However, many rice fields are above the critical levels as a result of the repeated use of phosphorous fertilizers. Phosphate fertilizer may also be topdressed with good results when a deficiency occurs in the crop. Topdressing should be made early, preferably in the seedling, or not later than the midtillering stage. In zinc-deficient soils, rice seedling growth may be reduced and, in severe cases, stand loss may occur. Preflood surface applications of 2 to 16 pounds per acre of actual Zn, depending on the source, have effectively corrected this deficiency.
Zinc deficiency occurs more frequently in cool weather during stand establishment.
Zinc fertilizer in the form of zinc sulfate, zinc oxide, or zinc chelate is broadcast or sprayed on the soil surface after the last seedbed tillage for maximum effectiveness. Zinc should not be incorporated into the soil. Iron Although rare in California, iron chlorosis can be induced on soils that contain free calcium carbonate and where irrigation water contains more than 4 meq of bicarbonate per liter. Iron compounds have been inconsistent in correcting this nutrient deficiency, but lowering the soil pH can solve it.