Effect of Bicarbonate Ion and Root Aeration on Lime-Induced Chlorosis

Chlorosis in plants has been recognized as a devastating disease for over one-hundred and fifty years. It is easily recognized by a yellowing of the plant foilage and is associated with a reduced chlorophyll synthesis. Chlorosis is found so frequently on calcareous soils that its cause is attributed at least in part to this soil factor. It is therefore referred to as lime-induced chlorosis. This type of chlorosis has become a serious problem, especially in many of the fruit-growing regions of Europe and the Western United States where the soils are calcareous. In Utah the disease is more destructive than any other nutritional disease that affect horticultural crops. The exact cause of lime-induced chlorosis is not known, nor has a satisfactory control been developed as yet. Many conditions found in high-lime soils have been studied with regard to chlorosis, yet the problem is not so simple as it may appear. Green and chlorotic plants are frequently found growing in the same field, and yet a chemical analysis of the two soils fails to indicate a significant difference between them. It is common in chlorotic orchards to see trees where some of the major branches produce chlorotic foilage while the remainder of the tree is healthy and green. The severity of chlorosis varies from tree to tree as well as from season to season. It has been found that the heavier textured soils which are poorly drained are more conductive to chlorosis than are the lighter textured soils. If the soil moisture is held near field capacity, chlorosis is much more severe than if the soil is allowed to approach the wilting point before irrigation. Recent studies have also shown that the bicarbonate ion in solution cultures can induce chlorosis and retard the uptake of iron by plants. Since these factors--high moisture, soil aeration, and bicarbonate ion concentration--are all interrelated, it seems that their relationship to chlorosis should be investigated more thoroughly. Workers in the past have failed to show a consistent correlation between either the oxygen or the carbon dioxide can be measured at the soil-root interface. this is after all the only place in the soil that is of great importance as far as the living plant is concerned. Since the severity of chlorosis increases under poorly aerated conditions on calcareous soils, this hypothesis has been advanced: The respired carbon dioxide is given off at the plant root as carbonic acid which in a calcareous soil reacts with calcium and magnesium carbonate to give an increased bicarbonate ion concentration in the root environment. The resulting increased concentration of bicarbonate may reduce the effective oxygen at the root and thereby disturb the normal respiration and active-ion absorption of the roots or the increased concentration of bicarbonate may affect the roots absorption and plant metabolism in some other manner. The purpose of this investigation was to study the effect of the bicarbonate ion and different aeration mixtures on chlorosis. A preliminary study was made to relate the composition of the soil solution taken from the field with different conditions associated with chlorosis