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Soil available phosphorus pedotransfer function for calcareous soils of varamin region.

Introduction

In recent years, there has been increased interest in agricultural practices associated with the application phosphorus fertilizers. Phosphorus in plants performs unique function of energy transfer via formation of pyrophosphate bond. Phosphorus compounds (ADP and ATP) act as energy currency within the plants and involve in wide range of plant processes from permitting cell division to developing good root system (Meena et al., 2007). Phosphorus is removed from the soil by plant uptake or lost by soil erosion and runoff. Crops remove varying amounts of phosphorus from the soil (Manunta et al., 2001). Also, the availability of phosphorus in soils is often limited by fixation reactions, which convert the monophosphate ion into various insoluble forms (Di et al., 1994).

The importance of organic matter and accordingly organic carbon in the soil has been recognized for centuries as the key to soil fertility and productivity. Organic manures and other products of farming and related industries contribute to plant growth through their favorable effect on physical, chemical and biological properties of soil (Reddy et al., 2005; Meena et al., 2007). Besides, previously researches report that the availability of soil phosphorus is enhanced by adding organic matters, due to chelating of polyvalent canons by organic acids and other decay products (Jama et al., 1997; Reddy et al., 2005; Mohanty et al., 2006).

Precise information on the quantity of soil available phosphorus can be obtained only with the aid of almost laborious, costly and time consuming standard test methods (Bray and Kurtz, 1945; Spratt et al., 1980). However, for almost 50 years many attempts have been made to predict some complex soil properties from some easily available soil properties using empirical models. In soil science, such empirical models are named pedotransfer functions (MacDonald, 1998; Krogh et al., 2000).

So far many of the pedotransfer functions have been developed to predict various soil properties. MacDonald (1998) developed two pedotransfer functions to predict soil cation exchange capacity (CEC) based on soil organic carbon (OC) and clay (CL) as CEC = 2.0 OC + 0.5 CL and CEC = 3.8 OC + 0.5 CL for Quebec and Alberta soil state in Canada, respectively. Rashidi and Seilsepour (2008) studied Varamin soils in Iran and proposed a pedotransfer function to predict soil CEC based on soil organic carbon (OC) and pH (PH) as CEC = 26.76 + 8.06 OC - 2.45 PH with [R.sup.2] = 0.77. Seilsepour and Rashidi (2008a, b) also predicted soil CEC from organic carbon using a pedotransfer function as CEC = 7.93 + 8.72 OC with [R.sup.2] = 0.74. Moreover, the United States Salinity Laboratory (USSL) proposed one of the earlier pedotransfer function to predict soil exchangeable sodium percentage (ESP) from soil sodium adsorption ratio (SAR) as ESP = - 0.0126 + 0.01475 SAR for United States soils (Richards, 1954). Furthermore, Al-Busaidi and Cookson (2003) suggested a pedotransfer function to predict soil sodium adsorption ratio (SAR) based on soil electrical conductivity (EC) as SAR = 0.464 EC + 7.077 with [R.sup.2] = 0.83 for saline soil in Oman.

Since, the above pedotransfer functions have been derived from different saline-zone soils, the general pedotransfer functions between soil properties may be assumed to be similar to those. However, these pedotransfer functions have been shown not to be constant, but to vary substantially with both solution ionic strength and the dominant clay mineral present in the soil (Shainberg et al., 1980; Nadler & Magaritz, 1981; Marsi & Evangelou, 1991; Evangelou & Marsi, 2003). Therefore, the pedotransfer functions are not constant and should be determined directly for the soil of interest.

As previously researches report that there is a relationship between the availability of phosphorus in soil and soil organic matter (Jama et al., 1997; Reddy et al., 2005; Mohanty et al., 2006), soil organic carbon (OC) can be used to estimate soil available phosphorus (AP). Despite the considerable amount of research done, which shows the relationship between soil AP and soil OC, very limited work has been conducted to develop a soil AP pedotransfer function based on soil OC. Therefore, the specific objective of the study presented here was to develop a soil AP pedotransfer function based on soil OC for calcareous soils of Varamin region in Iran, and to verify the developed pedotransfer function by comparing its results with those of the laboratory tests.

Materials and methods

Experimental procedure

Forty-eight soil samples were taken at random from different fields of experimental site of Varamin, Iran. The site is located at latitude of 35[degrees]-19'N and longitude of 51[degrees]-39'E and is 1000 m above mean sea level, in grid climate in the center of Iran. The soil of the experimental site was a fine, mixed, thermic, Typic Haplocambids clay-loam soil. In order to obtain required parameters for determining soil AP pedotransfer function, some physical and chemical properties of the soil samples i.e. sand, silt, clay (% by weight) and pH were measured using laboratory tests as described by the Soil Survey Staff (1996). The method of Walkley and Black (1934) by oxidation with potassium dichromate using the heating-block modification of Heanes (1984) was used to measure organic carbon (% by weight) of the soil samples. The method of Olsen and Sommers (1982) was used to measure available phosphorus of the soil samples. Physical and chemical properties of the forty-eight soil samples used to determine the soil AP pedotransfer function are shown in Table 1. Also, in order to verify the soil AP pedotransfer function by comparing its results with those of the laboratory tests, twelve soil samples were taken at random from different fields of the experimental site. Sand, silt, clay (% by weight) and pH of the soil samples were measured using laboratory tests as described by the Soil Survey Staff (1996). Again, the method of Walkley and Black (1934) by oxidation with potassium dichromate using the heating-block modification of Heanes (1984) was used to measure organic carbon (% by weight) of the soil samples. Also, the method of Olsen and Sommers (1982) was used to measure available phosphorus of the soil samples. Physical and chemical properties of the twelve soil samples used to verify the soil AP pedotransfer function are shown in Table 2.

Regression model

A typical exponential regression model is shown in Equation 1:

Y = a [e.sup. b X] (1)

Where:

Y = Dependent variable, for example AP of soil

X = Independent variable, for example OC of soil

e = Base of the natural logarithm, 2.71828182845904

a, b = Regression coefficients

In order to develop the soil AP pedotransfer function based on soil OC, an exponential regression model as above was suggested.

Statistical analysis

A paired samples t-test and the mean difference confidence interval approach were used to compare the soil AP values predicted using the soil AP pedotransfer function with the soil AP values measured by laboratory tests. The Bland-Altman approach (1999) was also used to plot the agreement between the soil AP values measured by laboratory tests with the soil AP values predicted using the soil AP pedotransfer function. The statistical analyses were performed using Microsoft Excel (Version 2003).

Results and discussion

Results

The p-value of the independent variable, Coefficient of Determination ([R.sup.2]) and Coefficient of Variation (C.V.) of the soil AP pedotransfer function is shown in Table 3. Based on the statistical result, the soil AP pedotransfer function was judged acceptable due to statistical results. The [R.sup.2] value and C.V. of the soil AP pedotransfer function were 0.92 and 23.8%, respectively. The soil AP pedotransfer function is given in Equation 2.

AP = 0.7927 [e.sup.4.9922 OC] (2)

Discussion

A paired samples t-test and the mean difference confidence interval approach were used to compare the soil AP values predicted using the soil AP pedotransfer function with the soil AP values measured by laboratory tests. The Bland-Altman approach (1999) was also used to plot the agreement between the soil AP values measured by laboratory tests with the soil AP values predicted using the soil AP pedotransfer function.

[FIGURE 1 OMITTED]

The soil AP values predicted by the soil AP pedotransfer function were compared with the soil AP values determined by laboratory tests and are shown in Table 4. A plot of the soil AP values determined by the soil AP pedotransfer function and laboratory tests with the line of equality (1.0: 1.0) is shown in Figure 1. The mean soil AP difference between two methods was 1.57 ppm (95% confidence interval: -2.88 and 6.03 ppm; P = 0.453). The standard deviation of the soil AP differences was 7.01 ppm. The paired samples t-test results showed that the soil AP values predicted with the soil AP pedotransfer function were not significantly different than the soil AP measured with laboratory tests (Table 5). The soil AP differences between these two methods were normally distributed and 95% of the soil AP differences were expected to lie between [mu]+1.96[sigma] and [mu]-1.96[sigma], known as 95% limits of agreement (Bland and Altman, 1999). The 95% limits of agreement for comparison of soil AP determined with laboratory test and the soil AP pedotransfer function were calculated at -12.17 and 15.32 ppm (Figure 2). Thus, soil AP predicted by the soil AP pedotransfer function may be 12.17 ppm lower or 15.32 ppm higher than soil AP measured by laboratory test. Figure 2 also shows that for soil OC ranged from 0.30 to 0.60%, the soil AP predicted by the soil AP pedotransfer function is almost equal to soil AP measured by laboratory test. As the soil OC increased, for soil OC ranged from 0.60 to 0.65% the soil AP pedotransfer function overestimated the soil AP while for OC more than 0.65% the soil AP pedotransfer function underestimated the soil AP. The average percentage differences for soil AP prediction using the soil AP pedotransfer function and laboratory test was 19.6%.

[FIGURE 2 OMITTED]

Conclusions

An exponential regression pedotransfer function based on soil organic carbon (OC) was used to predict soil available phosphorus (AP) of calcareous soils of Varamin region in Iran. The soil AP values predicted using the soil AP pedotransfer function was compared to the soil AP values measured by laboratory tests. The paired samples t-test results indicated that the difference between the soil AP values predicted by the soil pedotransfer function and measured by laboratory tests were not statistically significant (P > 0.05). Therefore, the soil AP pedotransfer function can provide an easy, economic and brief methodology to estimate soil AP.

Acknowledgments

The financial support provided by the Agricultural Research and Education Organization of Iran under research award number 100-15-76048 is gratefully acknowledged.

References

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Bland, J.M. and D.G. Altman, 1999. Measuring agreement in method comparison studies. Stat. Methods Med. Res., 8: 135-160.

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Di, H.J., R. Harrson and A.S. Campbell, 1994. Assessment of methods for studying dissolution of phosphate fertilizers of different solubility in soil. I. An isotopic method. Fertilizer Research, 38: 1-9.

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(1) Majid Rashidi and (2) Mohsen Seilsepour

(1) Department of Agricultural Machinery, Faculty of Agriculture, Islamic Azad University, Takestan Branch, Iran.

(2) Varamin Soil and Water Research Department, Soil and Water Research Institute, Iran.

Corresponding Author: Dr. Majid Rashidi, Ph.D., Department of Agricultural Machinery, Faculty of Agriculture, Islamic Azad University, Takestan Branch, Iran Tel: 0098-9123376127 E-Mail: majiarashidi8l@yahoo.com, m.rashidi@aeri.ir
Table 1: The mean values, Standard Deviation (S.D.) and Coefficient
of Variation (C.V.) of soil physical and chemical properties of
the forty-eight soil samples used to develop the soil AP
pedotransfer function.

Parameter            Minimum   Maximum   Mean   S.D.   C.V. (%)

Sand (%)                14.0      44.0   33.1    6.3       19.1
Silt (%)                30.0      56.0   45.3    4.1        9.1
Clay (%)                 9.0      50.0   22.0    6.7       30.2
pH                       7.0       8.1    7.5    0.3        3.6
Organic carbon (%)       0.2       0.7    0.5    0.1       25.5
Available
  phosphorus (ppm)       2.7      43.6   15.3   10.9       71.1

Table 2: The mean values, Standard Deviation (S.D.) and Coefficient
of Variation (C.V.) of soil physical and chemical properties of the
twelve soil samples used to verify the soil AP pedotransfer
function.

Parameter      Minimum   Maximum   Mean    S.D.    C.V. (%)

Sand (%)          10.0      34.0    24.1    5.87       24.4
Silt (%)          40.0      56.0    48.2    4.40       9.13
Clay (%)          18.0      50.0    28.2    7.90       28.0
pH                 7.0       8.0    7.31    0.33       4.51
Organic
  carbon (%)      0.31      0.72    0.56    0.13       23.4
Available
  phosphorus
  (ppm)            4.4      49.3    17.3    13.7       78.9

Table 3: The p-value of independent variable, Coefficient of
Determination ([R.sup.2]) and Coefficient of Variation (C.V.)
of the soil AP pedotransfer function.

Model               Independent   p-value    [R.sup.2]   C.V. (%)
                     variable

AP = 0.7927             OC        2.44E-26     0.92        23.8
[e.sup.4.9922 OC]

Table 4: Chemical properties of soil samples used in evaluating
the soil AP pedotransfer function.

                   Available phosphorus (ppm)
         Organic
Sample   carbon    Laboratory   Pedotransfer
 No.       (%)        test        function

1           0.31          4.4            3.7
2           0.40          5.4            5.8
3           0.46          7.0            7.9
4           0.47          8.3            8.3
5           0.50          9.4            9.6
6           0.60         11.2           15.8
7           0.62         13.2           17.5
8           0.65         14.8           20.3
9           0.66         22.5           21.4
10          0.68         29.6           23.6
11          0.70         32.6           26.1
12          0.72         49.3           28.8

Table 5: Paired samples t-test analyses on comparing soil
available phosphorus determination methods.

                               Standard             95% confidence
                 Average     deviation of            intervals for
Determination   difference   difference      p-     the difference
methods           (ppm)         (ppm)       value   in means (ppm)

Laboratory         1.57          7.01       0.453     -2.88, 6.03
test &
pedotransfer
function
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Title Annotation:Original Article
Author:Rashidi, Majid; Seilsepour, Mohsen
Publication:American-Eurasian Journal of Sustainable Agriculture
Article Type:Report
Date:Jan 1, 2008
Words:2933
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