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Effects of Deep Tillage on N Response by Corn on a Sandy Coastal Plain Soil¹

Tillage pans in sandy coastal plain soils often restrict utilization of subsoil moisture and nutrients and may limit corn (Zea mays L.) yields. Field experiments were conducted for 2 years on a Wagram loamy sand (Arenic Paleudult) with a tillage pan to evaluate deep tillage treatments and preplant N applications containing nitrapyrin (NI) on corn. Experiments were arranged in a split plot design containing three tillage treatments as the main plots with four N-treatments as the subplots. The three tillage treatments were: a) disking to a 15 cm depth, designated as conventional, b) chisel-plowing to a depth of 30 cm at 30 cm intervals, and c) subsoiling in the row to a depth of 45 cm and bedding over the subsoil slit. Four N-treatments consisted of preplant applications of 0, 112, and 168 kg N/ha broadcast as urea and a urea-NI solution at a rate of 112 kg N/ha plus 1.12 kg NI/ha. Measurements were made of soil NH⁺₄ and NO^-₃ concentrations, leaf N, stover weight, stover N concentration, grain yield, grain N content, and recovery of fertilizer N.

Use of NI in 1978 resulted in more of the total inorganic soil N in the 0 to 15 cm depth being in the NH⁺₄ form; however, this did not significantly increase total inorganic soil N content at any depth. Nitrapyrin had no significant effect on inhibition of nitrification in the top 45 cm depth in 1979. Inorganic soil N was moved below the tillage pan into the subsoil by leaching rains. Ineffectiveness of NI to significantly suppress nitrification in 1979 was probably due to the differential movement of NH⁺₄ and NI. Subsoiling significantly increased leaf N concentration above that obtained from conventional tillage in 1979, suggesting root extraction of soil N below the tillage pan. Chisel-plowing and subsoiling increased corn stover dry matter production above that of the conventional tillage in both seasons.

Disruption of the tillage pan by subsoiling or chisel-plowing increased corn yields above that obtained with conventional tillage. Highest yields were obtained with subsoiling. In 1978, higher grain yields with the deep tillage treatments were thought to have resulted from the utilization of moisture below the tillage pan. Increased yields with deep tillage in 1979 were due to utilization of both moisture and N below the tillage pan. Yields in 1978 were significantly higher with 168 kg N/ha as compared with 112 kg N/ha rate. Yields in 1979 were not significantly different at the 112 and 168 kg N/ha rates. Nitrapyrin had a positive effect only on yields with the chisel-plow treatment.

Key Words: Nitrapyrin • Subsoiling • Chisel-plowing • Nitrification inhibitor • N accumulation • Urea

² Former graduate research assistant and professor of soil science, respectively, North Carolina State Univ., Raleigh, NC 27650.

Received for publication May 28, 1981.