Research Article IJAAER (2017); 3(1): 86-105 International Journal of Agricultural and Environmental Research FREE AND OPEN ACCESS Available online at www.ijaaer.com ISSN 2414-8245 (Online) ISSN 2518-6116 (Print) ASSESSMENT OF SOIL FERTILITY STATUS OF NATIONAL RICE RESEARCH PROGRAM, HARDINATH, DHANUSHA, NEPAL DINESH KHADKA 1, SUSHIL LAMICHHANE 1, DEV N. TIWARI 2 AND KULANAND MISHRA 2 1 Soil Science Division, NARC, Khumaltar, Lalitpur, Nepal 2 National Rice Research Program, NARC, Hardinath, Dhanusha, Nepal Corresponding author s email: dinesh.khadka92@gmail.com Abstract Soil fertility evaluation is most basic decision making tool for effective sustainable plan of a particular area. Thus, the present study was carried out to evaluate the soil fertility status of the National Rice Research Program (NRRP), Hardinath, Dhanusha, Nepal. The study area is situated at the latitude 26 0 47 46.5 N and longitude 85 0 57 49.35 E as well 75masl altitude. A total of 50 soil samples were randomly collected based on the variability of land at a depth of 0-20 cm. A GPS device was used to identify the location of the soil sampling points. Soil samples were analyzed for texture, ph, OM, N, P 2O 5, K 2O, Ca, Mg, S, B, Fe, Zn, Cu and Mn status following standard analytic methods in the laboratory of Soil Science Division, Khumaltar. The Arc-GIS 10.1 was used to prepare the soil fertility status maps. The data revealed that soil structure was sub-angular blocky and grayish brown in colour. The sand, silt and clay content were 37.17±2.3%, 42.0±1.40% and 20.83±1.28%, respectively and varied six textural classes as loam, clay loam, sandy loam, silt loam, silty clay and silty clay loam. The soil ph (6.59±0.11) was nearly neutral and very low in available sulphur (0.79± 0.06 ppm) and available boron (0.37±0.05 ppm). The organic matter (1.02±0.007%), total nitrogen (0.07±0.002 %), extractable potassium (42.49±2.52 ppm), available zinc (0.83±0.21 ppm) and available manganese (6.75±0.64 ppm) status were low. While, extractable calcium (1120.80±67.30 ppm) and available copper (0.89±0.08 ppm) were medium in status. Similarly, extractable magnesium (207.60±21.10 ppm) and available phosphorus (30.40±2.93 ppm) status were high. Furthermore, available iron (57.79±5.95 ppm) contains very high in status. From this study, it can be concluded that for enhancing efficacy of the rice research, future research strategy should be built based on the soil fertility status of the research farm. Key words: Nutrient management; Rice research; Soil fertility maps and Sustainable planning INTRODUCTION Soil, is a natural dynamic body acts as a medium for growth and development of plants (Brady and Weil, 2004). Soil fertility is an unseen factor plays important role for making soil alive. Among the various challenges in soil system, soil fertility improvement has become major concerned day to day. Evaluation of soil fertility is now becoming a routine work for sustainable soil management and crop production. There are various techniques for soil fertility evaluation, among them soil testing is an indispensible tool in soil fertility management for sustained soil productivity (Havlin et al., 2010). Soil analysis is helpful for better understanding of the soils Citation: Khadka, D., S. Lamichhane, D.N. Tiwari and K. Mishra. 2017. Assessment of soil fertility status of national rice research program, hardinath, dhanusha, Nepal. Int. J. Agri and Env. Res., 3(1): 86-105.
to increase the crop production and obtaining sustainable yield. The different soil physical parameters like texture, structure, colour etc. and chemical parameters as soil reaction (ph), organic matter, macro and micronutrients etc. are key components of soil fertility. The determination of these properties is prerequisite for assess knowledge about soil fertility and determined after testing in the laboratory. Spatial variation across a field become great challenge for assesses soil fertility of an area. Describing the spatial variability of soil fertility across a field has been difficult until new technologies such as Global Positioning Systems (GPS) and Geographic Information Systems (GIS) were introduced. GIS is a powerful set of tools for collecting, storing, retrieving, transforming and displaying spatial data (Burrough and McDonnell, 1998). Nepal Agricultural Research Council (NARC) was established to strengthen agriculture sector in the country through agriculture research. National Rice Research Program (NRRP), Hardinath, Dhanusha, Nepal is an important wing among the research farms of NARC, in order to generate appropriate rice crop production technologies for the Nepal. Studies related to the soil fertility status of National Rice Research Program (NRRP), Hardinath, Dhanusha, Nepal are scant. Therefore, it is important to investigate the soil fertility status and may provide valuable information relating rice research. Keeping these facts, the present study was conducted with the objectives: i) assessment of the different physical (texture, structure and colour) and chemical (ph, organic matter, macro and micronutrients) parameters, ii) preparation of fertility maps of the studied soil parameters of National Rice Research Program (NRRP), Hardinath, Dhanusha, Nepal. MATERIALS AND METHODS Study area: The study was carried out at National Rice Research Program, Hardinath, Dhanusha, Nepal (Figure 1). The research farm is situated at a latitude 26 0 47 46.5 N and longitude 85 0 57 49.35 E as well altitude 75masl. Soil sampling: Surface soil samples (0-20 cm depth) were collected from National Rice Research Program, Hardinath, Dhanusha, Nepal during 2015. A total of 50 soil samples were collected from the research farm (Figure 2). The exact locations of the samples were recorded using a handheld GPS receiver. The random method based on the variability of the land was used for soil sample collection. Laboratory analysis: The collected soil samples were analyzed at laboratory of Soil Science Division, Khumaltar. The different soil parameters tested as well as methods adopted to analyze is shown on the Table 1. Table 1. Parameters and methods adopted for the laboratory analysis at Soil Science Division, Khumaltar S.N. Parameters Methods 1 Soil texture Hydrometer (Bouyoucos, 1927) 2. Soil colour Munshell-colour chart 3. Soil structure Field-feel 4. Soil ph Potentiometric 1:2 (Jackson, 1973) 5. Soil organic matter % Walkely and Black (Walkely and Black, 1934) 6. Total N % Kjeldahl (Bremner and Mulvaney, 1982) 7. Available P2O5 ppm Olsen (Olsen et al., 1954) 8. Extractable K2O ppm Ammonium acetate (Jackson, 1967) 9. Extractable Ca and Mg ppm EDTA Titration (El Mahi, et.al.,1987) 11. Available S ppm Turbidimetric (Verma, 1977) 12. Available B ppm Hot water (Berger and Truog, 1939) 13. Available Fe ppm DTPA (Lindsay and Norvell, 1978) 14. Available Zn ppm DTPA (Lindsay and Norvell, 1978) 15. Available Mn ppm DTPA (Lindsay and Norvell, 1978) 16. Available Cu ppm DTPA (Lindsay and Norvell, 1978) 87
Figure 1. Location Map of National Rice Research Program, Hardinath, Dhanusha, Nepal Figure 2. Distribution of soil sample points during soil sampling 88
Statistical analysis: Descriptive statistics (mean, range, standard deviation, standard error, coefficient of variation) of soil parameters were computed using the Minitab 15 package. Rating (very low, low, medium, high and very high) of determined values were based on Soil Science Division, Khumaltar. The coefficient of variation was ranked according to the procedure of (Aweto, 1982) where, CV 25% = low variation, CV >25 50% = moderate variation, CV >50% = high variation. Arc Map 10.1 with geostatistical analyst extension of Arc GIS software was used to prepare soil fertility maps, while interpolation method employed was ordinary kriging with stable semi-variogram. Similarly, the nutrient index was also determined by the formula given by Ramamoorthy and Bajaj (1969). Nutrient index (N.I.) = (N L 1 + N M 2 + N H 3) / N T Where, N L, N M and N H are number of samples falling in low, medium and high classes of nutrient status, respectively and N T is total number of samples analyzed for a given area. Similarly, interpretation was done as value given by Ramamoorthy shown on the Table 2. Table 2. Rating Chart of Nutrient index S.N. Nutrient Index Value 1. Low <1.67 2. Medium 1.67-2.33 3. High >2.33 RESULT AND DISCUSSION The soil fertility status of the study area was evaluated with respect to texture, colour, structure, ph, organic matter, primary nutrients, secondary nutrients and micronutrients such as B, Fe, Zn, Cu, and Mn, and the results obtained are presented and discussed in the following headings. Soil texture: Soil texture determines a number of physical and chemical properties of soils. It affects the infiltration and retention of water, soil aeration, absorption of nutrients, microbial activities, tillage and irrigation practices (Gupta, 2004). The results in Figure 3 shows that soil texture of the study area was predominantly loam type, while others clay loam, sandy loam, silt loam, silty clay and silty clay loam also present. The % sand were ranged from 5.0 to 67.6 % with a mean of 37.17 % and that of % silt were 20.5 to 64.5 % with a mean of 42.0% while the range of % clay were 9.6 to 40.8 % with a mean of 20.83% (Table 3). The coefficients of variation between the soil samples were 43.79%, 23.63% and 43.30% for sand, silt and clay contents, respectively. Table 3. Soil Separates Status of National Rice Research Program, Hardinath, Dhanusha, Nepal Soil Separates Descriptive Statistics Sand Silt Clay Mean 37.17 42.00 20.83 SE Mean 2.30 1.40 1.28 StDev 16.28 9.93 9.02 Minimum 5.00 20.80 9.60 Maximum 67.60 64.50 40.80 CV% 43.79 23.63 43.30 % 89
Figure 3. Soil texture status of National Rice Research Program, Hardinath, Dhanusha, Nepal Soil colour: Soil colour reflects on the transformation and translocation occurred in the soil due to chemical, biological and physical attributes (Ponnamperuma and Deturck, 1993). It shows water drainage, aeration and organic matter content in soil. In the majority of the study area, grayish brown coloured soil was observed. Soil structure: Soil structure refers to the size, shape and arrangement of solids and voids, continuity of pores and voids, their capacity to retain and transmit fluids and organic and inorganic substances, and ability to support vigorous root growth and development (Lal, 1991). In the majority of the area, sub-angular blocky structured soil was detected. Table 4. Soil fertility status of National Rice Research Program, Hardinath, Dhanusha, Nepal Soil Fertility Parameters Descriptive Statistics ph OM N P2O5 K2O % ppm Mean 6.59 1.02 0.07 30.40 41.95 SE Mean 0.11 0.07 0.002 2.93 2.52 StDev 0.77 0.51 0.01 20.69 17.83 Minimum 4.60 0.13 0.04 0.51 12.00 Maximum 8.26 2.54 0.11 103.03 87.60 CV% 11.74 50.61 21.78 68.06 42.49 90
Soil reaction (ph): Soil ph has a dominant effect on availability of plant essential nutrients (Clark and Baligar, 2000). The ph of soil was ranged from 4.6 to 8.26 with the mean value of 6.59 (Table 4). The results have shown that soil ph was nearly neutral (Figure 4). For normal growth, a ph range 5.5-8.0 is suitable for rice growth. Therefore, observed ph is suitable for rice cultivation in the majority of the area. Soil ph showed low variability (11.74%) in the investigated soil samples. Figure 4. Soil ph status of National Rice Research Program, Hardinath, Dhanusha, Nepal Soil organic matter: Soil organic matter is a complex mixture which contributes positively to soil fertility, soil tilth, crop production, and overall soil sustainability. It minimizes negative environmental impacts, and thus improves soil quality (Farquharson et al., 2003). The organic matter content was varied from 0.13 to 2.54% with a mean value of 1.02% (Table 4).It indicates that the organic matter content was low (Figure 5; Table 7). Therefore, incorporation of organic matter adding materials is important for improvement of organic matter in the soils. Organic matter showed high variability (50.61%) in the investigated soil samples. 91
Figure 5. Organic matter status of National Rice Research Program, Hardinath, Dhanusha, Nepal Total nitrogen: Nitrogen (N) is essential for rice, and usually it is the most yield-limiting nutrient in irrigated rice production around the world (Ladha and Reddy, 2003; Samonte et al., 2006). It is pivotal in yield realization of rice. The total nitrogen content was ranged from 0.04 to 0.11% with the mean value of 0.07% (Table 4). This indicates low status of total nitrogen (Figure 6; Table 7). There may have high response of nitrogen application on rice cultivation. Therefore, proper method of nitrogen management is crucial for reducing nitrogen mining in soils. Low variability (21.78%) in total nitrogen was observed among the sampled soils. Available phosphorus:phosphorus is known as the master key to agriculture because lack of available P in the soils limits the growth of both cultivated and uncultivated plants (Foth and Ellis, 1997). Rice, like any other cereal, requires a considerable quantity of phosphorus for vigorous growth and high grain yield. The available phosphorus was ranged from 0.51 to 103.03 ppm with the mean value of 30.40 ppm (Table 4). This showed high status of available phosphorus (Figure 7; Table 7). There may possibility of low response of applied phosphorus on rice cultivation. The variation in the available phosphorus of the soil is high, with coefficients of variation of 68.06%. Extractable potassium: Potassium (K) is an essential nutrient that affects most of the biochemical and physiological processes that influence plant growth 92
and metabolism (Wang et al., 2013).The extractable potassium content was ranged from 12.0 to 87.6 ppm with a mean value of 41.95 ppm (Table 4). This indicates low status of extractable potassium (Figure 8; Table 7). There may possibility of high response of potassium application on rice cultivation. Therefore, different organic and inorganic sources of materials should be essential for adequate supply of potassium. Moderate variability (42.49%) in extractable potassium was observed among the soil samples. Figure 6. Total nitrogen status of National Rice Research Program, Hardinath, Dhanusha, Nepal 93
Figure 7. Available phosphorus status of National Rice Research Program, Hardinath, Dhanusha, Nepal Table 5. Soil fertility status of National Rice Research Program, Hardinath, Dhanusha, Nepal Soil Fertility Parameters Descriptive Statistics Ca Mg S B ppm Mean 1120.80 207.60 0.79 0.37 SE Mean 67.30 21.10 0.06 0.05 StDev 475.60 149.10 0.45 0.33 Minimum 200.00 1.20 0.10 0.003 Maximum 2002.00 720.00 2.15 1.33 CV% 42.44 71.84 57.19 87.84 94
Figure 8. Extractable potassium status of National Rice Research Program, Hardinath, Dhanusha, Nepal Extractable calcium Calcium plays a pre-dominant role in the composition of cell wall and protoplasm. It has been associated with carbohydrates and various organic acids (Mahajan and Billore, 2014). The extractable calcium content was ranged from 200 to 2002 ppm with the mean value of 1120.80 ppm (Table 5).In overall, medium status of extractable calcium was observed (Figure 9; Table 7). Moderate variability (42.44%) in extractable calcium was observed among the soil samples. Extractable magnesium: Magnesium is a water soluble Cation and it is necessary for chlorophyll pigment in green plants (Mahajan and Billore, 2014). The extractable magnesium content was ranged from 1.2 to 720 ppm with the mean value of 107.60 ppm (Table 5). This revealed high content of extractable magnesium (Figure 10; Table 7). The variation in the extractable magnesium of the soil is high, with coefficients of variation of 71.84%. Available sulphur: Sulphur is an essential plant nutrient and plays a vital role in the synthesis of amino acids (methionine, cystein and cystine), proteins, 95
Figure 9. Extractable calcium status of National Rice Research Program, Hardinath, Dhanusha, Nepal 96
Figure 10. Extractable magnesium status of National Rice Research Program, Hardinath, Dhanusha, Nepal chlorophyll and certain vitamins (Tiwari and Gupta, 2006).The available sulphur was varied from 0.10 to 2.15 ppm with a mean value of 0.79 ppm (Table 5). In overall, available sulphur was very low in status (Figure 11; Table 7). There may have possibility of high response of sulphur application on rice cultivation. Therefore, regularly sulphur rich organic and inorganic sources of materials should be incorporated for supplement sulphur in adequate amounts. Available sulphur showed high variability (57.19%) in the soil samples. 97
Figure 11. Available sulphur status of National Rice Research Program, Hardinath, Dhanusha, Nepal Available boron Boron is one of two non-metal micronutrients required by plants for their cell wall structural integrity (Havlin et al., 2010). The available boron content was ranged from 0.003 to 1.33 ppm with a mean value of 0.37 ppm (Table 5). This indicates very low content of available 98
boron (Figure 12; Table 7). Due to this, there may have very high chance of boron deficiency stress on rice growth and development. Plants fail to produce panicles if they are affected by B deficiency at the panicle formation stage. Similarly, tips of emerging leaves are white and rolled (Das, 2000). Therefore, regularly boron rich organic and inorganic sources of materials should be incorporated for supply boron in adequate amounts. Moderate variability (87.84%) in available boron was observed among the soil samples. Figure 12. Available boron status of National Rice Research Program, Hardinath, Dhanusha, Nepal Table 6. Soil Fertility Status of National Rice Research Program, Hardinath, Dhanusha, Nepal Soil Fertility Parameters Fe Zn Cu Mn Descriptive Statistics ppm Mean 57.79 0.83 0.89 6.75 SE Mean 5.95 0.21 0.08 0.64 StDev 42.06 1.47 0.55 4.50 Minimum 10.01 0.02 0.28 1.06 Maximum 209.82 7.32 2.10 19.70 CV% 72.78 175.87 61.83 66.60 99
Available iron Iron is the fourth most abundant element, comprising about 5% of the earth s crust. In plants, it functions both as a structural component and as a co-factor for enzymatic reactions (Das, 2000). The available iron content was ranged from 10.01 to 209.82 ppm with the mean value of 57.79 ppm (Table 6). This indicates very high content of available iron (Figure 13; Table 7). There may have high probability of iron toxicity stress on rice due to very high content of iron on soils, called bronzing in rice. The main important toxicity symptoms of iron are tiny brown spots on lower leaves starting from tip and spread towards the leaf, base or whole leaf coloured orange-yellow to brown. Spots combine on leaf inter-veins and leave turn orange brown and die. Similarly, If Fe toxicity is severe stunted growth extremely limited tillering. The different Iron toxicity controlling methods like seed treatment with Ca peroxide at 50% - 100% seed wt., intermittent irrigation at tillering stage and by balanced fertilization are advisable. Available iron showed high variability (72.78%) among the soil samples. Figure 13. Available iron status of National Rice Research Program, Hardinath, Dhanusha, Nepal 100
Available zinc Zinc is one of the eight trace elements which are essential for the normal healthy growth and reproduction of crop plants (Alloway, 2008). The available zinc was varied from 0.02 to 7.32 ppm with the mean value of 0.83 ppm (Table 6). In overall, available zinc was low in status (Figure 14; Table 7). There may have possibility of zinc deficiency disorder in rice known as Khaira at young age usually in nursery. Therefore, regularly zinc rich organic and inorganic sources of materials should be incorporated for adequate supply zinc for rice. The available zinc showed high variability (175.87%) among the soil samples. Figure 14. Available zinc status of National Rice Research Program, Hardinath, Dhanusha, Nepal Available copper: Copper is one of the oldest known metals. It is the 25 th most abundant element in the Earth s crust. The available copper content was ranged from 0.28 to 2.1 ppm with the mean value of 0.89 ppm (Table 6). This revealed medium status of available copper (Figure 15; Table 7). High variability (61.83%) in available copper was observed among the soil samples. 101
Figure 15. Available copper status of National Rice Research Program, Hardinath, Dhanusha, Nepal Available manganese: Manganese is the 10 th most abundant element on the surface of the earth. It is involved in many biochemical functions, primarily acting as an activator of enzymes such as dehydrogenases, transferases, hydroxylases, and decarboxylases (Graham, 1983; Burnell, 1988). The available manganese content was ranged from 1.06 to 19.70 ppm with the mean value of 6.75 ppm (Table 6). This indicates low content of available manganese (Figure 16; Table 7). Therefore, regularly manganese rich organic and inorganic sources of materials should be incorporated for supply manganese in adequate amounts. The available manganese showed high variability (66.60%) among the soil samples. 102
Figure 16. Available manganese status of National Rice Research Program, Hardinath, Dhanusha, Nepal Table 7. Nutrient indices of studied parameters of National Rice Research Program, Hardinath, Dhanusha, Nepal % Sample category S.N. Parameters VL L M H VH NI Remarks 1. OM 24 62 14 6-1.14 Low 2. N - 42 58 - - 1.58 Low 3. P 2O 5 10 12 20 38 20 2.36 High 4. K 2O 22 48 30 - - 1.3 Low 5. Ca - 44 54 2-1.58 Low 6. Mg - 20 26 54-2.34 High 7. S 100 - - - - 1.0 Low 8. B 22 64 10 4-1.18 Low 9. Fe - - 6 12 82 2.94 High 10. Zn 14 68 12 2 4 1.24 Low 11. Cu 10 46 18 26-1.7 Medium 12. Mn 28 42 18 12-1.42 Low *VL= Very low; L= Low; M= Medium; H= High, VH=Very High; NI= Nutrient Index 103
CONCLUSION Overall, the soil structure was sub-angular blocky and grayish brown in colour. The soil was neutral in reaction. The boron and sulphur status were very low. The organic matters, nitrogen, potassium, zinc and manganese contains low in status. The calcium and copper were medium in status. The phosphorus and magnesium contains high in status. The iron was very high in status. The crop may suffer from deficiency of low and toxicity of very high plant available nutrients. Thus, proper nutrient management strategy should be adopted especially for these nutrients. Considering low status of organic matter, the practices like manure or compost incorporation, crop residue retention, green manuring etc. can be suggested for its improvement. For enhancing research efficacy of the rice, future research strategy should be built based on the soil fertility status of the farm. ACKNOWLEDGEMENT The authors would like to acknowledge Nepal Agricultural Research Council (NARC) for funding this research. We are very much thankful to National Rice Research Program, Hardinath, Dhanusha, Nepal for their cooperation. Similarly, support of the Soil Science Division, Khumaltar, Lalitpur, Nepal for providing laboratory facilities to analyze the soil samples as well as other technical support is highly acclaimed. REFERENCES Alloway, B.J. 2008. Zinc in soils and crop nutrition, 2 nd edition, International zinc association Brussels, Belgium, 139p. Aweto, A.O. 1982. Variability of upper slope soils developed under sandstones in South-western Nigeria. Georg. J., 25: 27-37. Berger, K.C. and E. Truog. 1939. Boron determination in soils and plants. Ind. Eng. Anal. Ed.; 11: 540 545. Bouyoucos, G. J. 1962. Hydrometer method improved for making particle-size analysis of soils. Agron. J., 53: 464-465. Brady, N.C. and R.R. Weil. 2002. The nature and properties of soils, 13 th edition, Pearson Education, New Jersey. Bremner, J.M. and C.S. Mulvaney. 1982. Nitrogen total, In: Page, A.L. (editor) Methods of soil analysis. Agron. No. 9, Part 2: Chemical and microbiological properties, 2 nd edition, Am. Soc. Agron., Madison, WI, USA, 595-624. Burnell, J.N. 1988. The biochemistry of manganese in plants, in: Graham R.D., Hannam R.J., Uren N.C. (eds.) Manganese in Soils and Plants, Kluwer Academic Publishers, Dordrecht, The Netherland, 125 137. Burrough, P.A. and R.A. McDonnell. 1998. Principle of geographic information systems, Oxford: Oxford University Press. Clark, R.B. and V.C. Baligar. 2000. Acidic and alkaline soil constraints on plant mineral nutrition, In: R.E. Wilkinson (ed.) Plant- Environment Interactions. Marcel Dekker Inc, New York, pp. 133-177. Das, D.K. 2000. Micronutrients: their behavior in soils and plants, Kalyani Publishers, New Delhi, pp. 113 121. El Mahi, Y., I.S. Ibrahim, H.M. AbdelMajid, and A.M. Eltilib. 1987. A simple method for determination of calcium and magnesium carbonate in soils. Soil Sci. Soc. Am. J., 51: 1152-1155. Foth, H.D. and B.G. Ellis. 1997. Soil fertility, 2 nd edition, Lewis CRC Press LLC., USA, 290p. Francioson O., C. Ciavatta, S. Sanche-Cortes, V. Tugnoli, L. Sitti and C. Gessa. 2000. Spectroscopic characterization of soil organic matter in long-term amendments trials. Soil Sci., 165: 495-504. Goovaerts, P. 1998. Geo-statistical tools for characterizing the spatial variability of microbiological and physicochemical soil properties. Biol. Fertil., Soil, 27: 315-344. Graham, R.D. 1983. Effect of nutrient stress on susceptibility of plants to disease with particular reference to the trace elements. Adv. Bot. Res., 10: 221-276. Gupta, P.K. 2004. Soil, plant, water and fertilizer analysis, Shyam Printing Press, Agrobios, India, 38p. Havlin, H.L., J.D. Beaton, S.L. Tisdale and W.L. Nelson. 2010. Soil Fertility and Fertilizers- an introduction to nutrient management. 7 th edition, PHI Learning Private Limited, New Delhi. Jackson, M.L. 1973. Soil chemical analysis, Prentice 104
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