Boron Uptake by Plants as Influenced by Soil Moisture

BORON-DEFICIENCY symptoms have been observed in alfalfa on some Indiana soils for several years. However, these symptoms have been observed in dry summers only. This corroborates the observations of Brown and King (3) and Kostyuchenko (5), in Connecticut and Russia respectively, that boron-deficiency symptoms were more prevalent in dry seasons. Recent investigations of Indiana soils (4) have indicated that the supply of "available" boron in the surface soils should be adequate for normal plant growth. Consequently normal development would be expected in humid seasons. However, as dry soil conditions have been shown to reduce nutrient uptake by plants (2, 7), boron absorption from the surface soil would be expected to be reduced in dry seasons. The occurrence of boron-deficiency symptoms under these conditions might be expected to depend on the boron content of the more moist sub-surface horizons. Investigations (4) have indicated that the available supply of boron in Indiana soils decreased with increasing depth. In the areas where boron-deficiency symptoms have occurred, the available boron content of the sub-surface horizons has been shown to be extremely low. Accordingly it was considered possible that the boron-deficiency symptoms which have been noticed in alfalfa in dry summers resulted not necessarily from a deficient supply of boron in the soil as a whole, but rather from a deficient supply of moisture in the surface soil combined with a deficient supply of boron in the subsoil. In order to test this hypothesis, two greenhouse experiments were set up to study the effect of dry surface soil conditions on the uptake of boron by plants.