Zinc deficiency symptoms may be very subtle and difficult to identify. Basal leaf of midrib chlorosis, bronzing of leaf blades, staking of leaf sheaths, reduced number of tillers and shorter internodes can be indicative of the pathology. Most of these symptoms, together with floating leaves can also be observed after flooding. Plants fail to grow properly and spikelets are sterile, even after application of nitrogen-rich fertilizers. In some cases, young or middle-aged leaves can develop small, dusty, brown spots, whereas brown blotches or streaks appear on older leaves. Plant growth is uneven and it takes longer to reach crop maturity. In the worst cases, the symptoms are only observed after flooding and characterized by high plant mortality.
Zinc deficiencies usually occurs in poorly drained soils, sandy soils, calcareous soils and soils with high phosphorus and silicon. They can be due to a shortage of zinc or a reduced availability to the plant. Other factors like pH, organic content or presence of other micronutrients also affect the uptake. The particular conditions in paddy fields after submergence may further hinder the availability of zinc to the plants. Higher plant density in the field and enhanced root development lead to greater uptake of zinc. The main difference between zinc and phosphorus deficiencies is that in the latter case, leaves generally do not show the basal chlorosis and they remain erect instead of floating on the water.
Application of organic manure to the seedbed, or to the field a few days after transplanting reduces the odds of observing zinc deficiency. Drain permanently flooded fields periodically.
To avoid deficiencies, products containing zinc sulphate (ZnSO4) can be spread in the nursery seedbed or in the fields (in the latter case, 20kg/acre once every second season). Alternatively, seeds or seedlings can be immersed in a 2−4% zinc oxide (ZnO) solution before sowing or transplanting. After noticing the first symptoms of deficiency, zinc sulphate (2g/l) can be applied twice at 5 days interval. When zinc deficiency is observed after flooding, the best solution is to drain the soil and apply zinc fertilizers when the soil is dried and new roots and shoots have formed. Application of zinc products into the floodwater is not effective.
Make sure to choose plants with an efficient uptake of zinc or good tolerance to deficiency.,Avoid planting rice in intensively cropped soils.,Allow permanently flooded fields to drain and dry out periodically.,Use fertilizers based on urea (that generate acidity) rather than on ammonium sulfate.,Apply organic manure before seeding or transplanting.,Monitor irrigation water quality regularly.,Make sure not to over-fertilize with phosphorus.