Illinois
Fertilizer Conference Proceedings
January 25-27, 1993
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Michael J. Mainz and Glenn A. Raines1
Phosphorus and potassium soil test levels at the Northwest (Monmouth) and Orr (Perry) Agricultural Research Centers have not always increased or decreased at predicted levels. Nor have the various crops grown on these soils always produced yields in response to the existing soil fertility levels or added fertilizer. In 1990, the highest wheat yields at Perry in the phosphorus rate study occurred in the plots with the lowest P1 test and the lowest yields occurred in the most fertile plots. The following 2 years the results were reversed. Similar patterns have been observed in both corn and soybeans at both locations. Weather or more specifically, rainfall patterns, will influence crop yields to the extent that soil fertility levels and fertilizer applications may be detrimental. It is impossible to predict crop yield responses in relationship to fertilizer applications and soil fertility. Soil test changes in response to crop removal and fertilizer have varied with soil moisture and temperature at sampling time. The year to year variability makes it difficult to monitor exact changes with any certainty. Leaf tissue analysis and its associated DRIS index may be used in the future to monitor soil P & K levels and their associated yield response.
Starting in the spring of 1990 the existing phosphorus (P) and potassium (K) rate studies at the Northwest and Orr Research Centers were split into 4 subplots, each of which received a different rate of either P or K. Phosphorus rates of 0, 40, 80, 120 and 0, 60, 120, and 180 pounds of P2O5 per acre, are annually applied to all crops at Monmouth and Perry, respectively. Potassium rates of 0, 40, 80, 120 and 0, 100, 200, 300 pounds of K2O per acre are used at the respective centers. Nitrogen and either P or K is applied at recommended rates to each crop. No effort has been made to maintain equal PK ratios at either location. Ag lime has been added to selected plots at Perry when needed to bring the pH into agreement with the other plots.
Conservation tillage practices were used at both locations to incorporate the fertilizer and produce the crops. Except that once every 3 years both the P & K rate studies at Perry are moldboard plowed. Soybean and wheat stubble is shallow tilled with either a disk, field cultivator or the DynaDrive prior to planting. Generally, all fertilizer applications, except nitrogen, have been made in the fall. High yielding hybrids and varieties that are best adapted to each location have been used. The same hybrid/variety has been used these past 3 years for each respective study. Weed and insect control measures are used when needed. This included pesticides, mechanical cultivation and occasionally hand weeding.
Soil samples were taken following harvest each year in 8 inch increments to
a 24 inch depth, from every plot in all studies and locations. Soil samples
were analyzed for pH, P1, P2, and K at all 3 depths plus organic matter (OM)
and cation exchange capacity (CEC) for the surface 8 inch depth. Ear leaf samples
are taken at pollination as are soybean and wheat leaves. A grain sample is
saved for visual and nutrient evaluations. All tissue and grain samples are
analyzed for N, P, K, Ca, Mg, S, B, Fe, Mn, Cu, Zn and Al. A DRIS index is calculated
for all leaf tissue samples. Limited space prohibits the inclusion of all the
data in this report.
Wheat yields at Perry had the greatest response to P rates and soil P levels in 1991 & 1992. Wet weather occurring early in 1990 increased disease levels in response to increasing soil fertility and fertilizer applications, thereby reducing yields. Highest yields in 1990 were in the zero-zero P rate plots. Except for 1990, optimum yields were obtained at soil test P levels above 45 or annual applications of 120 lb/a of P2O5 (Table 3).
Corn yields at Perry responded to the low P testing soil by increasing yields with the addition of P fertilizer (Table 2). Initial soil P1, tests at Perry can be found in Table 7 for the old 0, 60, 120 and 180 lb/a rates, respectively. Only a minimal yield response has occurred for any crop on the old 120 and 180 pound plots because of the high soil test levels. Optimum yields, 95 or better "Relative yields", generally occurred only at the highest testing soils or those with additional P fertilizer (Table 2). A soil P level of 40+ has produced optimum yields on the forest soils found at Perry and surrounding area.
Phosphorus soil test levels, both P1, & P2 tests, are much lower at Perry than Monmouth. Prior to the University of Illinois acquiring the NW Research Center in 1980, a previous owner must have made generous applications of rock phosphate and/or livestock manure. The high P2 tests at Monmouth have virtually eliminated any yield response to P fertilizer. Relative yields of 90% or greater have occurred these past 3 years for all P rates and soil test levels. Highest corn and soybean yields have frequently occurred at the lowest soil test levels when some P was added. Soil test P levels have dropped to a low of 29 at Monmouth.
Potassium rate study
Soybean yields at Perry were generally the lowest at the lowest soil K level (old zero rate treatments) all three years of the study (Table 5). Yield increases were obtained on the old' zero and 60 lb/a plots through the addition of potash fertilizer. Initial K test levels can be found in Table 7. Only on the old 200 lb/a plots were the K soil test levels above 300.
Corn and wheat yield responses to soil K levels and potash didn't vary much (Tables 4 & 5). Weather patterns during the three years of the study appear to have affected corn and soybean yields greater than soil fertility levels. Wheat yields were generally lowest for the new zero rate treatments, but not always (Table 4). Corn yields didn't vary much in response to soil K levels, except that the lowest yields were generally at the lowest K levels.
At Monmouth very few of the plots have soil K test levels in the surface 0-8 inch depth below 2501bs/a. Corn and soybean yield response has been minimal, if any, throughout the duration of the study. Highest soybean yields generally occurred when soil K levels were at the highest levels and over 300 lbs/a. Relative corn and soybean yields only dropped to 85 % of the high yielding treatment in any particular year.
DRIS Index
Leaf tissue samples have been analyzed for several nutrients including P and K. In addition to the nutrient analysis the lab also gives a DRIS index value for each nutrient. DRIS analysis can be used to determine the nutrient balance in the plant. These past 2 years the DRIS index for either P or K follows soil test levels very accurately. Lowest DRIS values occurred at the lowest soil test levels and increased with increasing fertility levels. Leaf samples from 1992 have not been analyzed when this report was written, but with 2 years of data it looks promising to utilize DRIS as another tool for monitoring P & K fertility levels.
Soil Sampling
Soil sampling has been done in 8 inch increments to a depth of 24 inches at both locations. Previous sampling, prior to 1990, had only been done on the surface 0-8 or 0-6 inch layers at Monmouth and Perry, respectively. Deep sampling is being done to help monitor movement of nutrients in the soil profile. With only 2 years data it is difficult to detect any patterns that may be developing in soil fertility levels below 8 inches.
Deep soil testing has shown high to very high P and K levels at both locations with the exception of the 8-16 inch depth at Perry. This depth is very low in K for the potassium rate study. Closer examination of this depth has found a "shaley" layer a few inches thick. This shaley layer appears to be dense enough to restrict roots from growing through it most years. Either the roots have mined all the K from this depth or the shaley layer has tied it up, making it unavailable to the plant and soil tests. Deep subsoiling has helped loosen it, but not eliminate it. Additional testing and subsoiling will help to identify its effect on K fertility and crop growth.
There is a slight increase in soil test P & K levels in the 8-16 inch depth at Monmouth that follow the pattern of surface applications. Since deep chisel plowing or moldboard plowing have not been done to these fields since the study began, it can be assumed that the fertilizer is moving through the profile. The high to very high P2 levels are masking out any possible effect of added P fertilizer.
Potassium levels in the surface 8 inches are not changing at anticipated rates for either location, but especially at Monmouth. The K test has a good response the first year and then there occasionally is a significant drop in K test levels. Since the K rate study started in 1982 at Monmouth, an effort has been made to drawdown K levels so that a negative yield response for corn and soybeans would occur when K is not applied. As K levels approach the low 200's there appears to be the start of yield decreases, especially for soybeans. In 1992 K deficiencies were visible for most of the growing season in the zero-zero plot border. Yields were decreased, but not as much as indicated by plant growth.
Efforts are underway to calculate the amount of P & K removed from both studies since their inception. Since grain analysis has been done on a limited basis a common value is being used rather than actual numbers. This effort should yield a drawdown estimation for each soil type. The influence of the deeper layers on the surface 8 inch layer will be difficult, at best, to figure into a formula. We have been using the values of 9 and 4, found in the Illinois Agronomy Handbook, for amounts of P2O5 and K2O, respectively needed to increase soil tests 1 pound per acre. These values are close, but once a certain level is reached, the values do not work. It will be useful to know a value that could be used for removal and drawdown of soil test levels. Soil tests are not drawdown at the same rate they are built up.
Following analysis of the 1992 soil, leaf tissue and grain samples we will be better able to make calculations on buildup and drawdown of soil P & K levels. The unusually wet and dry years we have been experiencing have significantly affected soil test results. The drastic increases in P & K levels in some years have made it impossible to develop patterns with crop removal and soil test drawdown. We have also gone back and done additional calculations on pre 1990 soil, leaf and grain analysis data when we have it. This long term data will help us develop the drawdown and buildup formulas for the soils in these two areas of the state.
Yield data from these past 3 years and others from the first 8 and 10 years
of the P & K rate studies at Monmouth and Perry, respectively, could help
to reinforce fertility data currently found in the Illinois Agronomy Handbook.
A P, soil test of 40+ and K test of 300+ should produce optimum yields on these
soil types when used with a maintenance PK program.
Table 1: Relative Corn and Soybean Yields at Monmouth, 1990-92. Phosphorus rate study
Table 2: Relative Corn and Soybean Yields at Perry, 1990-92. Phosphorus rate study
Table 3: Relative Wheat Yields at Perry, 1990-92. Phosphorus rate study
Table 4: Relative Wheat Yields at Perry, 1990-92. Potassium rate study
Table 5: Relative Corn and Soybean Yields at Perry, 1990-92. Potassium rate study
Table 6: Relative Corn and Soybean Yields at Monmouth, 1990-92. Potassium rate study
Table 7: Soil test levels for the 0-8 inch depth at Perry, Fall 1989
Table 8: Soil test levels for the 0-8 inch depth at Monmouth, Fall 1989
1Agronomists with the University of Illinois and Superintendents of the Northwest and Orr Agricultural Research Centers, respectively.
Mainz, J.J., J. Sutor, T. Tinkham and R. Caulkins. 1983-91. Phosphorus and Potassium Rate Study. Northwestern Illinois Agricultural Research and Demonstration Center, Report of Research Results.
Peck, T.R., and et.al., 1991-92. Illinois Agronomy Handbook. Circular 1311. University of Illinois, College of Agriculture, Cooperative Extension Service.
Raines, G.A., T.E. Hallock and M. Vose. 1981-91. Phosphorus and Potassium Rate Study. Orr Agricultural Research and Demonstration Center, Report of Research Results.
