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Utilization of Anhydrous Ammonia Fixed by Clay Minerals and Soil Organic Matter¹

ABSTRACT

A portion of anhydrous NH₃ fertilizer applied to soil can be rendered nonexchangeable through fixation by clay minerals and soil organic matter. The plant availability of anhydrous NH₃ fixed by these two soil fractions can be important agronomically if such fixation limits plant uptake of the fertilizer N. In this study, three soils (Drummer, Typic Haplaquoll; Blount, Aeric Ochraqualf; Cisne, Mollic Albaqualf) with clay and organic C contents ranging from 120 to 310 and 7.8 to 30.1 g kg^–1, respectively, were injected with ¹⁵N-labeled (2 atom % ¹⁵N) liquid anhydrous NH₃ at a rate equivalent to 245 kg N ha^–1. Soluble and exchangeable N were removed by leaching and the soil was cropped to ryegrass (Lolium multiflorum Lam.) in pots. Soils were analyzed before and after cropping for clay fixed N and organic matter fixed N. Four cuttings (harvests) were made at 3- to 4-week intervals and roots were collected at the termination of the experiment. Aboveground dry matter, total N uptake, and fertilizer-derived fixed N uptake (mg N pot^–1) increased from the first to the second harvest and declined thereafter. Nitrogen recovered in the roots accounted for <11% of the total N and <7% of the fixed N utilized, and root dry matter accounted for 13 to 14% of the total dry matter produced. The ratio of fertilizer-derived fixed N uptake to total N uptake declined with harvest suggesting that the fixed N became less available to the ryegrass with time. Fertilizer-derived fixed N recovered in the ryegrass ranged from 19 to 26% of that originally fixed by the soil. The percentages of fertilizer-derived clay fixed N removed from the soils during cropping (35–72%) were much larger than those of the fertilizer-derived organic matter fixed N (<12%) suggesting that a majority of the plant uptake of fixed N originated in the clay fraction. Overall, fertilizer-derived fixed N removal from the soils (21–30%) agreed well with plant uptake data.

¹ Contribution from the Univ. of Arkansas Rice Research and Extension Center, Stuttgart, AR 72160, and the Dep. of Agron., Univ. of Arkansas, Fayetteville, AR 72701. Published with the approval of the Director of the Arkansas Agric. Exp. Stn.

² Assistant Professor and Professor and Head of Agronomy, respectively.

Received for publication November 12, 1986.