Yield and protein content of cereals and oilseed as influenced by long-term use of urea and anhydrous ammonia

Abstract

Nitrogen use on the Canadian Prairies has gradually increased in recent years, and producers have switched from ammonium nitrate to urea and anhydrous ammonia. Ammoniacal N is known to acidify soil, and some segments of society question the efficacy of using anhydrous ammonia, suggesting it might adversely affect soil productivity. A 10-yr experiment conducted on a moderately acid, medium-textured, Orthic Dark Brown Chernozem at Scott, Saskatchewan, enabled us to assess the impact of annual rates of N (0–180 kg ha−1), applied as urea or anhydrous ammonia, on yields of hard red spring wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), and canola (Brassica campestris L.). The soil was gradually acidified by the fertilizers, with the effect being greatest for anhydrous ammonia. Yields of cereals reflected the positive influence of added nutrients in the case of urea, but because of soil acidification, response to anhydrous ammonia decreased in the later years. Yield of canola (only grown in two later years, when the soil was severely acidified) was negatively related to N rate and was lower for anhydrous ammonia than for urea. Acidification increased soil solution Mn, and this probably contributed to the suppressed yield responses. Grain protein was positively related to rate of N, except when yield increases caused a dilution effect. Different responses to the two N sources may have been due to loss of N from the urea via volatilization, and, thus, urea acidified the soil to a lesser extent than anhydrous ammonia. We concluded that if producers use recommended rates of N, there should be little deleterious influence of acid-forming N fertilizers on cereal production; however, more care will be necessary when growing canola. Key words: Nitrogen rate, canola, wheat, soil acidity, Mn toxicity