N U T R I E N T S IN S O I L S
Heterogeneity of Soil of Agricultural Land in Relation to Soil Sampling
MICAH W. M. LEO' Soils Department, Rutgers University, N e w Brunswick, N. J.
The heterogeneity of soil fertility in apparently uniform, agricultural lands was studied. One-hundred composite soil samples were taken from four New Jersey farms. Their available N, PI K, and Mg were determined, and statistical analysis was employed. The coefficient of variability of various available nutrients ranged from 10 to 537& The findings indicate that the heterogeneity of soil fertility of agricultural soils i s far beyond that commonly anticipated. Optimum sample sizes for various nutrients calculated from the data raised a question as to the validity of the present, generally accepted methods of soil sampling. New approaches should therefore be taken to meet the ever-changing soil fertility levels due to modern farming practices, especially intensive use of concentrated chemicals. Further investigation in this direction appears to be urgently needed.
S
has been considered as a heterogeneous body, a mixture of organic and inorganic materials. Naturally, the composition of soils varies from location to location and from site to site ( 7 , 2, 5, 7). However, to a certain extent, soil can also be considered as a homogenous body, in a physical sense, within a limited area ( 7 . 2). For these reasons. the knowledge of sampling has been recognized as one of the most important keys to soil tests. The validity of soil analysis depends on the fairness of samples as well as the technique employed in studying the samples. Tisdale and Nelson ( 9 ) recommend that each composite sample should consist of borings from 15 to 20 locations over an average field of 10 acres. Graham ( 2 ) suggests that a final sample consisting of 5 to 10 of the composites of cores per 40 acres would give quite sufficient and accurate information in that field for most soil management practices. The purpose of taking composite samples is to attempt to minimize the influence of nonuniformity of a given soil. Tisdale and Selson ( 9 ) : however, point out that the composite sample which is approximately equivalent to a pound from an average field of 10 acres (1 pound per 20,000.000 pounds of surface soil a t plow depth) is usually extremely small and would likely cause considerable error. But for labor and time saving, a practical operation must be adapted. The inherent soil fertility of virgin soils, as a rule, is less heterogeneous than the soil fertility of farmed lands where various agricultural activities such as liming, manuring, and fertilization have been conducted. This study reveals OIL
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432
that the soil sampling methods widely used a t present require further critical tests. Materials and Methods
Four apparently uniform lands about 10 miles apart were selected from New Jersey farms ( d ) which had been cultivated since the early settlement of the area. T h e crops of the year before soil sampling were: oats-red clover mixture, corn. corn, and sod for the College farm, Forsgate farm, A. Pinter farm, and E. Pinter farm, respectively. An area of 60 X 250 feet was chosen from each of the farms and subdivided into 25 plots of 12 X 50 feet. Soil samples were taken from the plots a t a depth of about 7 inches with a Hoffer soil-sampling tube. Each composite sample from a plot consisted of 12 borings from different sites selected a t random. Soil samples were air dried and mixed well to eliminate possible errors which may arise from subsampling. The soils were passed through a 2-mm. sieve and analyzed for their available P205, K20. and M g content with the methods described by Hanna and Purvis ( 3 ) . The soils passed through a 1-mm. screen were tested for their potentially available nitrogen by the procedure of Purvis and Leo (4, 6 ) . Statistical analyses of data were employed (8). The optimum number of randomly selected samples assumed with a precision level of the sample-mean within P = 5% were calculated from the coefficients of variation a t the 0.10, 0.05, and 0.01 levels of significance. Results and Discussion
Distributions of the available nutrients in the soils from the four New Jersey farms are shown semidiagrammatically in Tables I to IV.
AGRICULTURAL AND FOOD CHEMISTRY
Nitrogen contents ranged from 62 pounds per acre at the Forsgate farm to 155 pounds per acre a t the A. Pinter farm; PeOj. from 1 pound per acre a t the E. Pinter farm to 244 pounds per acre a t the Forsgate farm; K20, from 35 pounds per acre a t the A. Pinter farm to 600 pounds per acre a t the Forsgate farm; Mg, from 15 pounds per acre a t the E. Pinter farm to 180 pounds per acre a t both the Forsgate and A. Pinter farms. There were definite differences in soil fertility among the various farms. Table V shows the means, standard deviations, standard errors of means, calculated t-values. and coefficient of variation of the available nutrients from each of the four New Jersey soils. The standard deviations of all nutrients of the soils were beyond the confidence limits of significance a t the 1% level, and the variability of the nutrients in soils was remarkably great. As a whole, N tests showed the least variation, M g the greatest, with K 2 0 and PZOSin between according to the order as listed. McKenzie (5) found that all nutrients in a soil tend to rise and fall together in virgin soils. Since there is no such tendency in the farmed soils studied here, it would be reasonable to assume that agricultural activities, such as intensive use of chemical amendments, have drastically changed the nutrient status of the soils. In this experiment, as mentioned previously, one composite sample of 12 borings was taken from each plot of 12 x 50 sq. ft. (or 1 j 7 2 . 6 acre). i.e., about 1 pound of soil sample from 27,550 pounds of surface soil. In comparison with the amounts of soils recommended by Tisdale and Nelson ( 9 ) and by Graham (21, i.e., 1 pound from 20 million pounds and from 80 million pounds. respectively, the size of the
Table 1.
Distribution of Available Nutrients in Soil in the College Farm (Values in Dounds D e r acre)
N.. , I122 PzOb.. . 6 K2O.. . 80 M E . . . . . 40
100 5 75 45
111 5 90 40
117 7 95 50
99 8 65 40
N..... P20,. . . K20.. . Mg.. , ,
96
101 9 110 75
108 9 100 45
101 12 90 45
N..... P,Oa K20. . .
132 11 125 45
119 9 110 40
100 8 85 100
104 23 140 100
N.....
120 9 90 75
110 10 90 60
95 9 100 35
99 7 75 30
N..... PgOa.. . K 2 0 . .,
138 5 100 30
105 6 100 30
92 5 130 30
106
N.. .... Piob.. . KZO.. ,
4 80 45-
N.....
124
.Mg. . . .
N... .
K20.. .
120 60
Mg.. . .
Table 111.
I..
Mg..
P205
~
255 180
180
45 180
N..... Pzob. .. KZO.. , Mg.. , .
180
N..... P20,. . . K20.. .
125
Mg.. . .
N PZOS... KZO. . . Mg . . . .
~
I
l:i 120 40
180 130
~
1
180 635
125 150
121
~
I
180 6;
125 75
1
1 I
55 110
180
180
127
118
180 3;
180 90
N..... P 2 0 6 . .. K20. . . Mg . . . .
70 180
65 180
60 120
80 90
135 5;
~
I
150 55
Mg.,. .
100
104
Table IV.
Mg..
..
87 88 560 120 82 76 500 150
1
87 184 460
82 176 44 0
116 420 160
116 I320 165
480 135
+E-
!
i
73 128 400 150
1
80 164 520
’
I 1
~
156 540 125
; :2 540 135 89 244 520
1 I ~
400 140
w
Distribution of Available Nutrients in Soil of the E. Pinter Farm (Values in pounds Der acre)
25
‘ 8 : 70 30
1 1 7;
8;
80 30
105 30
~
105 30
~
N..... P20a.. . KZO.. .
I
Mg.. . .
VI
0
N..... P206..
-
.
KzO.. .
Mg . . . .
12 ft.
sample used in this study should be considered more reliable with less error introduced by sampling. Holyever, the statistical analysis indicates that there were still tremendous variations in various nutrients in the soils from different plots a t each location. A stratified sampling method is suggested for a land with inherent heterogeneity in soil fertility. HoLvever, it is neither practical nor possible to apply the method in most agricultural soils which exhibit apparent uniformity but actuall>-are heterogeneous. Table VI shows the calculated optimum sizes of sample which are required to satisfy the probability at the 0.10, 0.05, and 0.01 significance levels; respectively.
164 360 135
42 5 180
71 120 500 125
87 276 520
N..... 19: P205... K20.. . 80
90 180
500 125
.
Mg.. . .
30
80 180
450 135
K20. . , Mg.. , ,
Distribution of Available Nutrients in Soil of the A. Pinter Farm (Values in pounds per acre)
...
Distribution of Available Nutrients in Soil of the Forsgate Farm (Values in pc nds per acre)
, ,
P205..
N.. ...
KZO . . . Mg.. . .
Table II.
12 ft.
The optimum size of sample for each nutrient varies significantly from location to location, suggesting that there is no definite pattern of optimum size of sample for various nutrients studied in any single location. For instance. should the 0.05 level of significance be considered, 336 samples should be taken from the College farm for a reliable PaOs test: while only three samples would be enough from the same farm for a representative M g test. These results also reveal that there is no definite pattern of a n optimum size of sample for any one nutrient in various locations. For example, should the 0.01 level of significance be demanded, 1331 observations should be required for a S test a t the
Forsgate farm. and only 2- should be needed for both the A. Pinter and E. Pinter farms. I t is. therefore. concluded that the optimum size of sample for various nutrients of the soils studied ranged from three to more than a thousand observations. The results strongly suggest that the sampling methods currently used are far from being ideal for rhose agricultural lands similar to the ones studied here. T h e sampling sizes of the methods currently in use hardly approach the lowest extremes of the optimum sampling size calculated. If a piece of land is fairly homogeneous in its available nutrients, as shokvn in rhe case of the E. Pinter farm where little
VOL. 1 1 , NO. 5 , SEPT.-OCT.
1963
433
Table V.
Statistical Analysis of Four Agricultural Soils in New Jersey
Available Sail Stofistical Nutrients Measurea
College Farm
Forsgate Farm
A. Pinter
108.96 12.38 2.48h 43.94 11.36
79.00 17.53 3.516 22.51 22.19
124.32 12.36 2.47b 50.33 9.94
90.16 8,94 1.79b 50.37 9.91
8.16 3.74 0.756 10.88 45.81
142.88 49.79 9.966 14.34 34.85
5.48 2.89 9,585 9.45 52.73
1.80 0.41 0.08b 22.50 22.67
100.00 20.41 4.08b 24.50 20,41
474.00 73.21 14.646 32.38 15.45
77 . O O 31.42 6.28‘ 12.26 40.81
79.20 14.91 2.986 26.58 18.82
50.00 2.17 0.436 116.27 4.34
147.80 22.96 4.59b 32.20 15.54
162.40 28.36 5.76h 28.64 17.47
25.20 6.99 1.406 18.00 27.75
Farm
E. Pinfer Farm
Table VI. Optimum Size“ of Sample Calculated from Data of Four N e w Jersey Farms at the 0.1, 0.05, and 0.01 Levels Available Sail Nutrienfs
fertilizer \vas used in the previous years. the eight to 16 composile samples may be adequate for PzOb and N tests. O n the other hand, in a farmed land. such as the Forsgate farm. where fertilizers were heavily banded in rows for years, the variation of available nutrients Ivould be expected to be high, and therefore the size of samples should be appropriately increased. I n vieiv of progress made in chemical techniques for soil resting and increasingly extensive and intensive use of soil amendments on agricultural lands, further study on proper methods of soil sampling \i.ould appear to be urgently needed.
434
College Farm
Forrgate Farm
A. Pinter
E. Pinfer
Farm
Farm
0.10 0.05 0.01 0.10 0.05 0.01 0.10 0.05 0.01 0.10 0.05 0.01
15 21 35 243 336 567 48 67 113 2 3 5
569 788 1331 140 194 328 28 38 65 28 39 65
11 16 27 321 445 752 193 266 450 35 49 83
11 16 27 6 8 14 41 57 96 89 123 208
N PZOS
K?O
Mg 5
?. mean; S. standard deviation; S f , standard error of mean, C. coefficient of \,ariation; t , calculated t - value. .411 units are in pounds per acre except C which is in percentage of the mean. h Significant at the 0.01 lekel.
Precision Level as Percent o f Mean
Formula used to obtain optimum sampling sizes is:
where, n
= slze
of a sample; t
Acknowledgment
The author wishes to thank A. Owens and J. Trio, the Kew Jersey Agricultural Experimental Station, New Brunswick, N. J.?for most of the chemical analyses. Grateful acknowledgment is also expressed to Martin H. Yeh. the University of Manitoba, LVinnipeg, Manitoba, Canada, for his suggestions on statistical analyses, and William J. Zwerman, LTniversity of Alberta, Edmonton, ,41ta., Canada, for his final review of the paper. Literature Cited
Sci. 58,275 (1944). (2) Graham, E. R.: Llniu. Mo. Agr. Eupt. Sta. Bull. 734 (1959).
(1) Cline, M. G., Soil
AGRICULTURAL A N D FOOD CHEMISTRY
=
t-values at the 0.1, 0.05, and 0.01
levels, respectively: C = coefficient of variation (C p = a precision level, e. g., 5yc of the mean
=
i)
; and
(3) Hanna, W.J.. Purvis, E. R.. S.J . Agr. Expt. Sta. Bull. 780 (1955). (4) Leo, M. LV. M., Ph. D. thesis, Rutgers Vniv.. New Brunswick, N. J. (1 960).
(5) ~, McKenzie. R. M., Australian J . Aer. -
Res. 6, 699 (1955). (6) Purvis, E. R.. Leo, M . W. M., J. AGR. FOODCHEM.9, 15 (1961). (7) Reed, J. F.. Rigney, J. A., J . Am. SOC.Agron. 39, 26 (1947). (8) Snedecor, G. W. “Statistical Methods,” 4th ed., Iowa State College Press, Ames, Iowa. 1960. (9) Tisdale, S. L., Nelson, W. L., “Soil Fertility and Fertilizers,” Macmillan, New York, 1956. Received f o r review February 20, 7 962. Accepted n’ouember 28, 1962.
