Immobilization of Fertilizer Nitrate Applied to a Swelling Clay Soil in the Field

Abstract

AbstractLow recoveries of N fertilizer by crops on Houston Black clay prompted this investigation. Our objectives were to determine how much fertilizer nitrate can be biologically immobilized under field conditions when fertilization exceeds crop requirements and to determine when immobilization occurs during the growing season. Calcium nitrate tagged with 9.6 atom % 15N was applied to field microplots at 328 kg N/ha and grain sorghum (Sorghum bicolor L.) was planted in rows across each microplot. The various components of the N balance were measured by sampling microplots at different times during the season.Only 2.1 kg N/ha was immobilized during the first 47 days after fertilization and planting. The rate of immobilization increased rapidly during the next 60 days and an additional 60 kg N/ha was immobilized. This increase was apparently in response to an increase in soil temperature above 22°C.About 91% of the fertilizer N was recovered, indicating that no more than 9% was denitrified, in spite of ample water that was received as rain or irrigation water during the study. A net immobilization (an increase in soil organic N relative to an unfertilized plot) of 53 to 73 kg N/ha was measured on the fertilized plot. This net immobilization suggests that the organic N content of swelling clay soils that have historically been subjected to exhaustive cropping may increase slowly and gradually reach a higher equilibrium level due to N fertilizer use.