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Coşkun Gülser, Nutullah Özdemir, Tayfun Aşkın, Feride Candemir, Ahmet Korkmaz Ondokuz Mayıs University, Faculty of Agriculture, Soil Science Department, Samsun, Turkey Abstract The n value characterizes the relationship between the percentage of field moisture capacity and the percentage of clay and humus. In this study, n value was investigated to determine whether it might be used as an indicator of soil structural stability or not. The n values of 129 soil samples gave the significant positive correlations with their soil erodibility factors (K) and erosion ratios (ER). Also, the n values of the soils including clay textural class showed significant negative correlation with soil structural stability index (SSI). It seems that the n value may be used as an indicator of soil structural stability. Introduction As the world population rate increases, cultivation of agricultural lands becomes increasingly necessary to feed this high population. Therefore, soil management and cultivation practices should be improved to sustain soil fertility and to prevent high erosion vulnerability. It is known that soil properties limit the land use and management or establish the severity of the limitation. An abundance of nutrients in soil does not always indicate high crop production. Soil structure one of the most important soil physical properties is known as an indicator of the productivity of a given soil and also controls the severity of soil erosion. Structure can be improved or destroyed readily through our choice and timing of soil management practices (Hillel, 1982). Soil structure influences some soil erodibility indices such as, dispersion ratio (DR), erosion ratio (ER), erodibility factor (K) and soil structural stability index (SSI). These indices have been developed to determine soil erosion susceptibility and used to assess sustainable soil use and management (Leo, 1963; Bryan, 1968; Karagül, 1999; Özdemir ve Aşkın, 1999). The n value characterizes the relationship between the percentage of field moisture capacity and the percentage of clay and humus in soil. It is useful to predict whether a soil can be grazed by livestock or can support other loads and to predict what degree of subsidence occurs after drainage (Soil Survey Staff, 1998). It may be used for a rapid and quantitative method to assessing soil structure. The objective of this study was to determine whether n value might be used as an indicator of soil structural stability or not. Material and Methods 129 soil samples used in this study were taken from 0 to 20 cm depth around Samsun. Some soil physical and chemical properties were determined as follows; particle size distribution by the hydrometer method (Demiralay, 1993); lime content by Scheibler Calsimeter (Soil Surv. Staf, 1993); soil reaction, pH in 1:2.5 (w:v) soil-water suspension by pH meter (Black, 1965); organic matter content by Walkley-Black method (Kacar, 1994). Field capacity (FC) was measured at 33 kPa on a ceramic plate after soil samples were passed from 2 mm sieve, saturated for 24 hours and then equilibrated for another 24 hours (Klute, 1986). The indices such as dispersion ratio (DR), erosion ratio (ER), soil erodibility factor (K), structural stability index (SSI) and n value were estimated by the following standard techniques: DR (%) = (a/b) * 100
where, a is the percentage of silt plus clay in suspension, b is the percentage of silt plus clay dispersed with Calgon agent (Bryan, 1968).ER (%) = (a/b) * (A/c) * 100
where, a is the percentage of silt plus clay in suspension, b is the percentage of silt plus clay dispersed with Calgon agent, A is the field capacity, c is the percentage of clay dispersed with Calgon agent (Akalan, 1967).K = [(2.1*10-4 (M)1.14 (12 - a) + 3.25 (b - 2) + 2.5 (c - 3)) 1.292] / 100
where, M is the particle size parameter (% silt + % very fine sand)*(100 - % clay), a is the percentage of organic matter, b is the soil structure code and c is the profile permeability class (Wischmeier and Smith, 1978).SSI (%) = S b - S a
where, S b is the percentage silt plus clay dispersed with Calgon agent, S a is the percentage of silt plus clay in suspension (Leo, 1963).n value = (A - 0.2 R) / (L + 3 H)
where, A is the percent moisture in field capacity, R is the percentage of silt plus sand, L is the percentage of clay and H is the percentage of organic matter (Soil Survey Staff, 1998).
Descriptive statistics of n Value and soil erodibility indices was calculated by using SPSS. The correlations between n value and the other indices, DR, ER, K and SSI were also estimated (Steel and Torrie, 1982).
Results and Discussions Some Physical and Chemical Properties of Soil Samples : Some physical and chemical properties of the soil samples used in this study are given in Table 1. According to soil particle size distribution, 129 soil samples were subdivided into five different textural classes such as, 25 soil samples in clay (C), 46 in clay loam (CL), 20 in sandy clay loam (SCL), 22 in loam (L) and 16 in sandy loam (SL). Field capacity, lime (CaCO3) and organic matter contents of the soil samples varied from minimum 20, 0 and 0.85 % to maximum 63, 49.81 and 5.47 % with the means of 36.9, 4.82, and 2.15 % respectively. Soil reaction (pH) of the samples was generally alkaline and changed from extremely acid, minimum 4 to strongly alkaline, maximum 8.9 (Soil Survey Staff, 1993).  n Value and Soil Erodibility Indices of The Soil Samples : Descriptive statistical results for n value and soil erodibility indices; dispersion ratio (DR), erosion ratio (ER), erodibility factor (K) and structural stability index (SSI) in different textural classes are given in Table 2. The n value describes the relationship among the field capacity, clay and humus in soil. Regardless of textural classes, the n values of all soil samples varied from 0.12 to 1.52 with a mean of 0.63 and 39.7 % coefficient of variance (CV). While the texture class changed from fine to coarse, the lowest CV 22.8 observed in C changed to the highest CV, 71.2 obtained in SL. Positive values of the skewness or third central moment suggest tailing to the right, while negative values of the skewness suggest tailing to the left on the horizontal axis of a plot. A symmetrical distribution always has zero for the value of skewness (Isaaks and Sarivastava, 1989). Therefore, the n values in C and L texture classes showed almost a symmetrical distribution due to their skewness values becoming close to zero. The lowest mean n value (0.57) and standard deviation (0.13) were obtained in C among the other textural classes, except the SL textural class. But, the highest skewness, 1.43 and standard deviation, 0.37 for n value were obtained in the SL textural class. Dispersion ratio was used to evaluate soils erodibility by the amount of silt plus clay in a dispersed state (Bryan, 1969). Dispersion ratio for all soil samples varied from 9 to 50 % with a mean of 27.4 % and 28.5 % coefficient of variance (CV). While the texture class changed from fine to coarse, mean dispersion ratio increased 22.9 % in C to 35.8 % in SL. Erosion ratio is the form of dispersion ratio that is combined with the ratio of "colloidal content / moisture equivalent" (Bryan, 1968). Erosion ratio for all soil samples changed from 8 to 138 % with a mean of 37.6 %. While the texture class changed from fine to coarse, mean erosion ratio increased 20.9 % in C to 74.1 % in SL. Erodibility, the vulnerability or susceptibility of the soil erosion, is a function of both the physical characteristic of the soil and the management of the soil (Hudson, 1995). Erodibility factor (K) for all soil samples varied from 0.06 to 0.63 with a mean of 0.25. The lowest mean K (0.16) was obtained in C, the highest mean K (0.37) was determined in L among the texture classes. Structural stability index (SSI) by the sum of the difference between mechanical and aggregate analyses of silt plus clay fractions was introduced as a rapid technique for estimating structural stability of soils by Leo (1963). SSI for 129 soil samples varied from 12 to 74 % with a mean of 44.3 %. While the texture class changed from fine to coarse, mean SSI decreased 56.1 % in C to 26.2 % in SL.  Relationships Between n Value and Soil Erodibility Indices : The relationships between the n value and the soil erodibility indices for different texture classes and all soil samples are given in Table 3. When 129 soil samples were evaluated together, the n value gave the statistically significant positive correlations with erosion ratio (0.361** in Figure 1 ) and erodibility factor (0.490** in Figure 2 ) and did not show any relationship statistically with dispersion ratio and structure stability index. If the texture classes are considered individually, the n values in C textural class gave the significant correlations with the most erodibility indices such as, ER, K and SSI. Except SL textural class, n value showed the significant positive relations with almost all ER and K values in the other texture classes too. On the other hand, significant relationships were obtained among the erodibility indices for 129 soil samples. SSI in only C and CL texture classes were significantly correlated with DR (-0.676**, -0.482**), ER (-0.735**, -0.390**) and K (-0.691, -0.527**) respectively. Also, n values only showed significant negative correlations with SSI (-0.479*) in C and DR (-0.578*) in SL texture classes.   Conclusion The critical n value is accepted as 0.7. In the field by a squeezing a soil sample in the hand, if the soil flows between the fingers with difficulty the n Value is accepted between 0.7 and 1.0. If the soil flows easily between the fingers the n Value is 1.0 or more (Soil Survey Staff, 1998). Therefore, while the n value is getting smaller, soil sample becomes more resist to detachable. In the study on Daphan Plain soils by Akgül (1994), soil texture class of A horizons was mostly clay and the n values of surface soils varied from 0.47 to 0.73. For this study, n values in C texture class varied symmetrically between 0.42 and 0.82 were similar to his results. n Values in this study usually gave higher correlation with erosion ratio and erodibility factor than the other erodibility indices. It may be explained with that there are some similar parameters used in calculation of highly correlated these indices. For example, field capacity and organic matter content used in estimation of erosion ratio and erodibility factor were only different parameters from the other parameters used in calculation of dispersion ratio and structure stability index. Because, silt plus clay content were only parameters used in estimation of DR and SSI. Organic matter content and field capacity other than particle size distribution were also used in estimation of n value. Therefore, it seems reasonable that n value would probably give higher correlation with ER and K due to having similar parameters in their calculation. As a result, due to significant relationships between n value and the other erodibility indices, the n value may be used as an indicator of soil structural stability especially in soils including finer texture class. Besides particle size distribution, using field moisture and organic matter content in estimation of n value gives more details about soil structure than DR and SSI. There is not much study about this subject. It will be useful that further studies in field and laboratory conditions should be made along this line. References . Akalan, İ., 1967. Toprak Fiziksel Özellikleri ve Erozyon. Ankara Üniversitesi Ziraat Fakültesi Yıllığı, 3-4, 490-503. . Akgül, M. 1994. Basic Soil Survey of the Daphan Plain Soils. I. Some Physical and Chemical Properties. Atatürk Univ. J. of Agricultural College Vol: 25, Number:2, p.223. . Black, C.A., 1965. Methods of Soil Analysis. Part I, American Society of Agronomy , No:9. . Bryan, R.B., 1968. The development, use and efficiency of indices of soil erodibility. Geoderma, 2, 5-25. . Demiralay, İ. 1993. Toprak Fiziksel Analizleri. Atatürk Ün. Ziraat Fak. Yay. No:143 Erzurum . Hillel, D. 1982. Introduction to Soil Physics. Academic Press, Inc. San Diego, CA. USA. . Hudson, N. 1995. Soil Conservation.Third Edition.Iowa State Univ. Press. Ames.USA . Isaaks, H.E. & Sarivastava, M.R. 1989. Applied Geostatistics. Oxford Univ. Press. New York, USA. . Kacar, B., 1994. Bitki ve Toprağın Kimyasal Analizleri III. Ankara Üniv. Ziraat Fak. Eğitim, Araştırma ve Geliştirme Vakfı Yayınları No.3. Ankara. . Karagül, R. 1999. Trabzon-Söğütlüdere Havzasında Farklı Arazi Kullanım Şekilleri Altındaki Toprakların Bazı Özellikleri ve Erozyon Eğilimlerinin Araştırılması. Tr. J. of Agriculture and Forestry. 23:53-68. TÜBİTAK . Klute, A. 1986. Water Retention: Laboratory Methods. In A. Klute(ed) Methods of Soil Analyses. 2nd ed., Part I, Agronomy 9, p: 635-662, ASA and SSSA, Madison, WI. . Leo, W.M., 1963. A rapid method for estimating structural stability of soils. Soil Science, 96, 342-346. . Özdemir, N & Aşkın, T.1999. Ordu İli Topraklarının Strüktürel Dayanıklılığının ve Aşınıma Duyarlılığının Belirlenmesi Üzerine Bir Araştırma. Karadeniz Bölgesi Tarım Sempozyumu, 4-5 Ocak, Bildiriler Cilt:1 p.141-151. . Soil Survey Staff, 1998. n Value. Keys to Soil Taxonomy. Eighth Edition. NRCS-USDA, p.25. . Soil Survey Staff, 1993. Soil Survey Manuel. USDA Handbook No:18, Washington, USA. . Steel, R.G.D. & J.H. Tore, 1980. Principle and Procedures of Statistics. Mc Graw-Hill Company, New York. . Wischmeier, W.H. & Smith, D.D., 1978. Predicting rainfall erosion loses a guide to conservation planning. U.S.D.A. Agriculture Handbook No:557. |