Twice Monthly Field Soil Sampling For Soil Testing To Evaluate Reproducibility Of Soil Test Levels

Illinois Fertilizer Conference Proceedings
January 23-25, 1995

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Twice Monthly Field Soil Sampling For Soil Testing To Evaluate Reproducibility Of Soil Test Levels

Ted R. Peck and Marilyn E. Sullivan¹

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Introduction

Two previous monthly field soil sampling studies have been done in Illinois. The results indicated relatively stable soil pH and phosphorus soil test levels on soil samples collected from the field during all months of the year. The potassium soil test showed a cyclic nature with low K soil lest levels in late summer (August), relative to high soil test levels in midwinter (February) but great variability of K test levels occurred. These studies involved a four year crop rotation (corn, corn, wheat, hay) at Brownstown in southwestern Illinois with an unfertilized plot and a modest K fertilized plot ahead of corn and wheat. The other location was a three year crop rotation (corn, wheat, hay) with an unfertilized plot and a modest K fertilized plot ahead of corn. Tillage involved the mold board plow. Studies were done 1960-64 and 1964-66 respectively. Low soil phosphorus levels, low usage of nitrogen, and great variation in K test from month to month were the basis for doing a third soil sampling seasonality study.

The objectives of this study were to more clearly characterize the seasonal trends of field sampling for soil testing.

Materials and Methods

Soil samples were collected twice monthly, first and mid-month. Soil sampling started April 1, 1986. Care would be taken to obtain a 7-inch depth core of soil. Plot area was 20 feet wide by 80 feet long, and for the first five years, separate soil samples were collected form each third of the plot. The plot area had a history of annual K application of 0, 50, 100, 150, 200, and 250 lb K₂O/acre on plots 205, 204, 206, 202, 201, and 203, respectively, during the years 1970 through 1983. On the date of field sampling a portion of the field moist soil was analyzed for field condition K level and moisture level. Beginning in April 1991, the annual K and P rates were resumed on the north one/third portion of the respective plots. Sampling the south one/third portion of each plot was discontinued. Beginning in mid-June 1991, a separate sampling of the surface 0-1 inch depth from the center one/third area which continues to be residual of fertilization since the last application in 1983 was made.

Results and Discussion

Typical soil moisture levels are depicted in Figure l. The serious drought in the summer of 1988 is shown in figure 2. In this soil type, plant growth might be irreversibly stopped at about 10 % moisture level. Release of K from the clay inter-layer positions starts at about 8' and progressively increases with further decrease in moisture. This is a factor in soil K test increase to be observed in later figures.

Soil pH levels are generally stable as shown in figure 3. Our laboratory policy is to read the pH meter to the closest one/tenth unit. Experienced laboratory technicians will attest to drift and uncertainty of pH meter operation and in routine work accept one/tenth pH unit. These plots are cropped to corn and soybean in alternate years. Figure 4 shows a strong decline in soil pH in early summer believed to be associated with nitrification of anhydrous ammonia applied as the N carrier. The upward spike of pH in mid-May may be the recently applied anhydrous ammonia.

Soil phosphorus is not a variable in the potassium study. These P test levels shown in figures 5 and 6 are low and consistent through the year. Higher P test levels in the surface inch relative to the 0-7 inch layer are probably due to exudation and leaching of P from the crop plants.

Separate samples were collected from the north third, center third, and south third of each plot with the intent to average the results to reduce variation. However as shown in figures 7 and 8, the different parts of each plot are consistently different throughout the sampling period.

Higher K test levels in the surface 0-1 inch depth relative to the surface 0-7 inch depth are shown in figures 9, 10, 11, and 12. Due to the 14 year history of different K rate, each plot depicted in the respective figure, has a different K soil test. The differential K test level between the surface inch and surface 7 inch composite in the four figures 9, 10, 11, and 12, are similar. In the fall, the differential is at the greatest range. Three mechanisms that likely contribute to the differential are 1) plant exudation and leaching, 2) death of the plant and complete leaching of K from residues, and 3) dryness of the surface soil which may include the freeze/dry effect that may release K from clay inter-layer positions.

The relationship of the soil K test level in the field moist soil condition as compared to the air dry soil condition are shown in figures 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, and 24. The series of graphs shown in figures 13, 14, 15, 16, 17, and 18 represent increasing residual soil K levels as the result of differential increasing annual K rates during 1970 through 1983 and unfertilized since 1983. The difference in K test level between the field moist and air dry soil condition decreases as the residual soil K level increases. The series of graphs shown in figures 19, 20, 21, 22, 23, and 24 represent portions of preceding plot where annual K rates were resumed in early April 1991. Current K fertilization decreased the differential between the K test of filed moist and air dry condition. In general, the field moist K level is higher.

The trend of soil K levels from the "14 year annual rate of 100 K20 for 1970 through 1983" are shown in figures 25, 26, 27, 28, 29, 30, 31, 32, and 33. Three items are noteworthy.

Figure 25 showing the initial year of the study indicates a K soil test level about 400 while figure 33 shows the ninth year for the residual portion of the plot with a test level about 300.
The first four years of the study, 1986, 1987, 1988, and 1989 (Figures 25, 26, 27, and 28) tend to show the expected seasonality with relatively lower soil K levels during the summer and relatively higher soil K levels during the winter. The next four years, 1990, 1991, 1992, and 1993 (Figures 29, 30, 31, 32) show minimal seasonal change.
The resumption of annual K rate fertilization in April 1991 is not noticeable in the soil K level until the 1994 April application.

The present study in its ninth year is showing more fluctuation in soil pH levels perhaps due to the higher N rate than used in previous studies, less seasonal change in soil K level perhaps due to portions of the plot having longer residual time from K fertilizer application.