Tropical Agricultural Research Vol. 14:224-232 (2002) Effect off Texture and Organic Matter on Soil Water Retention Parameters K.L.N. Rajapaksha, R.B. Mapa1 and A.R. Dassanayake2 Postgraduate Institute of Agriculture University of Peradeniya Peradeniya, Sri Lanka ABSTRACT. Soil water retention parameters such as field capacity, permanent wilting point (PWP) and available water are used in irrigation planning and management, soil water conservation, modelling of soil water and solute flow and in drought tolerance studies. The objective ofthis study was to characterize the soil water retention parameters for the soils ofthe Intermediate Zone (IZ) of Sri Lanka and to determine their relationship with texture and organic matter content. Field capacity, PWP, soil texture and organic matter contents were measured for 20 benchmark soil series from the IZ of Sri Lanka. The results indicated that the soils of the up and low country IZshowed moderate amounts of 'available water (180-120 mmlm) while soils of the mid country IZ showed low amounts of available water (< 120 mm/m) to crops. The field capacity and PWP increased with the increase of clay content but the rate of increase was not similar in all soil series. The organic matter content was positively related tofield capacity and PWP only in the topsoil. Therefore, increase in clay and organic matter content improved soil water retention thereby increasing soil and water conservation, but not necessarily the available water to plants. INTRODUCTION Agriculture is the single largest consumer of water amounting to 70% of the global freshwater resource. The water scarcity directly affects the rainfed and irrigated agriculture forcing millions of farmers to search for suitable lands and adequate water supplies for crop production. The high water requirement per unit of dry matter produced applies not only for crops but also to all plants for their transpiration (Kuruppuarachchi, 2001). Plants and soil micro-organisms depend on the water, which is retained in the soil. Knowledge in soil water retention is necessary for irrigation planing and management as well as for many soil related investigations such as, water conservation, modelling water and solute flow and evaluation of plant water stress (Saxton et al., 1985). When considering soil water retention, the field capacity (FC) and permanent wilting point (PWP) are the most important parameters. The field capacity is defined as the water retained after the drainage rate of a completely saturated soil becomes negligible (Hillel, 1971). The permanent wilting point is the soil moisture content at which plants wilt permanently. The water held between these two upper and lower limits is the water available to crops. This is used when estimating available water for irrigation requirements and soil aeration indexes as field air porosity (Bodinayake and Mapa, 1989). Field capacity depends on several factors such as soil texture, structure, type of clay present and organic matter content (Hillel, 1971). Many researchers have documented that the field capacity shows a positive relationship with clay content, clay mineralogy and organic matter contents (Uehara and Gilman, 1981). Sandy soils can quickly be recharged with soil moisture but is unable to hold as much water compared with soils of heavier textures. As texture becomes heavier, the wilting point increases because fine soils with narrow pore space hold water more tightly than soils with wide pore spacing. In soil classification studies, the PWP is estimated by dividing the clay content by 2.5 when such data is not available (Soil Survey Staff, 1992) indicating the close relationship between these two. Department of Soil Science, Faculty of Agriculture, University of Peradeniya, Peradeniya, Sri Lanka. Land Use Division, Irrigation Department, Colombo 7, Sri Lanka. Effect of Texture and Organic Matter Sri Lanka experience diverse agro-ecological conditions throughout the country. Based on the amount of rainfall received, the country is divided in to three major zones called the Dry Zone, Intermediate Zone and Wet Zone. The Intermediate Zone of Sri Lanka is the area receiving a mean annual rainfall of 1750-2500 mm which covers approximately 13.2% of the total land area amounting to 0.87 million ha. This zone is sub divided on the basis of elevation namely up country Intermediate (> 900 m), mid country Intermediate (900-300 m) and low country Intermediate Zone (< 300 m). The soil water retention parameters of the Intermediate Zone soils have not been studied in detail. Therefore, the objective of this study was to characterize the water retention parameters of the soils in the Intermediate Zone (IZ) of Sri Lanka and to determine its relationship with soil texture and organic matter content. MATERIALS AND METHODS This study was conducted using 20 benchmark soil series from the up-country, mid-country and low-country areas of intermediate zone of Sri Lanka (Nayakekorala and Mapa, 2002). The names of the soil series, their great soil groups according to De Alwis and Panabokke (1972) and the soil taxonomic equivalents are given in Table 1. Benchmark sites of these soil series where the soil samples were collected are shown in Fig. 1. The soil profiles were characterized according to the FAO system (1990) and the major horizons were identified. Soil samples were collected for measuring the soil water retention parameters, clay and organic matter contents from the surface and sub surface horizons from each of these benchmark sites. Table I. Names of the soil series of the Intermediate Zone of Sri Lanka used for the study. Agro Ecological Soil Series Cjreat Soil Group Soil Taxonomic Zone Cjreat Soil Group Equivalents Up Country Welimada Immature Brown Loams Typic Dystrochrepts Intermediate Zone Ragala Red Yellow Podzolic Typic Hapludults Mahawalathanna Red Yellow Podzolic Soils Typic Troporthents Mid Country lnf?rmf*rliafp 7 n n p derived from Quartzite Typic Troporthents Mid Country lnf?rmf*rliafp 7 n n p Kundasale Immature Brown Loams Typic Eutropepts l l l l v l I l l C U I t l l C £*VIK Ukuwela Reddish Brown Latasolic Typic Rhodusalfs Badulla Red Yellow Podsolic Typic Haplohumulfs Waligepola Immature Brown Loams Typic Dystrochrepts Low Country Kuiiyapitiya Red Yellow Podzolic Typic Troporthents Intermediate Zone Maho Reddish Brown Earth Pasmmentic Hapludults Kuda Oya Low Humic Clay Aqualfs Hembarawa Alluvils Typic Ustorthens Ulhitiya (Rolling) Reddish Brown Earth Typic Rhodudalfs Kurunagala Red Yellow Podzolic Plinthudulls Kurunagala with soft and hard latarites Ranugalla Immature Brown Loams Typic Dystropepts Ulhitiya Reddish Brown Earths Typic Haplustalfs Wariyapola Non Calcic Brown Typic Dystrochrepts Andigama Red Yellow Podzolic with soft Typic Troporthents and hard latarites Oombagahawela Immature Brown Loams Typic Dystropepts Mutukandiya Alluvial Typic Ustorthens Bibile Immature Brown Loams Typic Dystropepts 225 Rajapaksha, Mapa & Dassanayake Fig. 1. Sampling sites of the soil scries in the Intermediate Zone of Sri Lanka. (Note: Each square represents sampling locations]. Soil core samples of 6 cm in diameter and 3 cm in height were obtained for measuring the FC and PWP. Four replicates were obtained from each horizon. The core samples were saturated with water in the laboratory and the FC and PWP were measured using a pressure plate apparatus as described by Klute (1986). The field capacity was estimated as the water content at 10 kPa (Joshua, 1985) and the PWP as the water content at 1500 kPa (Cassel and Nielsen. 1986). The bulk density obtained from the same core sample was used to convert the gravimetric water to volumetric water content. Soil texture and organic matter content were determined using air-dried and sieved samples. Fifty grams of soil was used to determine the clay, silt and sand content using the Pipette method as described by Gee and Bauder (1986). Pipefitting times were calculated for clay and silt particles according to the USDA classification. Aliquots were removed from the soil suspension at calculated times and oven dried to obtain the clay and silt contents. Soil organic matter was determined using the dichromate method (Hesse, 1971). Clay and soil organic matter contents were determined for the three replicates from each surface and sub surface horizon from all the benchmark sites. 226 Effect of Texture and Organic Matter RESULTS AND DISCUSSION The soil water retention parameters including field capacity, and PWP, for the top and sub soil and the available water for the Intermediate Zone soils are shown in Table 2. Clay and organic matter contents of these soil series are shown in Table 3. Table 2. Soil water retention parameters obtained for soil series of the Intermediate Zone of Sri Lanka. Soil Series Top soil Sub soil Available water of the profile (mm) FC (%) PWP (%) FC (%) PWP (%) Available water of the profile (mm) Up Country IZ Walimada 27 11 24 11 80/m Ragala 34 24 35 21 132/m Mid Country IZ Mahawalathanna IS 10 17 9 69/m Kundasale 24 18 24 17 76 /m ' Ukuwela 31 22 34 28 66 Im Badulla 22 14 28 17 99 Im Waligepola 32 22 31 23 91/50 cm Low Country IZ Kuliyapitiya 28 15 18 12 85 Im Maho 23 16 18 13 65 Im Kuda Oya 23 10 27 14 132/50 cm Hembarawa 25 13 27 15 119/m Ulhitiya (Rolling) 23 14 26 17 81 Im Kurunagala 27 14 30 19 119/60 cm Ranugalla 23 13 25 15 96/50 cm Ulhitiya 21 11 23 17 106 Im Wariyapola 13 5 10 4 83 Im Andigama 20 14 18 13 73/65 cm Dombagahawela 20 10 25 14 97 Im Mutukandiya 23 12 22 14 90/40 cm Bibile 22 8 28 16 67/50 cm The available water is shown in the standard form of mm per meter depth of soil. When the soil profile was shallow with less than one-meter depth, the available water is shown for the depths of measurements only. According to Landon (1984), when the available water is higher than 180 mm/m it is classified high available water. When the available is 180-120 mm/m and less than 120 mm/m it is classified as moderate and low available water, respectively. I 227 Rajapaluha, Mapa A Dassanayake Table 3. Clay and organic matter contents of top and sub soils for soil series of the Intermediate Zone of Sri Lanka. Top soil Sub soil Soil Series Clay Organic matter Clay Organic matter (%) (%) (%) (%) Up Country IZ Walimada 14 2.1 14 1.2 Ragal IS 4.8 46 0.5 Mid Country IZ Mahawalathanna 26 2.1 26 2.1 Kundasale 15 2.0 14 0.9 Ukuwela 47 1.7 37 1.5 Badulla 21 3.1 33 2.5 Waligepola 16 3.3 18 1.5 Low Country IZ Kuliyapitiya 16 2.2 33 2.1 Maho 11 1.9 20 0.7 Kuda Oya 8 1.6 11 0.3 Hembarawa 7.5 0.9 8 0.3 Ulhitiya (Rolling) 17 0.7 14 0.2 Kurunegala K I.I 8 0.7 Ranugalla 11 0.9 17 0.2 Ulhitiya 6 1.9 6 0.5 Wariyapola 4 0.1 7 0.03 Andigama 9 2.2 9 2.0 Dombagahawela 5 1.7 20 0.6 Mutukandiya 3 0.1 3 0.03 Bibile 10 1.6 9 0.2 The soils of the mid country Intermediate Zone showed low available water while some of the soils of the low country and up country Intermediate Zone soils showed moderate amounts of available water. The soils holding moderate amounts of available water are more suitable for rainfed and irrigated agriculture than soils with low available water, as they can hold more water after a rainfall event or irrigation. Effect of clay content on field capacity From the textural separates analyzed, only the clay content showed a relationship with soil water retention parameters. The relationship between the volumetric water content at field capacity and clay content for the top and sub soils are shown in Figs. 2 and 3. According to these results the field capacity of these soils increased with increasing clay content. This positive relationship was significant at 95% probability level. The regression equations and correlation coefficients (R2) for top and sub soils are given in Fig. 2 and 3. As shown from this table the increase of the field capacity with the clay content was higher 228 Effect of Texture and Organic Matter in sub soil than the topsoil. There are several soil physical characters that could affect the water held at field capacity. Among those, water retention is strongly affected by soil texture. Hillel (1971) documented that the greater the clay content, in general, greater the water content at any given suction. The results show that this phenomenon is true for some soil series of the Intermediate zone of Sri Lanka. Furthermore, specific surface of soil particles also greatly influence the water retention. When specific surface is high the water held at any given suction is high. Clay is the finest particle in the textural component of the soil, and therefore, water holding capacity is much greater in clay than other particles (Marshar, 1959). In addition to clay and organic matter content, soil structure also influence the field capacity. In general, soils structure mainly influences the water held at low suctions by modifying the macro-porosity (Salter and Williams, 1965). Certain soils of the low country intermediate zone showed a high field capacity even with low clay content due to the influence of soil structure, which can only be characterized qualitatively. Fig. 2 . Relationship between field capacity (FC) and permanent wilting point (PWP) with clay content of top soil. Effect of clay content on PWP The relationship between the clay content and the volumetric water content at PWP for the top and sub soils are shown in Fig. 2 and 3, respectively. Permanent wilting point increased with the increasing clay content, which was significant at the 95% probability level for top and subsoil. As shown in Fig. 2 and 3, the increase of PWP with increasing clay content was higher in the topsoil than the sub soil. As documented by Marshar (1959), increasing clay content from 38-58% increased PWP only by 20-25%. He also showed that PWP is not affected very greatly by structure and therefore does not require any special precaution during soil sampling in the field. As shown in Fig. 2 and 3 the water held at field capacity in top soil increased at a slightly lower rate (slope = 0.23) with increase in clay content than that held at PWP (slope = 0.27). According to Fig. 2 and 3 there is no parallel increase of available water held between FC and PWP with the increase of clay content. Therefore, no significant relationship was found with the available water and clay content. Salter and Williams 229 Rajapaluha, Mapa & Dassanayake (1966) also showed that the increase in FC with clay content was not parallel to the increase in PWP and the difference in moisture content between these upper and lower limits did not increase in a constant manner for a sandy loam soil. Fig. 3. Relationship between field capacity (FC) and permanent wilting point (PWP) with clay content of sub soil. Effect of organic matter content on field capacity Fig. 4 shows the relationship between organ ic matter and volumetric water content at field capacity for the topsoils. The field capacity showed a significant positive relationship with organic matter content only for the topsoil. The relationship, between organic matter and volumetric water content at field capacity for the sub soils, is not significant at 95% probability level and showed a very low correlation coefficient value (R2), which was 0.03. The organic matter content of the sub soil did not show any relationship with field capacity, as it was low in organic matter. Hudson (1994) showed how the organic matter content increased the field capacity in three textural classes of soils, namely sandy, silt loam and silty loam textures. According to Hillel (1971), soil organic matter can help to retain more water, though the amount of the organic matter normally present in mineral soils are too low to have a significant effect. Effect of organic matter on PWP The positive relationship of soil organic matter content with volumetric water content at PWP for the top soils is shown in Fig. 4. As for the relationship with field capacity, the sub soil did not show any significant relationship with organic matter and PWP. Correlation coefficient of this relationship is 0.14, which is not significant value. According to Table 3 most of the sub soils contains low amounts of organic matter content when compared to topsoil. 230 Effect of Texture and Organic Matter (I I 1 0 1 2 3 4 5 6 Organic matter % Fig. 4. Relationship between Field capacity (FC) and permanent wilting point (PWP) with organic matter content of top soil. Fig. 4 shows the regression equations with soil organic matter content and soil water retention parameters and correlation coefficients (R2) for the topsoil. According to these data the PWP increased at a higher rate than the field capacity with the increase of organic matter content. This is mainly due to the high affinity of water molecules to organic matter. Therefore, even the increase of organic matter content increased the water held at field capacity and permanent wilting point, it may not necessarily increase the available water content. In a review article Macrae and Mehuys (1985) stated that organic matter increased available water content in soils only under specific circumstances, and that such increases are the exception rather than the rule. The increase of organic matter is still beneficial as the higher amount of water retained decrease the runoff, increase water conservation and thereby reduce soil erosion. CONCLUSIONS In this study the soil water retention parameters, namely field capacity and permanent wilting point, were determined for 20 benchmark soil series of the Intermediate Zone of Sri Lanka. The results showed that the available water was moderate in majority of soils in the low and up country Intermediate Zone soils while the soils from the mid country Intermediate Zone showed low available water. This study demonstrated that the field capacity and PWP increases with increasing clay content, but this increase was not parallel and therefore did not increase the available water at all times. The increase of soil organic matter content increased the field capacity and PWP for the topsoil. These increases were also not similar. Therefore, increase in clay and soil organic matter content may increase water retention and contribute to soil and water conservation while it may not necessarily contribute to the increase in available water as commonly believed. ACKNOWLEDGEMENTS This study was funded by the SR1CANSOL project which is the twinning project between the Canadian Society of Soil Science (CSSS) and the Soil Science Society of Sri Lanka (SSSSL). The assistance given in soil analysis by Mr. P.R. ldamekorala is greatly appriciated. 231 Uajapaksha, M a p s & Dassanayake REFERENCES Nodinavakc. W.L. and Mapa. R.U. (1989). Determination of insiiu field capacity of Noncalcic Brown soils. J.'Soil Sci. Soc. Sri Lanka. 6: 78-83. C'asscl. D.K. and Nielsen. DR. (1986). Field capacity and available water capacity, pp. 901-924. In: Klute, A. (Ed). Methods of Soil Analysis, Pari II. 2nd Edition. American Society of Agronomy. De Alwis. K.A.N, and I'anabokkc. CR. (1972). Handbook of the soils of Ceylon. J. Soil Sci. Soc. Sri Lanka. 2: 1-72. FAO. (1990). Guideline for soil description. 3"' Edition. Food and Agriculture Organization of the United Nations. Rome. Uee.G.W. and Bauder. J.W. (1986). Particle size analysis, pp. 383-409. In: Klute, A. (Ed). Methods of Soil Analysis. Part II. 2'"' Edition. American Society of Agronomy. Hesse. P.R. (1971). A Text Book of Soil Chemical Analysis. Johj Murray, London. Ilillel. I).(I971). Soil und Water. Physical principles and processes. Academic Press. New York. pp. 162-165. I ludson. B.D. (1994) Soil organic matter and available water capacity. J. Soil and Water Cons. 49: 189-194. Jasmison. VC. (1953). Changes in air-water relationships due to structural improvement of soil. Soil Sci. 76: 143-151. Jasmison. VC. and Kroth. H.M. (1958). Available moisture storage capacity in relation to textural composition and organic matter content of several Missouri soils. Soil Sci. Soc. Amer. Proc. 22: 189-192. Joshua. W.D. (1985). Physical properties of Reddish Brown Earth soils (Alfisols) and their relationship to agriculture. J. Soil Sci. Soc. Sri Lanka. 5: 1-42. Klute. A. (1986). Water retention. Laboratory methods, pp. 635-660. In: Klute, A. (Ed). Methods of Soil Analysis, Part II. 2'"' Edition. American Society of Agronomy. Kuruppuarachchi, D.S.P. (2001) Water use efficiency in agriculture: oppertuniiies and limitations. J. Soil Sci. Soc. Sri Lanka. 13: 1-8. Landon. J.R. (1984). Hooker tropical soil manual. Booker Agricultural International Limited. Pp. 441. Marshar. T.J. (1959). Book of relations'between water and soil. pp. 55-61. In: Commonwealth Agriculture Bureaux Faruham Roxal. Bucks. England. Macrae. R.J. and Mehuys. (i.R. (1985). The effect of green manuring on the physical properties of temperate area soils, pp. 71-94. In: Stewart. B.A. (Ed). Advances in Soil Science. Na\ akekorale. II.B. and Mapa. R.B. (2002). Soils of the Intermediate Zone of Sri Lanka: morphology, characterization and classification. Special Publication No. 3. Soil Science Society of Sri Lanka, Sarvodaya Vishwalckha Press. (In Press). Salter. P.J. and Williams. J.ll. (1965). The influence of texture on the moisture characteristics of soils. II. Available water capacity and moisture release characteristics. J. Soil Sci. 16: 310-317. Salter. I'.J. and 1 laworth. I'. (1961). The available water capacity of a sandy loam soil. I. A critical comparison of methods of deierminiin; the moisture content of soil at Field capacity and the permanent wilting percentage. J. Soil. Sci, 12": .126-334. Saxton. K.l:.. Raw Is. W.J.. Rombcrgcr. J.S. and Oappcndick. R.I. (1985). Personnel Communication. Agric. Res. Center. Washington Slate University. Pullman. WA. USA. Soil Survey Stall". (1992). Keys to soil taxonomy. Soil management support service. Tech. Monogr., No. 19. S* Edition. Pocahontas Press. Blaekburg. VA. Uehara. G. and Oilman. Ci. (1981). The mineralogy, physics and chemistry of tropical soils with variable charge. Wcslvicw Press. Boulder. Colorado. Pp. 159.