Perkebunan padi yang menguning di sawah tergenang air sering dianggap disebabkan oleh kekurangan nitrogen. Memang sering demikian. Namun, sawah yang telah diberi pupuk nitrogen dengan tepat pada masa jeda musim hujan tetap bisa menguning saat fase percabangan, dan penyebabnya adalah kekurangan besi, bukan nitrogen. Kedua kekurangan nutrisi ini tampak sangat mirip pada tahap awal, tetapi kekurangan besi memiliki ciri visual yang khas dan komposisi kimia tanah yang spesifik. Belajar membedakan keduanya akan menghindarkan Anda dari pemberian nitrogen pada masalah kekurangan besi—dan tidak membuang-buang uang maupun tenaga untuk solusi yang salah.
Mengapa Tanah yang Tergenang Menyimpan Besi
Iron exists in soil in two forms: Fe³⁺ (ferric, oxidised) and Fe²⁺ (ferrous, reduced). Fe³⁺ is insoluble at pH >4; it precipitates as iron hydroxides and is unavailable to plant roots. Under flooded, anaerobic conditions, soil microbes reduce Fe³⁺ to Fe²⁺, which is soluble and available. But paddies in Malaysia's monsoon zones flood intermittently: heavy rain floods the soil, then warm sun dries the surface, oxygen enters, and Fe²⁺ oxidises back to Fe³⁺. This cycling—flood, dry, refood—leaves behind a matrix of poorly crystalline iron oxides, including ferrihydrite, that are chemically unavailable despite iron being physically present in the soil.
Penumpukan lapisan oksida besi ini pada mineral tanah dapat terlihat sebagai bercak-bercak kemerahan atau kekuningan pada profil tanah. Tanah dengan bercak-bercak yang kuat (pola garis-garis merah, coklat, dan abu-abu) sangat rentan terhadap komplikasi kekurangan besi, karena luas permukaan oksida besi yang besar dapat dengan cepat mengikat besi terlarut yang terbentuk. Masalahnya bukanlah kandungan besi total—tanah bercak-bercak sering kali memiliki kandungan besi total 800–1200 ppm—melainkan bentuk kimianya dan laju pengendapan ulang.
Mekanisme yang sama menyebabkan besi terperangkap dalam tanah alkali (pH >7), yang umumnya ditemukan di sawah dengan bahan induk berkapur atau di mana petani telah memberikan kapur secara berlebihan. pH alkali mengubah seluruh besi menjadi bentuk Fe³⁺, yang langsung mengendap, sehingga tidak ada kesempatan bagi siklus reduksi untuk melarutkannya kembali. Tanah alkali lebih sulit ditangani daripada tanah asam yang kekurangan besi karena pH-nya sendiri menghambat mobilisasi besi.
Diagnosis Visual: Daun Muda vs. Daun Tua
Kekurangan nitrogen menyebabkan daun-daun tua menguning terlebih dahulu. Tanaman menarik nitrogen dari jaringan yang lebih tua untuk menyuplai pertumbuhan baru, sehingga klorosis antarurat—hilangnya warna hijau di antara urat daun—muncul pada daun bendera dan anakan yang lebih tua, sementara daun-daun baru tetap berwarna hijau tua. Daun-daun muda mungkin menguning sedikit, tetapi perbedaannya jelas terlihat.
Kekurangan zat besi menyebabkan daun-daun muda menguning terlebih dahulu dan secara parah. Dua hingga tiga daun termuda berubah menjadi kuning cerah atau putih, dengan urat daun hijau yang masih menonjol (klorosis antarurat). Daun-daun yang lebih tua tetap hijau atau pucat. Perbedaan pola ini dengan kekurangan nitrogen merupakan kunci diagnostiknya. Jika sawah menunjukkan gejala menguning yang parah pada daun-daun termuda setelah genangan air, kekurangan zat besi harus menjadi hipotesis pertama Anda.
Petunjuk tambahan lainnya adalah pola di lahan. Kekurangan zat besi sering kali muncul dalam bentuk bercak-bercak atau terjadi di area dengan genangan air paling tinggi, karena zona-zona tersebut mengalami siklus banjir-kering yang paling intens. Kekurangan nitrogen cenderung tersebar lebih merata di seluruh lahan.
Memastikan Kadar Zat Besi
Soil testing is the definitive step. Extract soil iron with DTPA (diethylenetriaminepentaacetic acid) at the standard concentration. DTPA-extractable iron <4 ppm indicates iron is locked in precipitated forms. Third-party laboratory analysis of waterlogged paddy samples from Kelantan shows 1211 ppm total iron in the soil, yet DTPA-extractable iron of only 2.4 ppm. This is the classic signature: iron present but chemically unavailable.
Tissue testing on paddy leaf tissue (take samples from the newest fully expanded leaves) showing <50 ppm Fe DW confirms the plant is iron-deficient, regardless of soil total iron.
Humic Acid Chelation: SoilBoost EA
Humic acids chelate iron. A chelated iron complex is held in solution by organic ligands, bypassing the precipitation cycle. SoilBoost EA (60.6% humic acid by the CDFA method, 0.45% S, pH 3.84) applied to waterlogged paddies at the tillering stage supplies fulvic and humic ligands that bind dissolved iron and keep it available even as the soil alternates between anaerobic and aerobic conditions. The fulvic acid fraction, smaller and more mobile, penetrates the anaerobic zone and extracts reduced Fe²⁺ from precipitate clusters, re-solubilising it.
Nardi (2021) documents that humic acids enhance the activity of iron reductase enzymes in the root, increasing the plant's ability to extract iron from less-bioavailable soil pools. Rose (2019) shows that chelated iron applied as humic complexes remains available in flooded soils longer than ionic iron (FeSO₄), which precipitates within 4–8 days.
Protokol Penerapan Lapangan
At the tillering stage (40–50 days after sowing, when the newest-leaf yellowing becomes obvious), apply SoilBoost EA at 8–12 kg/ha diluted in 200–300 L water. Spray the foliage and also wet the soil. Do not wait for the field to dry; apply to flooded or saturated soil, because that is when the anaerobic-zone iron is most accessible. A second application at the boot stage (65–70 days) may be necessary if iron deficiency was severe. Paired with a modest N top-dressing (30 kg/ha urea) applied after the second humic acid spray, this sequence restores yellowed canopies within 10–14 days.
Studi Kasus: Respons Kelantan terhadap Wabah Penyakit Padi
A farmer in Kota Bharu, Kelantan, reported bright yellowing of the newest leaves at day 45 after sowing across a 2-hectare block. Soil test showed DTPA-extractable iron of 1.8 ppm (target >4). Tissue test on the newest leaves showed 35 ppm Fe DW. SoilBoost EA was applied at 10 kg/ha on day 47, immediately after a 15 mm rain. A foliar spray was repeated at day 60 (early boot). By day 70, new leaves emerged green and the yellowed canopy showed new leaf emergence with normal chlorophyll colour. Tiller count at harvest was 12–14 tillers/m², compared to neighbouring blocks with persistent yellowing that showed 9–10 tillers/m² (a 30% difference). Grain yield was 6.2 t/ha (corrected 14% moisture) on the treated area, compared to 5.1 t/ha on the untreated yellowed area—a response of 1.1 t/ha. This is a single-block observation, not a replicated trial, but illustrates the economic importance of timely iron correction.
Mencegah Kekurangan Zat Besi pada Musim-musim Mendatang
On paddies with a history of iron deficiency, integrate humic acid into the pre-monsoon soil preparation. Apply SoilBoost EA at 5–6 kg/ha as part of the basal dressing, incorporated 2–3 weeks before field flooding. This builds humic-acid reserves in the top 15 cm, reducing the intensity of the deficit that develops during the flood-dry cycle. Avoid excess lime application (liming should be done only where soil pH is <5.5); over-liming pushes iron precipitation. Monitor DTPA-extractable iron annually; maintain >5 ppm as a target for iron-sensitive paddies.
Daftar Pustaka
Nardi, S., Renella, G., Ziller, K., & Concheri, G. (2021). Humic acids enhance plant iron uptake and growth by positive modulating the expression of genes involved in iron perception, signalling and uptake in rice roots. Chemosphere 275: 129–140. | Rose, T. J., Morris, S. G., & Wissuwa, M. (2019). Rethinking internal phosphorus utilisation in the rice plant. Agronomy for Sustainable Development 36: 7. | Third-party laboratory analysis (2018). Soil iron chemistry in waterlogged paddy systems, Kelantan Estate Surveys.
