Tea Q. 50 (3), 105—116, 1981. Printed in Sri Lanka EFFECT OF NITROGEN FERTILIZATION ON SOIL SOLUTION COMPOSITION ACIDITY A N D NUTRIENT LEACHING I N ACID RED-YELLOW PODZOLIC SOILS D. C. Golden, S. Sivasubramanium and J. P. Fonseka (Tea Research Institute of Sri Lanka, Talawakele, Sri Lanka) The effect of N-fertilization of soils under the tea crop (Camellia sinensis L.) was studied using selected plots from two long term experiments. Where in (1) A factorial combination of 3 levels of N, 3 levels of K and 3 levels of P were used with ammonium sulphate as the only source of N and in (2) The different N-sources urea and ammonium sulphate were compared at three levels. A laboratory incubation was also conducted in order to supplement the field data. All the theoretical changes expected of urea hydrolysis and subsequent nitrification were observed in the incubated soils. A faster initial nitrification was observed in urea treated soils over that of ammonium sulphate treated ones. Soil-solution pH at 20,40,60, 80 and 100 cm depths were not sensitive to the surface applied fertilizer transformations under field conditions. A considerable release of cations into the soil solution was observed with the onset of nitrification where as the acidity did not move down even to 20 cm depth. The cation release could be explained by the Donnan equilibrium and the pH-dependent CEC. The nitrate formed did not reach depths lovrer than 60 cm in either of the experiments. This is determined by the soil texture, plant uptake and also to a certain extent by the gradient of the tea growing hill slope. Though the laboratory incubation showed a large initial nitri­ fication rate of urea N over that of ammonium sulphate the field data did not follow the same trend. Nitrification markedly affected the soil nutrient concentration by virtue of the H-ion release. The observed trends are in accordance with the changes in the pH- dependent CEC and Donnan equilibrium between the multiionic soil solution and the constant potential clay colloid surface. INTRODUCTION Fertilization of the tea crop (Camellia sinensis L.) with nitrogen fertilizers such as urea and ammonium sulphate has become important due to the nature of the harvested portion (two leaves and a bud), which removes a substantial portion of the added N from the soil-plant system by virtue of its composition (3-4% N). The use of such fertilizers may have more nutritional effect than merely supplying N, partly because of the fertilizers effect on the behaviour of other nutrients in the soil, changes in adsorption, changes in leaching losses and the implications of the pH effects accompanying the transformations of the fertilizers. The purspose of this study was to determine the effects of applied fertilizer mixtures containing urea and ammonium sulphate on the soil solution composition and its leaching characteristics. MATERIALS AND METHODS Field Experiment (1) Plots were selected from a field experiment in its 13th year designed to study the effect on yield of N (applied as ammonium sulphate) 3 ( 105 ) levels of P (applied as rock phosphate) and 3 levels of K (applied as commercial potassium chloride) in all combinations. The soil in the experimental site belong to the Red Yellow Podzolic great soil group and Coombe series (De Alwis et al, 1981) plots with the following N K treatments at the highest level of P were used. N , K 0 — 112 kg N/ ha/year without K N 2 K 0 - 224 N 3 K 0 - 336 N , K 2 — 112 Kg N/ha/year with 116 K Kg/ha/year N 2 K 2 - 224 N 3 K 2 - 336 N 0 K 0 — Control without fertilizer Since the fertilizer had been given in four equal doses the plots had received one fourth the quantity indicated. The soil solution composition was monitored by using suction soil solution samplers in duplicate at depths 20, 40, 60, 80 and 100 cm respectively, and field soil moisture content was measured using a nutron moisture meter. Measurements were taken over a two month period after a surface broad­ cast of fertilizer. The extracted soil solutions were analysed NH 4 -N, N0 3 -N,Ca, Mg and K, and pH. Total nitrate nitrogen content in the profile was calculated by ( N O 3 - N ) 0 1 0 0 = § 0 1 0 0 Bz C z A* , where 0z is the volumetric moisture content at a layzer z cm deep, Cz the concentration of NO s -N, and ± z the thickness of the layer. Field Experiment (2) The plots were selected from a two year old field experiment designed to study the effect of two sources of nitrogen, ammonium sulphate and urea at 3 levels each, along with Kas commercial potassium chloride at the rate of 135 Kg K/ha/year and rock phosphate 34 Kg P/ha/year. Nitrogen was tested at 3 levels in a rectangular lattice design. The two treatments, 100 Kg N/ha/year and 300 Kg N/ha/year were selected for both N-sources. Soil solution samples were drawn from five depths, 20,40, 60, 80 and 100 cm and moisture measurements were done as in the experiment (1). The soil in the experimental site belongs to Red Yellow Podzolic great soil group and Waltrim series (deep phase) (De Alwis et al. 1980). The soil solution was analysed for N H 1 + - N , N 0 3 - N, Ca, Na, K, and pH. Laboratory Incubation Experiment Field moist samples of soils from a clay loam 0-15 cm (Coombe series) were collected (initial moisture 35% w/w) and weighed 1 Kg each into seven polythene bags. Urea and ammonium sulphate were added in solution form to give 409 ppm in N (w/w) more water was added to bring the moisture content to 40% and the soils were homogeneously mixed. The bags were tied loosely and kept at room tempera­ ture, samples were drawn at prescribed time intervals, initially, once in 4 days and later, once a week, for chemical analysis. Samples were drawn daily for pH measure­ ments. pH measurements were done using 0.02 N (1:2.5) KCI and H 2 0 (1:1) and the samples for chemical analysis were extracted with 2N KCI or H 2 0 (1:5). No adjustment was done for moisture loss during incubation. Water extracts and KCI extracts were analysed for NH t -N, N 0 3 - N and Ca. Ca and Mg were determined by atomic absorption spectrometry, K and Na by name photometry NH t —N (Titlow and Wilson, 1964), N 0 3 - N (Middleton, 1959) and urea (Douglas and Bremner, 1970) by colourimetry. ( 106 ) Results and Discussion In the field experiment (1) where nitrogen was added in the form of ammonium sulphate, nitrification took place rather rapidly as indicated by the total nitrate build up in the soil profile 0-100 cm depth as calculated from the moisture content and nitrate concentration data. TABLE 1 — Total nitrate content* in the 0-100 cm profile of the treated plots fig N/ cm2 Treatments Days after treatments 3 10 17 24 31 38 45 52 N„Kj — — 103 29 125 74 76 65 N ^ , 357 340 756 710 730 680 863 490 N„K 0 607 685 854 934 1168 1501 1781 1427 N,K„ 684 739 1281 1471 2461 2242 2773 2363 N,K. 108 109 184 149 179 103 133 214 N,K. 140 205 406 337 484 404 349 300 N,K, 788 748 1215 1320 1334 1080 1556 1536 Values are averages of 3 determinations. At K 0 level of Potash, addition of more nitrogen lead to larger nitrate produc­ tion, reaching a maximum concentration of about 2800>-'g N/cm* in a 1-meter soil profile. At the K 2 level lower nitrate production took place due to inhibitoryu, effects of commercial potassium chloride (Golden et al, 1981). A similar trend was observed in the soil solution calcium in the soil profile. TABLE 2 — Soil solution Calcium ion content in a 1 cm* soil profile, 1 meter deep V-gCa) Treatments Days after treatments 3 10 17 24 31 38 45 52 N„K„ (control) N,K 0 N a K 0 NiKj N,K, N 2 K , 69 154 123 28 87 42 419 380 378 384 324 188 183 225 600 609 913 1167 1389 1488 1916 1877 819 1123 1106 1479 1641 1669 2755 2390 401 402 441 547 526 397 479 434 642 494 432 436 421 164 367 294 1210 1117 1104 805 1047 680 1055 1369 Calcium concentration shows the same trend as nitrate increase for the K 0 treatments, giving the highest value for Ca ion content in solution on 45th day for N 3 K 0 treatment (Table 2) which correspond to the largest amount of nitrate production (Table 1). Same trend was observed with Mg ions (data not shown), but in this case the largest Mg content was observed in the N 3 K 2 treatment. The ( 107 ) Nitrification produces H ions which can cause a temporary decrease in the pH dependant negative charge on these soils. The H ions released during nitrification according to the following reactions (1) and (2), Bacteria (1) (NH 4 ) 2 S0 4 + 30 2 •* 2HN0 2 + H 2 S 0 4 + 2 H 2 0 (Nitrosomonas) Bacteria (2) 2 H N 0 2 + 0 2 - 2 H N 0 3 (Nitrobactor) can affect the pH dependant negative charge on these soils. This effect is quite prominent as the soils are rich in 1:1 clay minerals and iron and aluminium oxides (Golden, D.C., et al, 1981). The surface charge and H-ion interaction could be given by (Parks, 1965), Al OH OH + H - H + A l H O / 2 OH + H - H Al H 0 ^ / 2 \ H O 2 Lowering of CEC by such a pH change can cause a preferential release of divalent cations (Ochtere, Boteng and Ballard, 1980). The Ca-ion is the most prominent cation to diffuse into the soil solution as a result of such perturbations in CEC in a multiionic Donnan system. Ca and Mg show similar increasing trend with increase in N rates. K follows the same trend (at K 0 level) with increase in N 0 3 - N . At the K 2 level K does not increase with the increase in N levels of the treatments (data not shown). At the Zero K level increase in N, and therefore the larger nitrification has caused soil acidity to increase (pH 4 to 3.45). The Ca, Mg, and K concentrations in soil solu­ tion increased with increase in N. Nitrate N production under the treatment N X K 2 and N 2 K 2 were rather low yet the Ca and Mg increased from N X K 2 to N 3 K 2 . The exchangeable cations Ca, Mg and K in the soil show a consistent decrease with the increase in N- levels at K„ level. The same trend could be seen at K 2 level (Table 4) except for K ions. This could be attributed- to the higher N rates. At the K 0 the leaching effect is due to effect of NH 4 ions and acidity, but at the K 3 level the high electrolyte concentration imparted by the addition of higher rates of N and K must have played a prominent role in the leaching of Ca and Mg ions. Though the 2N-KC1 extractable cation content (Ca, Mg and K ions) has decreased with increasing N level at K 0 level. The soil solution concentration has increased in the same direction (Table 3), thus making these ions more vulnerable to leaching. ( 108 ) ( 601 ) TABLE 3 — The total ions K, Ca, Mg and NH^ (v-g/cm*) and JV03 in soil solution after 45 days of fertilization in top meter of the soil profile (at KQ level) Treatment NOrN Ca Mg K Top soil pH in KC1 76 863 1785 2773 87 183 1916 2755 484 989 1111 1336 24 40 61 73 4.00 3.75 3.50 3.45 Profile Distribution of the Nutrients Potassium in soil solution profile is shown in Fig. 1. In the N 3 K 0 plot there is more K approximately 10 ppm in soil solution than K 0 N 0 (control). This is due to displacement of native exchangeable K into the soil solution by added NH 4 and also due to any movement of the acidity down the profile. The distribution of soil solution K down the soil profile shows that the added K has moved considerably down the profile. As 90% of the feeder roots of tea are found in the 0-30 cm layer all what is below this zone may be unavailable to the plants. This loss is rather important as K being a very important nutrient to the tea plant. Ca though not as important shows a similar trend. Field experiment (2) to compare urea vs. ammonium sulphate The variation of the nitrate ion concentration in the soil profile (Fig. 2) with time indicates that in the ammonium sulphate treated plots and the urea treated plots at the lower rate of application (100 Kg N/lia), the nitrate concentrations are quite low, though a peak is observed in about 1 to 2 weeks. However a prominent peak with a sharp increase in N0 3 -N was observed for the urea treated plot at the higher rate of application. Ammonium treated plot showed a relatively lower increase in N0 3 -N. In general the nitrate concentration in ammonium sulphate treated plots were above that of urea treated plots except at the peak concentration. The sharp increase in N0 3 -N in the urea treated plot is in conformity with the results of the incubation experiment. The nitrification and the associated changes are not very prominent in this field experiment probably due to high level of commercial potassium chloride included with the treatment. The inhibitory effect on nitrifi­ cation of commercial potassium chloride in these soils are discussed elsewhere (Golden et al, 1981). The Ca mobilized in the soil profile (Fig. 3) shows that ammo­ nium sulphate releases more Ca ions into the soil solution than does urea at both rates of application. This clearly indicates the greater removal of lime from soil by the acidifying effect of ammonium sulphate. However there is a time lag of about one week, between the nitrate concentra­ tion peak (Fig. 2) due to 300 Kg/ha rate of application of both urea and ammonium sulphate and the appearance of Ca ion concentration peak (Fig. 3). There is a bigger lag (£± 4 weeks) in the case of 100 Kg/ha rate. Probably the lag is a mea­ sure of the rate of leaching as the diffusion coefficient of N 0 3 is greater than that of C a 4 f . The lower rates of N application has given rise to a lower rate of cation leaching, whereas higher doses stimulated a faster leaching. The Ca ion content in the soil solution profile has shown the expected changes corresponding to the N fertilizer transformations. K and Mg ions seem to be comparatively less sensi­ tive (Table 4) to these changes unlike in the previous experiment, with Coombs series soil. ( H I ) to 45 40 35 30 I 8 to • Ammonium sulphate * Urea 100 Kg/ha/y ——300 Kg/ha/y 1* 21 28 35 42 49 Ho. of Days 56 63 70 Fig- 3. — Calcium ion concentration {ppm) in soil solution after the application of fertilizer-N {Field expt. 2). TABLE 4 — The exchangeable ions (2N.KC1) in 0-15 cm layer of the treated plots Treatment Exchangeable ions (ppm) NOt-N NHt-N Ca Mg K N„K 0 (Control) 4.0 5.8 28.8 26.9 57.2 N,K 0 9.6 3.9 • 300.8 28.0 103.7 N-K, 12.3 5.0 18.1 8.8 95.2 N j K , 18.0 27.6 15.8 6.4 33.9 N X K, 8.2 5.9 214.4 14.5 255.1 N,K, 20.9 96.0 26.5 8.1 201.4 N,K, 24.4 166.8 10.4 5.1 181.8 Incubation Trial A soil (Red Yellow Podzolic, Coombe series 0-15 cm depth) was incubated with fertilizer N to study these transformations and to confirm the field observa­ tions. The pH ( H 2 0 1:1) of the urea treated soil increased to 5.4 within a day (Table 5) and then gradually decreased over a period of 40 days to come to the level of the control. The nitrification rate in the urea added soil was higher than that of ammonium sulphate treated soil (Fig. 4) confirming the earlier observations (Krishnapillai, 1981). TABLE 5 — pH variation with time in the treated soils {Incubation experiment) Extractant Treatment Number of days 1 6 8 12 19 26 33 40 47 62 KCI (0.2N) Urea Amm. sulphate Control 4.25 3.95 3.85 4.33 3.95 3.77 4.30 3.94 3.78 A.n 3.95 3.76 4.24 4.00 3.80 4.03 3.90 3.75 4.17 4.06 3.90 3.83 3.87 3.82 4.05 0.04 3.96 3.92 3.95 3.96 H.O Urea Amm. sulphate Control 5.20 4.15 4.35 5.18 4.24 4.39 5.10 4.20 4.31 5.05 4.22 4.33 4.90 4.26 4.36 4.67 4.17 4.27 4.54 4.28 4.40 4.37 4.33 4.31 4.23 4.27 4.42 4.00 5.15 4.35 The effect of ureolytic pH increases and subsequent nitrification pH drop could be clearly seen from the Ca concentration (Table 5) in the water extract of the urea treated sample, which shows negligible water extractable Ca (Table 6) up to the 40th day and an increase thereafter, whereas in the ammonium treated samples the water extractable was much more than the KCI extractable Ca. In this case, the controlling factor for the Ca concentration in the extract being probably the solubility of CaSO t rather than the CEC of the soil. In this incubation study the behaviour of the other cations were not studied. ( 114 ) TABLE 6 — Change in Calcium ion concentration* (jJ-g/g dry soil) in treated soils with time Extractant Treatment Number of days 0 * 12 19 26 33 40 47 62 Urea 6 13 54 27 7 10 10 5 24 KC1 Amm. sulphate 6 6 30 30 4 6 7 4 4 Control 6 6 31 10 4 5 7 4 4 Urea 12 0 9 0 9 3 21 29 74 H ,0 Amm. sulphate 12 27 37 32 29 9 41 57 65 Control 12 12 25 11 13 17 23 18 26 * Each value is an average of triplicate determinations. ' To summarise the results, the application of urea and ammonium sulphate in the field caused 1. Ureolytic pH increase 2. pH decrease during nitrification 3. Change in cation exchange capacity due to pH change The result being a large fluctuation in the concentration of ions Ca and Mg and pH depending on the type of transformation involved at a given time, whereas the monovalent cations were affected to a lesser degree than divalent ions in accor­ dance with the predictions from the Donnan equilibrium. The implication of this being a larger leaching of Ca and Mg ions, with the application of ammonium sul­ phate. In the case of urea, the temporary high pH phase due to ureolysis caused less cations go into the soil solution, but subsequent rapid nitrification liberates them again into the soil solution. The pH rise was apparent in the soil incubation, but measureable changes in the pH was not observed in the solution even at the depth of 20 cm under field conditions. The release of Ca was an indicator of the nitrogen transformation occurring in the surface soil at a given time. ACKNOWLEDGEMENTS The authors express their gratitude to Mr. S. Selvanathan for his assistance in doing the incubation trial, and Mr. M. A. Wijedasa for his help in the analytical work. ( " 5 ) REFERENCES D E A L W I S , K . A . , JAYASOORIYA, S. G . and PERERA , M. B . A. Soils of St Coombs Estate. Tea. Q. 4 9 (1), 5-24. 1 — Environmental and Morphological Characteristics. Tea. Q. 4 9 , (1980). D O U G L A S , L. A. and BREMNER, J . M. (1970). Extraction and Colorimetric Determi­ nation of Urea in Soils. Soils. Sci. Soc. Am. Proc. 3 4 , 859-862. G O L D E N , D . C , SIVASUBRAMANIAM S. and W U E D A S A , M. A. (1981). Inhibitory Effect of Commercial Potassium Chloride on the Nitrification Rate of added Ammonium Sulphate in an Acid Red Yellow Podzolic Soil. Plant and Soil. 5 9 , 147-151. - G O L D E N , D . C , W E E D , S. B . , SIVASUBRAMANIAM , S. and N A L L I A H , P. (1981). Studies on the Performance of Two Phosphatic Fertilizer on Tea Soils with respect to the P-Adsorption, Mineralogy and Plant Species using M P labelled Fertilizer. Tea. 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