Zero tillage : The extreme in conservation tillage is no-till (zero tillage) wherein the new crop is planted in the residue of the previous crop without any prior soil tillage or seed bed preparation and is usually possible when all weeds are controlled by the use of herbicides. Zinc deficiency : Symptoms appear 2-4 weeks after sowing as blanching of the midrib of the emerging leaf, especially at base. Brown spots appear on the older leaves which later on coahesee to give brown colour. Tillering and growth are depressed. In severe deficiency the plant dies. Zinc deficiency is associated with calcareous, alkali, peat, and volcanic soils and incidence is more severe where high rates of nitrogen and phosphorous are applied.