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A Tracer Study of Nitrogen Balance and Residual Nitrogen Availability with 12 Soils¹

ABSTRACT

Plantavailability of ¹⁵N-tagged residual N following oats (Avena sativa L.) fertilized with 50, 100, or 200 ppm N was studied under conditions offering maximum exploitation of the tracer N. These conditions were achieved by subsequent uniform additions of nontagged N, as well as other nutrient elements, to promote good plant growth. Total recovery of tracer N in the initial oat crop and two succeeding crops of sudangrass (Sorghum sudanensis), plus that remaining in the soil, averaged 94% for all soils and treatments, but ranged as low as 82% for the low N treatment of the Fargo soil. On most soils the percent total recovery was not influenced greatly by N rates. Tracer N remaining in the soils after cropping ranged from 10 to 36% of the original addition and decreased with increasing N rate.

Residual N uptake by the first cutting of sudangrass, as a percentage of the tracer N which was not removed by the previous crop, ranged from 10 to 41% and generally increased with rate. Availability of the residual N to the second cutting of sudangrass dropped to an average of 4.5% of the remaining tracer N, with few significant differences among soils or treatments. Retention and availability of residual N were related to the relative N-supplying capacities of the soils.

¹ Contribution from the US Soils Laboratory, Soil and Water Conserv. Res. Div., ARS, USDA, Beltsville, Md.

² Research Soil Scientist, and former Research Soil Scientist, now retired.

Received for publication October 31, 1966. Accepted for publication December 30, 1966.