Causes and Control of Lime-induced Fe Deficiency in California Fruit and Nut Crops

Although iron (Fe) is the fourth most abundant element in the Earth’s crust and upper mantle (lithosphere), Fe deficiency is among the most common plant micronutrient deficiencies. There are multiple contributing factors, but the most important for practical purposes in California orchard crops are those soil and water characteristics that cause lime-induced Fe chlorosis: soil and water with high pH, calcium carbonate and bicarbonate; poor soil aeration due to high clay content, poor drainage, or compaction; and extremely wet or dry soil moisture status in the spring. As in the greater than 30% of the world's soils that are calcareous, limeinduced iron deficiency is a challenge in numerous perennial crops, including: kiwifruit, grape, pear, apple, citrus, avocado, pecan, and stone fruit (almond, apricot, peach, nectarine, cherry) (Fig 1). Plants take up Fe from the soil solution. Fe oxides are the common source of Fe for plant nutrition in most soils. Solubility of Fe oxides is pH dependant; as pH increases, the free ionic forms (Fe+2, Fe+3).are changed to the hydroxy ions, and finally to the insoluble hydroxides or oxides. Dissolution, and hence availability, of insoluble Fedecreases 1000 times for every unit of pH increase. The bicarbonate ion (HCO3−) directly affects availability of Fe in calcareous soils by buffering pH. It also inhibits mobilization of accumulated Fe from roots to foliage. If irrigation water is also high in bicarbonate, conditions are enhanced for Fe deficiency because bicarbonate is continuously supplied to the soil and more importantly, the root environment (rhizosphere). Besides buffering soil pH, plant roots can become crusted with lime as water evaporates, preventing normal root growth and function needed to solubilize and take up ferrous (Fe+2) Fe. Inside the plant, bicarbonate inhibits movement of nutrient ions from roots, stems, petioles and veins to interveinal leaf spaces where they are needed for photosynthesis. Negative effects of naturally high bicarbonate levels are exacerbated in very saturated, very dry, or compact soils, under which conditions bicarbonate concentration increases at the same time the environment for root growth and nutrient uptake is diminished.