Micronutrients though needed in small amounts—are critical for the healthy growth and high productivity of coconut palms. Elements like iron, manganese, zinc, copper, molybdenum, and boron play essential roles in photosynthesis, enzyme activation, flowering, and nut development. Deficiencies in these nutrients lead to poor growth, leaf deformities, reduced flowering, and nut production loss. Proper identification and correction of these deficiencies through targeted soil and foliar applications are crucial to sustaining coconut yield and preventing long-term damage. An integrated nutrient management approach ensures that palms receive a balanced diet, enhancing resilience, productivity, and overall plantation health.
(i) Iron
Iron is crucial for nucleic acid metabolism and chlorophyll synthesis in plants. Although interveinal chlorosis is a common sign of iron deficiency, it is rarely observed in coconut palms. Iron deficiency typically occurs in palms grown in poorly aerated soils or when planted too deeply, where waterlogged conditions or deep planting suffocate the roots, reducing their ability to absorb nutrients like iron. The primary symptom is chlorosis, or yellowing, between the veins of new leaves. As the deficiency worsens, leaf tips become necrotic, and leaf size diminishes. To address iron deficiency, applying ferrous sulphate at 200 g per palm annually, or 0.25 to 0.5 kg per tree per year, is recommended.
(ii) Manganese
Manganese is a crucial component of the water-splitting enzyme associated with photosystem II and plays a role in oxidative and non-oxidative decarboxylation reactions in palms. Manganese deficiency first appears in the newest leaves, which emerge chlorotic with longitudinal necrotic streaks. As the deficiency worsens, newly emerging leaflets become necrotic and withered, except for the basal portions, causing the leaflets to curl around the rachis and giving the leaf a frizzled appearance, known as "frizzle top." In severe cases, growth halts entirely, and new leaves consist only of necrotic petiole stubs. Manganese deficiency is particularly common in alkaline soils. To address this, applying MnSO₄ at 200 g per palm per year is recommended, with a broader soil application rate of 25 kg/ha.
(iii) Zinc
Zinc is a vital component of metallo-enzymes and plays a crucial role in RNA and protein synthesis in plants. Zinc deficiency in coconut palms is characterized by a significant reduction in leaf size, sometimes by 50% or more. The leaflets become chlorotic, narrow, and shorter, giving the leaves a rosetted appearance. Zinc deficiency is a primary cause of pencil point disorder in coconut, where flowering is delayed and button shedding occurs. To address zinc deficiency, it is recommended to apply zinc sulphate at 200 g per palm annually, with a broader soil application rate of 25 kg/ha.
(iv) Copper
Copper is vital for the synthesis of enzymes involved in photosynthesis and plays a key role in disease resistance and the fertility of male flowers in plants. Copper deficiency is often observed in waterlogged conditions and is characterized by coppery bluish leaves, rolling of terminal leaves due to loss of turgor, and a bleached grey appearance of the leaves. In severe cases, palms may fail to produce flowers. To address copper deficiency, applying copper sulphate at 200 g per palm per year or a soil application of 25 kg per hectare is recommended.
(v) Molybdenum
Molybdenum plays a crucial role in the functioning of two important enzymes, nitrate reductase and nitrogenase, which are vital for proper nitrogen assimilation in plants. It is essential for nitrogen fixation in soils and influences the health of pollen grains and the development of anthers. To meet the molybdenum needs of palm trees, applying ammonium molybdate or sodium molybdate at a rate of 10 grams per palm annually is recommended. Symptoms of molybdenum deficiency include chlorotic (yellowing) leaf blades, small and slender leaves, rosette-like growth patterns, and whip tail. To manage these issues, root feeding with a 0.05% sodium molybdate solution (0.5 grams per liter of water) can be effective.
(vi) Boron
Boron is essential for pollen germination and the growth of pollen tubes, and it aids in potassium transport in guard cells, facilitating stomatal opening. Boron deficiency symptoms typically appear on newly emerging leaves and persist as these leaves mature and are replaced by younger ones. Common symptoms include leaf wrinkling and sharply bent leaflet tips, known as "hook leaf," which create a serrated zigzag appearance. In severe cases, newly emerging spear leaves may fail to open normally, leading to multiple unopened spear leaves visible at the canopy's apex. Boron deficiency can also affect the inflorescence and nuts, causing them to become necrotic, leading to shriveled and barren nuts. To alleviate boron deficiency, applying 250 g of borax per palm is recommended. Additionally, foliar sprays of 0.2% borax (2g in 1 liter of water) at a rate of 75-100 ml per seedling are effective, particularly in nursery-stage coconut palms. Dosage recommendations vary with age: 5-10 g per plant per year for one-year-old palms, 15-20 g per plant per year for two to three-year-olds, and 30-50 g per plant every two years for palms four years old and older.
(vii) Sodium and Chlorine
Sodium accumulates preferentially in the vacuoles and plays a role in maintaining the solute potential of the cell. It improves the water balance of plants under limited water supply. Chlorine is imperative for osmoregulation and charge compensation of palms. Chlorine in abundance suppresses grey leaf sport in coconut. Common salt (NaCl) @ 1 kg per palm per year can be applied under non-saline conditions. Fertilizer schedule for palms of varied ages is presented.
