Malhi, S.S.

Nyborg, M.

English

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Methods of reducing the soil acidification caused by nitrogen fertilizers

University of Saskatchewan Research Archive

- 112 -METHODS OF REDUCING THE SOIL ACIDIFICATION CAUSED BY NITROGEN FERTILIZERS M. Nyborg and S.S. Malhi Department of Soil Science University of Alberta Edmonton, Alberta T6G 2E3 INTRODUCTION The slow acidification of non-calcareous soils with the use of ammonium-based nitrogen (N) fertilizers is well known. In Alberta, this behavior has been documented with agricultural soils by several researchers (Cairns, 1971; McCoy and Webster, 1977; and Hoyt et al.; 1981). The acidification of soils by N fertilizers essentially is caused by nitrification of the ammonium in the fertilizers. Application of anhydrous ammonia, aqueous ammonium, ammonium nitrate, or urea results in formation of nitric acid after the nitrification in the soil. Theoretic-ally, 3.6 parts of CaCO . are needed to counteract 1 part of fertilizer N which is nitrified. in trte soil (Tisdale and Nelson, 1975). Application of ammonium sulphate results in formation of nitric and sulphuric acids on nitrification of the fertilizer, and the calculated value is 7.1 parts of caco3 needed to meet the acidity produced by 1 part of N which is nitrified. In practice, the amount of acidification in cropped soils is usually much less than that requiring 3.6 or 7.1 parts of Caco3 per 1 part of fertilizer N. This is so especially when rates of N fertilizers are modest (Pearson and Adams, 19u7). In northern or Central Alberta "typical" results for accumulated acidification from annual application of urea or ammonia at a rate of 100 kg per ha might be a soil pH depression of 0.05 units, annually. This estimate is given here merely as a conservative guideline. In Alberta, approximately 40% of the cultivated soils have a soil PH of 6.5 and less. Consequently, acidification of soils by N fertilizer is .a practical problem. However, liming is the obvious solution as the soils become acid. The slowing of nitrification of fall-applied ammonium-based N fertilizers has been shown to lessen losses of the N (Nyborg, Malhi, and Monreal; 1980). In the present paper, we report results showing that techniques for slowing nitrification may have the incidental advantage of reducing soil acidification, as well as the main advantage of reducing losses of the fertilizer N. - 113 -MATERIALS AND METHODS Field experiments were set out in randomized complete block design in four replicates. Sub-plots were 6. 8m by 1. 8m in size. In some experiments fertilizers were applied in the fall on certain sub-treatments and in the spring on others. In other experiments, fertilizers were applied only in the spring. All experiments were cropped to Galt barley. After harvest the soil was cultivated. On each sub-plot of each replication, the soil was sampled by compositing eight cores of the 0-15 em depth, and the 15-30 em depth. Soil pH was determined .by adding 50 ml of distilled water to 20 g of soil. The mixture was stirred for 30 minutes, let sit for 1 hour, the hydrogen electrode inserted into the sediment with the reference electrode in the supernatant, and the pH was read after 2 minutes. The "titration" of soils was made with increments of dilute H2so4 added to a series of containers with a 2.5:1 of water:soil m~xture, and pH was determined after 3 days. RESULTS AND DISCUSSION Field experiments were conducted to measure the rate of acidification in 1972, 1973, and 1974 at Breton and Ellerslie and only in 1972 at Craigmyle. The soils were a Gray Luvisol (loam), Black Chernozem (silty clay loam), and a Dark Brown Solonetz (loam), respectively. There was a depression in soil pH after application of ammonium nitrate, or urea, at a rate of 67 kg N per ha yearly for three years at the Breton site (Table 1). The difference in pH of the control treatment as compared to the pH of the two treatments was statistically significant, and the differences were 0.20 and 0.32 with the ammonium nitrate and with the urea, respectively. The ammonium sulphate, at 67 kg N per ha, gave more lowering of soil pH, as would be expected. At the Craigmyle site, with only one season's results, the 67 kg N/ha rate showed a statistic-ally significant depression of pH only on the ammonium sulphate treatment. A rate of 202 kg N per ha of any of the three fertilizers mixed into the soil produced marked lowering of soil pH. However, for the ammonium sulphate and for the urea this high rate was also applied by placement in bands (23 em apart) at time of seeding. The placement was made at 2.5 em to the side of the row by 2.5 em below the row. Application of either of the two fertilizers in bands, instead of mixing the fertilizers into the soil, resulted in much less soil pH depression. The amounts of Caco3 required to counteract the acidity produced per unit of fertilizer N varied a good deal for the different treatments on each of the soils (Table 2). The results with mixed and banded urea, at 202 kg N per ha, showed striking differences between those two methods. With mixed urea the values of parts of CaCO per part of fertilizer N ranged f1;om 2.0 to 3.4 for the three soils. Wiih banding, - 114 -the values ranged from 0.1 to 2.3. Apparently , with band placement (compared to mixing ) of the urea, or ammonium sulphate, there was less change in soil pH and less CaCO needed to restore the soil to its original pH. However, these exp~riments were made with fertilizer applied at the time of seeding, and not with fall-applied N. Table 1. Soil pH of the 0-15 em depth of three soils after annual fertilization and cropping to barley. Annual Soil EH at three locations* rate of Method of Breton Craigmyle Fertilizer N (kg/ha) Application (3 years) (1 year) Nil 0 ~ 5.55 e Amm. nitrate 67 Mixed 5.94 f 5.48 de Amm. nitrate 202 Mixed 5.40 c 5.33 be Amm. nitrate 67 Mixed 5.58 d 5.38 cd Amm. nitrate 202 Mixed 4.76 a 5.09 a Amm. nitrate 202 Banded 5.38 c 5.41 cd Urea 67 Mixed 5 . 82 ef 5.53 e Urea 202 Mixed 5.24 b Urea 202 Banded 5.53 e * In each column, the values are significantly different (95% of probability) when not followed by the same letter. ** Missing values Ellerslie (3 years) 5.88 d ** 5.34 b ** 5 . 14 a 5.42 be ** 5.51 be 5.61 c Banding, rather than m1x~ng, of ammonium-based fertilizers substantially delays nitrification (Malhi, 1978), and the apparent plant uptake of the ammonium-form of N before its nitrification is most probably the main reason for lessening soil acidification. From 1974 to 1980 forty field experiments were conducted to compare fall and spring application of urea (as well as aqua ammonium and ammonium sulphate) in their effectiveness in increasing the yield and N-uptake for barley. The results, excepting those for 1980, have been summarized (Nyborg, Malhi , and Monreal, 1980), and they point out that with fall-application of N fertilizers the increase in yield and N-- 115 -uptake was only about half as great as with spring application. Band application, and more so nest or pellet application, improved the performance of fall added fertilizer N. Before 1978, occasional determinations of soil pH indicated that there were reductions of pH with fall applied fertilizer mixed into the soil rather than banded or nested. Consequently, in 1979 and 1980, more intensive experiments were run. Table 2. Kilograms of Caco3 needed per kilogram of fertilizer nitrogen to overcome the acidity generated in the top 15 em of the soil by the fertilizers. Annual Kg of Caco3 per kg of N rate of Method of Fertilizer N (kg/ha) Application Breton Craigmyle Ellerslie Annn. nitrate 67 Mixed 1.2 1.2 * Annn. nitrate 202 8 1. 8· 1.4 . 5.2 Annn. sulphate 67 Mixed 3.9 3.2 * Annn. sulphate 202 4.6 3.3 7.1 Annn. sulphate 202 1.9 0.9 4.3 Urea 67 Mixed 1.9 0.4 * Urea 202 6 2. 7. 2.0 3.4 Urea 202 ~ 0.9 0.1 2.3 * Missing values The eight experiments in 1979 and 1980 gave soil pH values which were lower with the fall applied urea mixed into the soil as compared to the nil treatment (Table 3). The differences were usually statistically significant, and average difference in pH was 0.12 units. The average difference between the spring application and the nil treatment was 0.09 pH units. These depressions in soil pH from 56 kg N per ha were unusually large, considering that the amount of Caco3 needed per part of fertilizer N was more than the theoretical requirement. (However, for two of the soils, pH was determined in 0.01 M CaC12 solution,in lN KCl solution, and in water, and the amount of depression in soil pH values on treatments where the urea was mixed into were similar with the three methods of determination.) In this paper, - 116 -let us not concern ourselves with the unusually large changes in soil pH from the fertilizer, but instead with the effect of bands and nests . on soil pH. Table 3. The soil pH of the 0-15 em depth after harvest, with urea at 56 kg N/ha applied in the spring or previous fall. Soil pH with different fertilizer treatments Fall, Spring, Year Location Nil Mixed Mixed 1978-79 Buford 6.64 b 6.50 a 6.61 ab Thorsby 7.90 b *7.84 a 7.92 ab Egremont 7.36 a *7.27 a 7.31 a 1979-80 Breton 6.69 b 6.60 a 6.66 ab Hay Lakes 6.02 b 5.95 ab 5.91 a Viking 6.10 b 5.95 a Didsbury 7.40 b 7.24 a Olds 6.87 b 6.75 a AVERAGE 6.88 6.76 * Fertilizer was banded rather than mixed into the soil NOTE In each row, values not followed by the same letter are statistically different (P = 0.05). 5.96 a 7.18 a 6.79 a 6.79 In four experiments, the lowering of soil pH was 0.10 units from mixing (that is, incorporation) in the fall, 0.04 units from fall banding, and 0.08 units from mixing in the spring (Table 4). In six experiments, the lowering was 0.11 units for fall mixing, 0.04 units from fall nesting, 0.10 units for spring mixing. That is, fall banding or fall nesting produced less lowering of soil pH than did mixing of urea into the soil. This would be a benefit from banding and nesting, in addition to their different type of benefit in increasing the crop yield and crop N-uptake. - 117 -Table 4. The soil pH of the 0-15 em depth after harvest, with urea at 56 kg N/ha in the spring or previous fall, as influenced by band and "nest" placement and addition of nitrification inhibitors. Fall-aEElied N Spring N No N Incoq~ora ted Bands (45 em a:eart) incor:eorated Soil pHl 6.36 6.26 6.32 6.28 Fall-a:e:elied N Spring N No N Incor:eorated Nests incor:eorated Soil pH2 6.62 6.51 6.58 6. 52 Fall-a:e:elied N in bands Urea plus Urea plus Spring Urea No N · Urea ·N-Serve* . ATC* incor:eorated Soil pH3 7.66 7.56 7.68 7. 68 7.62 1 Average of 4 experiments 2 Average of 6 experiments 3 Average of 2 experiments * These are two nitrification inhibitors In two experiments, with two nitrification inhibitors fall-applied with banded urea solution, there was no depression in soil pH. Considering the different type of benefit in increasing crop yield and N uptake, there was only modest benefit from the nitrification inhibitors (Nyborg, Malhi, and Monreal; 1980). CONCLUSION In one set of experiments, with only spring application of N fertilizers, banding produced less lowering of soil pH than did mixing into the soil for urea and ammonium sulphate. In another set of experiments with fall applied urea, the band and nest placement gave less soil pH depression than did mixing of the fertilizer into the soil. While banding or nesting for fall applied urea (and for ammonium--based N fertilizers) increases crop yield and N-uptake; there ' is also the advantage of banding and nesting lessening soil acidification. - 118 -REFERENCES Cairns, R.R. 1971. Changes in a Solonetzic soil caused by the long-term heavy fertilization of bromegrass. Can. J. Soil Sci. 51:299-301. Hoyt, P. B. , Nyborg, M. , and H. Ukrainetz. 19 81. acidification. In Proceedings Alberta Soil c/o ASFTL, O.S.~ongman Building, 6909-116 Alberta. (in press) Degradation by Science Workshop. 1981. Street, Edmonton, Malhi, S.S. 1978. Ph.D. thesis~ University of Alberta, Edmonton, Alberta. McCoy, D.A., and Webster, G.R. 1977. Acidification of a Luvisolic soil caused by low-rate, long-term applications of fertilizers and its effects on growth of alfalfa. Can. J. Soil Sci. 57:119-127. Nyborg, M., Malhi, S.S., and Monreal, C. 1980. Unconventional methods of fertilizer placement to r.educe losses of fall applied nitrogen. In Proceedings of the 1980 Soils and Crops Workshop. Extension Division, University of Saskatchewan, Saskatoon. pp. 33-45. Pearson, R.W., and Adams, Fred. 1967. Soil Acidity and Liming. American Society of Agronomy, Publisher, Madison, Wisconsin, U.S.A. Tisdale, S.L., and Nelson, W.L. 1975. Soil Fertility and Fertilizer. Macmillan Pub_. Co. Inc., New York. ACKNOWLEDGEMENTS To the Alberta Agricultural Research Trust and Western Co-operative Fertilizers Limited for financial support of these investigations. 

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Methods of reducing the soil acidification caused by nitrogen fertilizers

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