It has been apparent to those working in soil testing laboratories in Arkansas and other states that seasonal variations occur in soil test data. This has been observed in results from experimental plots that were sampled three or more times during the year. It has also been measured in soils sampled a number of times during the year to determine the rate of lime reaction by changes in soil pH values. In these studies, the samples taken in the fall from unlimed plots showed the lowest pH values.
Seasonal variations in soil test values have been reported in the literature. Some of the factors influencing variations in soil pH are the presence of soluble salts and carbon dioxide.
The purpose of this study was to determine the nature and magnitude of variations in soil test values for some of the prominent soil types used for row crops in Eastern Arkansas. It was felt that knowledge of the variations could be used to adjust limestone recommendations.
Eighteen soil series from Eastern Arkansas were used for the study. They included the following texture classes: silt loam, sandy loam, clay and clay loam. Most of the soils were in cotton or soybeans during the two summer sampling season. At each site, permanent reference points were established so samples could be taken from the same location. Sampling was started in June 1965 and was repeated approximately every two weeks through November. From December 1965 through April 1966, samples were collected monthly, weather permitting. From May until December 1996, or until the rows were destroyed after the 1966 crop, samples were taken approximately every two weeks. The soil pH was determined by using a standard laboratory method which uses de-ionized water. The soil pH was also determined by using salt solutions containing either 1.0N KCl or 0.01M CaCl2 to see if this would suppress the seasonal variations.
Table 1. Average and range for water pH values for 18 soil sites.
Silt loam loess
5.40 - 5.90
5.04 - 5.48
4.56 - 5.08
4.54 - 5.02
7.26 - 7.94
Sandy loam alluvium
6.00 - 6.90
6.40 - 7.20
6.12 - 6.98
5.40 - 5.96
5.48 - 5.90
Silt loam alluvium
5.70 - 6.34
6.42 - 6.96
5.06 - 5.70
5.16 - 5.58
Clay and clay loam alluvium
6.10 - 6.80
5.44 - 5.74
5.40 - 5.96
5.52 - 6.36
Table 2 gives the distribution of soil pH values. The shift of plus and minus pH values by season is apparent for all soils. There are less plus values in the spring and summer samplings and relatively more in the fall and winter. Minus readings are more apparent in the spring and summer months with almost none in the fall months.
Table 2. Distribution of Water pH by Season.
Summer Fall Winter 1965 1965 1966 + Ave. - + Ave. - + Ave. - All
9 87 35 23 81 4 11 33 8 Spring Summer Fall 1966 1966 1966 All
+ Ave. - + Ave. - + Ave. - 2 43 19 6 72 19 20 50 1 + = greater than the average plus 1 standard deviation
Ave. = overall average plus or minus 1 standard deviation
- = less than the average minus 1 standard deviation
There was no consistency for pH variation by season within the soil texture classes. The only generalized statement that can be derived from study is the overall trend towards lower pH values in the spring and summer and higher pH values in the fall.
The soil pH values which were determined by using salt solutions containing either 1.0N KCl or 0.01M CaCl2 showed similar patterns of variation as the standard pH method.
By: Joseph Keogh and Richard Maples