THE PHYTOTOXICITY LIMITS. ANIL KR. SINGH. Water Technology Centre, Indian Agricultural Research Institute, New Delhi - 110012, India and. R.K. RATTAN.
A NEW APPROACH
FOR ESTIMATING
THE PHYTOTOXICITY
LIMITS
ANIL KR. SINGH
Water Technology Centre, Indian Agricultural Research Institute, New Delhi - 110012, India and R.K. RATTAN
Division o f Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New D e l h i - 110012, India
(Received January 1987) Abstract. A pot experiment was carried out on a Typic ustipsamment to study the effect of Cd concentration on the yield of wheat (Triticum aestivum) and soybean (Glycine max). Cd levels taken were 1,5, 10, 20, 40, 80, and 160/lg g - tof soil. Three different statistical procedures were employed to evaluate the phytotoxicity limits. The non-linear regression technique was found to be more effective in calculating C O(threshold concentration) and Ci00 (toxic concentration) in comparison to Cate and Nelson (1971) and Beckett and Davis (1977) procedures. This technique was unaffected by the nature of the distribution of the data and did not require any initial value of concentration as a starting point.
1. Introduction
One of the most deleterious effect of the hazardous chemicals particularly heavy metals released as waste products, is their impact on crop production. To prevent an excessive build up of such elements or for taking remedial measures, it is essential to work out the limits of tolerance of various crops to these elements. This would serve the dual purpose of maintaining both soil and crop productivity by keeping the concentration of these chemicals at a safe level. There is, therefore, a need for a technique to correctly evaluate the concentration of a particular element beyond which the yield of a crop starts declining. It is also imperative to estimate the concentration at which the yield becomes zero for long term planning. Most of the earlier work related to techniques which can be used for such an exercise were generally concentrated on nutrient deficiencies (Cate and Nelson, 1965; 1971) or salt tolerance studies (Maas and Hoffman, 1977; Feinerman et al., 1982). A linear regression technique was used by Beckett and Davis (1977) to estimate the concentration of a metal in plant tissue at which the yield is affected. Beckett and Davis (1978); Davis and Beckett (1978); Davis et al. (1978); Davis and Carlton-Smith (1984); and Burton et al. (1983, 1986) have used this procedure for calculating the critical limits for several heavy metals in different crops. However, the much simpler procedure suggested by Cate and Nelson (1971) is commonly used for delineating Environmental Monitoring and Assessment 9 (1987) 26%283. 9 1987 by D. Reidel Publishing Company.
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limits. In this paper, the usefulness of a non-linear regression technique suggested by Marquardt (1963) has been examined for evaluating the critical limits of cadmium for crops. The results obtained have been compared and discussed in relation to the other procedures. 2. Materials and Methods
2.1. EXPERIMENTAL A pot experiment was conducted to study the effect of cadmium (Cd) concentration on the yield of a crop. Different levels of Cd were obtained by the addition of cadmium sulphate enriched sludge to soil in pots to provide Cd levels of 1, 5, 10, 20, 40, 80, and 160 #g g - i of soil. Successively two crops, wheat (Triticum aestivum) and soybean (Glycine max) were grown in these pots. The soil used was a Typic ustipsamment rich in illite clay minerals with pH 8.3 and D T P A extractable zinc as 0.36 #g g - 1. All the treatments were replicated three times. Cd content of s t r a w a n d grain in wheat and soybean was determined at harvest along with D T P A - extractable Cd in soil by atomic absorption spectrophotometry (Varian Techtron AA 120). 2.2. TECHNIQUES The concentration of Cd at which the yield starts declining has been termed as threshold concentration (Co). No yield reduction is expected to occur till this concentration level. Beyond this level the yield decreases linearly with increase in the logarithmic concentration value. The concentration at which the yield reduces to zero has been termed as toxic concentration (C10o). The three procedures evaluated for determining Co and Cl0o are briefly described below. 2.2.1. Cate and Nelson Procedure (CN) The earlier procedure suggested by Cate and Nelson (1965) mainly consisted of drawing horizontal and vertical lines so that the maximum points lay in the negative quadrants (the positive quadrants are considered for nutrient deficiency limits e.g. Sakal et al. 1982). This method had certain drawbacks and was modified by Cate and Nelson (1971). In the modified procedure the data is initially arranged so that the yield (or relative yield) varies with increase in concentration. From the graph plotted a value of concentration is so chosen that it lies two or more points to the left of the vertical line dividing the data into two distinct sets as in Cate and Nelson (1965) procedure. The next step involves calculation of the corrected sum of squares of the deviation from the means of the two yield data sets. The sum of the two corrected sum of squares is then substracted from the total corrected sum of squares of deviations from the overall mean of all yield observations. This difference is then expressed as a percentage of the corrected sum of squares i.e. R 2. The next higher concentration value is now taken and the calculation repeated.
A NEW APPROACH FOR ESTIMATING THE PHYTOTOXICITY LIMITS
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This process is continued and a series of R 2 values obtained for the divisions in yield data made at each concentration level. These calculations are stopped when the concentration reaches a value which is distinctly greater than the apparent threshold concentration. The modified Cate and Nelson procedure essentially consists of dividing the yield data into two distinct sets at various concentrations. The concentration at which the maximum R 2 value is obtained, is considered as the threshold concentration Co. The analysis of the experimental data shown in Figure 3, using this procedure has been presented in Table I.
TABLE I Sample calculations of yield data using the modified Cate and Nelson Procedure C ~ g g - ~)
Y (g pot-~)
YI
CSS1
Y2
CSS2
R2
36.75 38.75 39.00 44.25 52.50" 52.75 58.00 64.60 72.00 75.30
4.30 4.55 4.84 2.43 5.49 2.05 0.31 0.48 3.03 0.54
5.34 5.31 5.30 5.21 5.22 5.12 3.98 4.86 4.81 4.70
15.92 16.52 16.73 24.69 24.77 34.49 56.98 76.70 79.96 97.71
1.44 1.27 1.06 0.98 0.68 0.58 0.60 0.61 0.39 0.37
56.70 46.52 33.03 31.04 9.31 7.29 7.21 7.20 7.93 7.68
0.7055 0.7444 0.7982 0.7740 0.8618 0.8306 0.7397 0.6598 0.6725 0.6006
a Threshold concentration (Co); Total number of observations =48. Y1 = M e a n yield in data set 1 representing Ym. CSSI =Corrected sum of squares of deviations from the mean of data set 1 Y2 = Mean yield in data set 2. CSS2 -- Corrected sum of squares of deviations from the mean of data set 2.
2.2.2. Beckett and Davis procedure (BD) In this procedure, the experimental data is divided into two sets, one of which is assumed to represent a horizontal line and the other a sloping line (Beckett and Davis, 1977). The concentration at which the yields starts decreasing is called the split point i.e. the threshold concentration, Co. Mathematically, the two lines are represented by: Y = Ym Y = Ym - Ym. S. (log C o - log C)
C < C o (la) Co < C (1 b)
In these equations Y is the yield; Ym is the maximum mean yield which is not
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influenced by the presence of the element; C is the concentration; C o is the threshold concentration or split point; and S is the slope of the regression line. As in the previous procedure first all the observations are plotted and a value of concentration distinctly greater than the apparent split point or threshold concentration is chosen. The residual mean square of regression is calculated of this pair of yield and logarithm of concentration values and all concentrations greater than this value. The mean of the plateau (horizontal line) and the variance of all yield observations corresponding to the concentrations less than the arbitrarily selected concentration value, is estimated. The threshold concentration (split point) is calculated as the point of intersection between the regression and the horizontal line for each concentration. Pooled standard error is then calculated from the residual mean squares and variance. This process is repeated with one more pair of concentration and yield values in the regression and one less on the plateau. This implies that now the next lower concentration is chosen. This process is continued till the value of concentration used in the regression obviously lies below the split point. The calculated split points, the pooled standard errors and corresponding concentrations are tabulated. The optimum split point or threshold concentration is the value at which the split point lies between the highest concentration value of the corresponding regression and the next highest concentration that is paired with the first yield observation on the plateau. In a situation where two such points occur, the value with the lower standard error is chosen. Table II summarizes the results obtained following this procedure for the data presented in Figure 3. T A B L E II S a m p l e c a l c u l a t i o n s using the Beckett a n d Davis p r o c e d u r e C (,ug g
21.25 20.75 19.50 19.25 19.00 18.50 17.50 15.00 13.50 7.00
a
1)
Calculated split p o i n t
19.32 19.03 17.70 16.66 16.43 17.93 a 17.35 16.61 16.44 15.75
Y (g pot
4.86 5.26 4.41 4.65 5.43 7.11 5.25 5.32 5.91 6.24
O p t i m u m split p o i n t (Co); R 2 = 0 . 8 0 8 9 .
~)
Mean of plateau Yrn (g pot
5.64 5.66 5.74 5.82 5.85 5.75 5.79 5.84 5.83 5.78
Pooled standard ~) error
48.10 47.84 47.72 47.33 46.75 46.59 47.13 47.15 47.57 57.76
Regression line Intercept
Slope
13.57 13.52 13.21 13.00 12.99 13.22 13.21 13.02 12.92 12.44
- 6.17 -6.14 - 5.99 -- 5.88 - 5.87 - 6.04 - 5.98 - 5.88 - 5.83 - 5.56
A NEW APPROACH FOR ESTIMATING THE PHYTOTOXICITY LIMITS
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2.2.3. Non-linear regression technique ( N L R ) In this technique, it is assumed that there is a threshold concentration at which the yield is unaffected. Beyond this concentration, the yield decreases linearly with concentration until it is reduced to zero i.e. toxic concentration. The response function is, therefore, described by a three piece-wise linear curve. This function can be represented mathematically by a set o f equations given below Y = Ym Y = Y m - Ym.S. (log C - l o g Co) Y= O
0
