Illinois
Fertilizer Conference Proceedings
January 25-27, 1993
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Ted R. Peck and Marilyn E. Sullivan1
Two previous monthly field soil sampling studies have been done in Illinois.
The results indicated relatively stable pH and phosphorus soil test levels on
soil samples collected from the field during all months of the year. However
the potassium soil test showed a cyclic nature with low K soil test levels in
late summer (August), relative to high K soil test levels in mid-winter (February)
and K soil test levels somewhere between during the rest of the year. While
the cyclic seasonal nature of K soil tests was consistent the K soil test levels
from year to year were inconsistent in reflecting a higher or lower soil K fertility
level relative to established acceptable critical levels. While there is some
understanding of the causes of K soil test fluctuation, manipulating the soil
sample and predicting the soil K test level are with little success. The objective
of this study into sample, twice monthly, field plots with different history
of potassium and phosphorus fertilization applications for a period of years
to better characterize the reproducibility of soil test levels over time to
improve the predictability of soil tests.
The study utilizes pre-existing land areas devoted to P and K rate build-up at the Agronomy/Plant Pathology South Farm, Urbana, IL. Soil type is Drummer silty clay loam. Plots being used are a series of twelve field plots, each 20 feet wide by 80 feet long with a history from 1970 through spring 1983 of six plots receiving annual rates of K ranging from 0 to 250 lb K2O/acre by 50 lb increments and six plots receiving annual rates of P ranging from 0 to 200 lb P2O5/acre in 50 lb increments. Since the spring of 1983 application of fertilizer, the plots have been residual and cropped to corn or soybeans in alternate years for a plant breeding nursery so nutrient uptake and removal were minimal. Beginning in the spring of 1991 the annual application of fertilizer was resumed on the north one/third of each plot.
Tillage on the plots has involved fall chiseling, disking and field cultivation.
On years of corn production, anhydrous ammonia has been the nitrogen carrier.
Early in the sampling study depth increment samples on the ridges and in the
furrows did not show serious stratification of test levels. This year and future
plans are for more intensive crop production to place more stress on the soil
source of nutrients. A data base of twice monthly soil sampling exists, beginning
in mid-march 1986 on the K rate plots and in mid-November 1986 on the P rate
plots. Three 5core composite soil samples to a depth of 7 inches have been taken
from each plot representing each end and the middle one/third. Immediately after
collection from the field, soil samples from the K rate history are passed through
a 4-mesh screen, mixed, a portion tested for exchangeable K level and the moisture
content measured. The remainder of the sample and samples from the P rate history
are air dried and analyzed in the usual manner. Beginning in mid-June 1991 on
the K history plots and June-first 1992 in the P history plots, a surface soil
sample of the 0-1 inch depth is "skinned" with a sharp-edged tin can.
Representative soil moisture conditions are shown in figures 1 and 2. The horizontal axis of the graphs(x axis) represents one year starting in April. The points on the graphs represented by the different symbols represent single composite soil sample levels. On this soil type, soil moisture levels above 35% would be a saturated and muddy condition. Field moisture capacity or the condition of excess water drainage would be approximately 28%. Permanent wilting point or the condition when a plant will wilt and not recover will be about 8 %. Potassium release from expanding clay mineral lattice positions with soil drying may start at 8 %, increasing at 3 %, and markedly increasing at 2 to 1 1/2 %. Figure 1 represents the average moisture in the 7-inch surface layer for six years of the study. Figure 2 shows 1991 and 1992 7-inch surface layer soil moisture represented by "square" and "plus" symbols and 1-inch surface layer soil moisture represented by "triangles" and "x" symbols. The 1-inch surface layer soil moisture shows more extreme than the 7-inch layer, reaching a low of 1-2% in June and July of both years.
The relationship of "field moist K test" and "air dry K test" is shown in figures 3, 4, 5, 6, 7, and 8. The differential K history from 1970 through 1983 has induced differential extractable K levels. Figure 3 shows the data from plots with no history of K fertilizer application, figures 4, 5, 6, and 7 having increasing annual K fertilizer application and figure 8 showing the highest annual rate used in this study of 250 lb K2O/acre . With no history of K fertilization and at low levels of annual use the tendency for the "field moist K test" to be lower than the "air dry K test" is consistent. This is an illustration of the well accepted and long know characteristic of soils with 2:1 clay mineralogy known as - K release on drying. Higher annual applications of K fertilizer generally bring the "field moist" and "air dry" test levels together. This is also consistent with a soil chemistry concept, that their is an equilibrium level beyond which, fixation will occur on drying. It is interesting to speculate whether the differential between "field moist" and "air dry" is a percentage of the extractable soil K or a specific form or an amount.
Representative water pH soil test data are shown in figures 9 and 10. Six years of data from a 20 foot wide by 26.7 foot long area totaling 144 soil samples are reported in figure 9. A preponderance of test levels fall in the range of pH 5.8 to 6.0. Considering this variation with reference to interpretation and agricultural limestone recommendation it is of concern. Part of the variation may be due to the long standing policy of reading the soil test pH to the nearest tenth of a pH unit. Considering the merits of reading the soil pH test to a hundredth of a pH unit is debatable from the standpoint of reading equilibration. Figure 10 shows the soil pH levels for the past two years of the 7-inch depth, along with the soil pH levels of the 1-inch depth. Greater range of soil pH levels occur in the 1-inch depth and this being a part of the 7inch depth is part of the explanation of 7-inch variation.
Air dry soil K tests from six years of twice monthly sampling of the 7-inch surface soil for the six different histories of K fertilizer are shown in Figures 11, 13, 15, 17, 19, and 21. Slight seasonality of the test levels occurs (i.e., lower K test levels in late summer relative to mid winter), but it is not of the magnitude shown in two previous studies. This study is different from those studies in that current K fertilization of the plots was occurring during the sampling period while this study involves field plots that last received K fertilizer in the spring of 1983. Application of K fertilizer rates in this study resumed in the spring of 1991 and again in 1992 on the north 1/3 of each plot but consistent effect on soil tests is not apparent. Figures 12 , 14, 16, 18, 20, and 22 shows the air dry soil K tests for the 7-inch surface sampling for 1991 and 1992 along with the K tests of the 1-inch soil sampling, also for 1991 and 1992. Generally the K test of the 1-inch soil sample reflects a higher test level than the 7-inch soil sample.
Phosphorus soil tests of the current and past two years of twice monthly soil
samplings are shown in Figures 23, 24, 25, 26, 27,
and 28. P test levels of the
7-inch depth soil samplings show slight annual decline. Plots with lower P fertilizer
rate history show test levels of the 1-inch depth soil samplings slightly increased
compared to P test levels of the 7-inch depth sampling.
Results of this study to date are tending to corroborate soil chemistry concepts,
providing illustrative examples and furthering understanding of soil testing.
The relationship of "field moist K test" and "air dry K test"
is a positive example. The lack of seasonal K soil test behavior of the magnitude
of the two previous studies needs further study. Also, the lack of soil test
response to resumption of fertilizer rates (data not shown in this report) needs
further study.
Figure 1. Soil moisture level in the 7-inch soil surface layer
Figure 2. Soil moisture level in the 7-inch and 1-inch soil surface layer
Figure 9. Soil pH test in the surface 7-inch layer
Figure 10. Soil pH tests in the 7-inch and 1-inch surface soil layers
1Professor and Research Specialist, Dept. of Agronomy, Univ. of IL, Urbana, IL.
