1 THE "NITRATE NITROGEN ELECTRICAL CONDUCTIVITY" RELATIONSHIP IN NON-SALINE SOILS UNDER FERTIGATION LA CONDUCTANCE ELECTRIQUE DU NITRAT DE NITROGENE EN RAPPORT AVEC DES SOLS NON-SALES SOUS LA FUMURE Kouman S. Koumanov, Georgi P. Stoilov and Docho V. Dochev Institute of Fruit Growing, 12 Ostromila, Plovdiv 4004, Bulgaria E-mail: ksoumanov@hotmail.com Abstract The subject of present article is the evaluation of nitrate nitrogen (N NO 3 ) concentrations from values of soil electrical conductivity in non-saline soils. The results presented are based on data from a lysimetric study on the migration and localization of fertilizers under fertigation of peach trees in three soils: alluvial meadow soil (Fluvisol) sandy loam, cinnamon forest soil (Luvisol) clay loam, and smolnitsa (Vertisol) clay. The experimental trees were supplied with water and fertilizers urea (CO(NH 2 ) 2 ) and phosphoric acid (H 3 PO 4 ), through a drip-irrigation system. The investigation objectives were addressed from laboratory analyses of soil samples taken in profiles through the wetted soil volume 20 hours after water applications. The content of nitrate nitrogen in soil samples was determined by the distillation method of Cotte und Kahane (1946). The zones and the levels of salt accumulation were determined indirectly by measuring the electrical conductivity of soil samples after adding of distilled water (1:1) and thirty-minute stirring on a shuttle. The zones of salt accumulation were found to coincide (by location, shape, and size) with the areas of increased nitrate content. After analyzing of 500 soil samples, it was found a strong correlation between the investigated characteristics (R=0.88). The regression equation derived was: y = 84.801x 2 10.059x. It allows the nitrate nitrogen concentrations (y, mg.kg 1 ) in non-saline soils to be evaluated from the values of soil electrical conductivity (x, ds.m 1 ), irrespective of soil type and with a satisfying, for practical purposes, exactness. Key words: soil electrical conductivity, nitrate nitrogen, drip-irrigation, fertigation, lysimeters. Introduction Soil nitrate content is an important parameter for soil status characterizing. Usually the nitrate-nitrogen concentrations are estimated through laboratory analyses of soil samples, which require expenditure of more or less labor, time, consumables and supplies. Subject of present publication is the evaluation of nitrate nitrogen (N NO 3 ) content in non-saline soils from the electrical conductivity (EC) values as far as the measurement of the EC is much more easier and cheaper. The results presented are part of a larger project on the migration and localization of fertilizers under fertigation, carried out at the Institute of Fruit Growing in Plovdiv during the period 1994 1997 (Koumanov et al., 1998; Stoilov et al., 1999a,b). Methods and material The results presented are based on data from a lysimetric study on three soils: alluvial meadow soil (Fluvisol), cinnamon forest soil (Luvisol), and smolnitsa (Vertisol). Texturally they are determined respectively as sandy loam, clay loam, and clay, after the USDAclassification (Soil Survey Staff, 1975). The values of some soil characteristics are given in Table 1.
2 Table 1 Physical and chemical properties of the investigated soils Soils Soil properties Alluvialmeadoforest Cinnamon- Smolnitsa Particle density, g/cm 3 2.7 2.6 2.7 Bulk density, g/cm 3 1.43 1.19 1.23 Porosity, % 47.0 54.2 54.4 Field capacity, kg/kg 0.16 0.24 0.35 Content of sand * (2 0.05 mm), % 64.8 40.0 34.7 Content of silt * (0.05 0.002 mm), % 24.3 25.8 14.2 Content of clay * (< 0.002 mm), % 10.9 34.2 51.1 Soil reaction, ph 7.9 7.5 7.3 Humus content, % 0.9 1.0 2.2 Mobile phosphorus (P 2 O 5 ), mg/100g 4.5 1.4 4.7 Mobile potassium K 2 O, mg/100g 9.4 7.8 10.0 Exchange capacity, mgeq/100g 25.4 47.4 62.1 * According to the USDA classification (Soil Survey Staff, 1975). In the spring of 1994, single peach trees (cultivar Redhaven on GF-677 rootstock) were planted in every one of the lysimetric cells. The experimental trees were supplied with water and fertilizers urea (CO(NH 2 ) 2 ) and phosphoric acid (H 3 PO 4 ), through a drip-irrigation system: one emitter per tree, with an average discharge of 4.6 l/h, and located 0.75 m apart from the tree trunk. Irrigation was scheduled on the basis of evapotranspiration values (ET) calculated in function of the average daily temperatures by the method of biophysical coefficients (Davidov and Gaidarova, 1983; Sharma, 1985): ET = ZΣt av.d. (1) where Z is biophysical coefficient and Σt av.d. is sum of the average daily temperatures for the period of consideration, usually ten days. The values of Z for peach in Plovdiv Region were adopted from Dochev (1972). The irrigation was realized on a daily basis, five days in a week except Saturdays and Sundays. The annual application rates per single plant and their partitioning by months are shown in Table 2. Urea [CO(NH 2 ) 2 ] and phosphoric acid (H 3 PO 4 ) were used to provide annual fertilization rates of 165 g N/tree (100 kg/ha ) and 84 g P 2 O 5 /tree (50 kg/ha). The annual fertilizer amounts were partitioned to monthly doses according to the scheme presented in Table 3. The nitrogen application was conformed to the average monthly values of peach evapotranspiration while the phosphorus was uniformly distributed along the vegetation period. Nutrient solution was injected in the irrigation system by an automatic dosing pump (DOSATRON INTERNATIONAL, Bordeaux, France) thus providing concentrations in the irrigation water of respectively 0.4 % CO(NH 2 ) 2 and 0.9 % H 3 PO 4.
3 Table 2 Annual application rates (liters per tree) and their distribution during the months of the irrigation season. Year Application Monthly parts of the application rate rate April May June July August September liters/tree liters/tree 1995 2228 420 540 542 436 290 1996 1890 39 100 241 754 608 148 1997 2330 432 701 1050 147* * The experiment was ceased on 02.08.1997. Table 3 Fertilization rates (grams/tree) supplied in each lysimeter and their partitioning during the months of the irrigation season (in % from the total) Year Fertilizers Fertilization Monthly parts of the fertilization rate, % rate, g/tree April May June July Aug. Sept. Envisaged N 165.0 12 18 28 18 14 10 scheme P 2 O 5 84.0 17 17 17 17 16 16 1995 N 115.5 28 18 14 10 P 2 O 5 55.4 17 17 16 16 1996 N 165.0 12 18 28 18 14 10 P 2 O 5 84.0 17 17 17 17 16 16 1997 N 125.4 12 18 28 18* P 2 O 5 57.1 17 17 17 17* * The experiment was ceased on 02.08.1997. The investigation objectives were addressed from laboratory analyses of soil samples taken 20 hours after the consecutive water application. In 1995 and 1996, soil samples were taken by drilling at radial distances of 10 cm, 25 cm, 50 cm, 75 cm and 100 cm from the dripper and by layers of 10 cm. In 1995 soil sampling was done weekly - four times after the maximal fertilization dose, and to 30 cm in depth. In 1996 it was done three weeks after the maximal fertilization doze, to depth of 80 cm. The average soil quantity provided by one sample was about 250 g. In 1997 soil samples were taken from a soil profile in 10-centimeter square grid, after digging a trench along the line tree dripper. In this case each of the soil samples weighed approximately 1000 g. The zones and the levels of salt accumulation were determined indirectly by measurement the electrical conductivity of soil samples after adding of distilled water (1:1) and thirty-minute stirring on a shuttle. The content of nitrate nitrogen (N NO 3 ) in the soil samples was determined by the distillation method of Cotte und Kahane (1946). Results and discussion The spatial distribution of nitrate nitrogen in the three soils as well as that of the soil electrical conductivity is shown on Figures 1, 2, and 3. The juxtaposition between the zones of salt accumulation and the areas with increased content of nitrate nitrogen revealed an impressive coincidence by location, by shape and by size.
4 03.07.95 03.07.95 10.07.96 10.07.96 Fig. 1 Fields of the electrical conductivity (µs.cm 1 ) left, and the nitrate concentration (mg.kg 1 ) right, in alluvial-meadow soil. The established relationship was subjected to correlation and regression analyses based on the data for 90 soil samples from the alluvial-meadow soil, 212 from the cinnamon-forest soil, and 198 from the smolnitsa, taken in the period 1995-1997. The obtained results are presented on Figure 4. The derived regression equation is of second degree and has the following form: y = 84.801 x 2-10.059 x, (2) where x is the soil electrical conductivity in ds/m, and y is the concentration of nitrate nitrogen in mg/kg. The high correlation ratio, R=0.88, proved a strong correlation between both characteristics. Thus, the concentration of nitrate nitrogen in non-saline soils can be estimated directly from soil electrical conductivity values, irrespective of the soil type and with a satisfying, for practical purposes, exactness.
Fig. 2 Fields of the electrical conductivity (µs.cm 1 ) and the nitrate concentration (mg.kg 1 ) in cinnamon forest soil. 5 a) Soil electrical conductivity, µs.cm 1 b) Nitrate nitrogen (N NO 3 ), mg.kg 1 10.07.96 15.07.97 10.07.96 15.07.97
Fig. 3 Fields of the electrical conductivity (µs.cm 1 ) and the nitrate concentration (mg.kg 1 ) in smolnitsa. 6 a) Soil electrical conductivity, µs.cm 1 b) Nitrate nitrogen (N NO 3 ), mg.kg 1 10.07.96 15.07.97 10.07.96 15.07.97
7 350 (N NO 3 ) concentration, mg/kg 300 250 200 150 100 50 0 y = 84,801x 2-10,059x R 2 = 0,7801 R=0,88 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Electrical conductivity of the soil, ds/m Fig.4 Correlation between the soil electrical conductivity and the nitrate-nitrogen concentration. Conclusions The applied by the irrigation water urea and phosphoric acid have not induced unfavorable for the peach plants changes in the salt concentration of the soil solution. The zones of salt accumulation coincide (by location, shape, and size) with the areas with increased content of nitrate nitrogen. Based on the established strong correlation and respectively on the derived regression equation, the concentration of nitrate nitrogen in non-saline soils may be estimated through the soil electrical conductivity values, irrespective of the soil type and with satisfying for the practical purposes exactness. Acknowledgments This investigation was carried out with the financial support by the National Science Fund, Ministry of Education and Science, Republic of Bulgaria, as well as with the kind equipment support by the "Dosatron International", Bordeaux, France. References: Davidov, D. and Gaidarova, S. (1983): Vurkhu tochnostta na formulite za izchislyavane na evapotranspiratshiyata. (in Bulgarian) V pomoscht na tekhnicheskiya progres vuv vodnoto stopanstvo 6, 12-22. Dochev, D. (1972): Investigation on some biological and physiological manifestations of peach under irrigation. (Abstract in English) Ph.D. thesis, Institute of Fruit Research, Plovdiv, 318 pp. Koumanov, K., Dochev, D. and Stoilov, G. (1998): Investigations on fertigation of peach on three soil types patterns of soil wetting. Bulg. J. Agric. Sci., 4: 745-753. Sharma, M.L. (1985): Estimating evaporation. In: Hillel, D. (Ed.), Advances in Irrigation, 3:213-281. Soil Survey Staff, (1975):
Soil taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA - SCS Agricultural Handbook 436, U.S. Government Printing Office, Washington, D.C. Stoilov, G., Dochev, D. and Koumanov, K. (1999b): Investigations on fertigation of peach on three soils.ii.migration and localization of phosphorus. Bulg. J. Agric. Sci., 5: 615-620. Stoilov, G., Koumanov, K. and Dochev, D. (1999а): Investigations on fertigation of peach on three soils.i.migration and localization of nitrogen. Bulg. J. Agric. Sci., 5: 605-614. 8