Zinc deficiency in black cotton soil, zinc sulphate soil dose, residual effect across crops.
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Black cotton (vertisol) soils are often low in available zinc because high pH and high calcium lock it up. For a soil test confirmed deficiency, a common soil dose is about 10 kg per acre of zinc sulphate heptahydrate (21 percent) or about 6 to 7 kg per acre of the monohydrate (33 percent), broadcast and mixed into soil at sowing. Do not apply zinc every season blindly; a soil application usually carries over for 2 to 3 crops, so apply once and monitor. For a quick correction in standing crop, a foliar spray of 0.5 percent zinc sulphate with half that quantity of lime works. Keep zinc away from phosphate fertiliser at placement so they do not react. Confirm the dose against your Soil Health Card and KVK and do not over apply.
In our area near Jalna zinc was the missing link. One good soil dose lasted me cotton then chana then cotton again before I needed to repeat.
Mixing the zinc sulphate well with some FYM before broadcasting helped me get even coverage and avoid burning spots. If you just throw the crystals dry they clump and the field gets patchy. The dung also seems to keep the zinc available a bit longer in our heavy soil.
Please test before you keep adding zinc every year. A neighbour kept dosing zinc out of habit and his cotton actually suffered, and the soil man said too much zinc can push down iron and other micronutrients. Once in two or three crops on a real test result is plenty in this black soil.
Mixing the zinc with a little FYM before broadcasting gave me even spread and the soil dose lasted three crops here. Throwing dry crystals left patchy spots last time. The dung carrier seems to keep it available longer in our heavy land.