ZINC DEFICIENCY is one of the most serious nutritional problems of walnut production in California, and has been very difficult to correct. The most common treatment in past years has been the use of zinc-coated sheet metal strips driven into the sapwood of the tree. This method has been laborious and expensive and has required periodic treatments (every three to four years) to maintain deficiency-free trees. In some soils, trees have responded well to soil applications of zinc, while in other soils they have responded poorly. Soil applications of zinc at levels sufficient to achieve correction have often been very expensive.