching and erosion risk. Olsen P at 6 mg/kg is genuinely low, so phosphate is needed, but the method is wrong: place it in a concentrated band near the emitters or apply it through the drip as MAP or phosphoric acid, where acidification and root concentration improve uptake.
Ferrous sulphate incorporated into soil. Ferrous sulphate oxidises quickly to ferric iron and precipitates as oxides and hydroxides at pH 8.3. Soil application will not correct the chlorosis. The correct products are Fe-EDDHA or Fe-EDDHMA chelates, which remain stable at high pH, applied in fertigation or as a soil drench; a foliar chelate or sulphate spray gives rapid but temporary greening. Acidifying the irrigation water to about pH 6.5–7.0 with nitric or phosphoric acid also improves iron and phosphate availability in the wetted bulb.
Cattle manure, 25 t/ha. Manure adds organic matter and some nutrients, but 25 t/ha is excessive here. At 0.6% N it supplies 150 kg N/ha, close to the 170 kg N/ha annual ceiling for manure in the Nitrates Directive, and the irrigation water already supplies about 135 kg N/ha (30 mg/L × 4,500 m3/ha). The total is far above crop demand and will drive nitrate leaching and salinity. Manure also adds salt and phosphate, which the soil does not need in that quantity.
The underlying problem is a high-pH, high-lime soil with induced iron deficiency and a poor root environment, not a simple shortage of broadcast fertiliser.
2. Design a fertigation and amendment programme for years 1–3, including product choice, rates, timing and monitoring.
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Year 1: correct the limiting factors. Stop the broadcast TSP and the soil ferrous sulphate. Apply iron as Fe-EDDHA/Fe-EDDHMA at label rate through the drip in two or three splits between March and June, plus two foliar sprays on the chlorotic flushes to give rapid greening. Acidify irrigation water to pH 6.5–7.0 using nitric or phosphoric acid, monitoring with a pH meter at the emitter. Apply phosphate as MAP or phosphoric acid through the drip, banded in the wetted zone, at a rate based on expected removal plus a moderate build-up; split it across spring and autumn. Reduce manure to 8–10 t/ha of well-composted material, incorporated in autumn, and account for its nitrogen. Use ammonium-based nitrogen rather than nitrate where possible, because ammonium uptake acidifies the rhizosphere.
Year 2: consolidate. Continue chelated iron but reduce the number of applications if leaf colour improves. Maintain water acidification. Repeat soil and leaf analysis. Adjust phosphate according to Olsen P and leaf P, and avoid building Olsen P above the agronomic optimum. Keep manure at 8–10 t/ha only if organic matter remains below 1.5%; otherwise use compost or a cover crop.
Year 3: monitor and refine. Test soil every second year and leaf tissue annually in July–August, sampling comparable leaves from treated and untreated rows. Record yield, fruit size, leaf colour score and chloride/sodium levels, because the water EC of 2.5 dS/m and the high manure rate both add salt. Watch for potassium and magnesium imbalance induced by excess phosphate or ammonium.
Monitoring indicators. Target a gradual rise in Olsen P into the adequate range, greener young leaves, improved shoot growth, yield moving towards 25–30 t/ha, and soil EC stable or falling. If chloride accumulates, increase the leaching fraction and reduce manure further.
3. Calculate the nitrogen contribution of the irrigation water and the manure, state whether the plan complies with the 170 kg N/ha manure limit, and recommend adjustments.
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Irrigation water nitrogen. 30 mg/L is 30 g/m3. At 4,500 m3/ha, the annual input is 30 × 4,500 = 135,000 g N/ha, i.e. 135 kg N/ha. This is a large, often overlooked contribution and must be counted when calculating crop nitrogen supply.
Manure nitrogen. 25 t/ha at 0.6% total N = 25,000 kg × 0.006 = 150 kg N/ha. That is below the 170 kg N/ha annual ceiling for livestock manure in the Nitrates Directive, so the manure application on its own is formally within the limit. The problem is that the limit is a ceiling, not a target, and the total nitrogen reaching the crop is 150 + 135 = 285 kg N/ha, far above the uptake of a lemon orchard producing 18–30 t/ha. The surplus will leach as nitrate into the vulnerable zone or accumulate as salinity.
Recommended adjustment. Reduce manure to about 10 t/ha, which supplies roughly 60 kg N/ha, and credit both the manure nitrogen and the 135 kg N/ha from water against the crop requirement. In practice the water already covers most or all of the nitrogen need, so fertiliser nitrogen should be minimal in year one and adjusted using leaf analysis and fruit yield. Do not apply manure in autumn if heavy rain is expected; incorporate it or compost it. Keep written records of manure quantities, nitrogen content and application dates, which are required in a Nitrates Vulnerable Zone.