Illinois Fertilizer
Conference Proceedings
January 28-30, 1991
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M.J. Mainz and G.A. Raines1
Shortly after the new Agronomy Research Centers at Perry (Orr Center) and Monmouth (Northwest Research Center) were formed in 1979 and 1981, respectively, Phosphorus and potassium rate studies were started. Since then soil fertility levels have been monitored annually for P&K buildup and drawdown. Corn, soybean and wheat (Orr); and corn and soybean (NW) yields have been taken to evaluate the effect of the changes in soil test levels. Starting in 1990, each of the original plots in these studies were split into four subplots, each of which received a different fertilizer rate.
Crop yields were generally not affected by the new rates with the exception of when fertilizer was applied to the old zero rate plots. Corn and wheat responded most to the P rates while soybeans responded to the K rates. Yield differences between the old or new rates were not as great as expected for the zero rate plots.
Interesting developments are showing up in deep soil sampling. Soil tests at the 8-16 inch depth are responding to fertilizer applications more than was expected, since the plots at Monmouth have never been moldboard plowed; and those at Orr only three times in nine years. Potassium soil tests in the 8-16 inch depth at Orr are much less than those at the 16-24 inch depth, even to the point of affecting crop yields.
Phosphorous (P) and potassium (K) rate studies were started in 1980 and 1982 at the Orr (Perry) and Northwest (Monmouth) Research Centers, respectively. Phosphorus rates of 0, 60, 120, and 180 lbs/a of P205 and potassium rates of 0, 100, 200, and 300 lbs/a of K20 have been applied annually to corn soybeans and wheat at the Orr Center. At Monmouth, phosphorus and potassium rates of 0, 40, 80, and 120 lbs/a of P205 and K20, respectively, have been applied only in years when the respective fields are planted to corn. No fertilizer was applied the alternate years when soybeans are grown. Generally, fertilizer rates have been applied in the fall at the Orr center and spring at Monmouth. Fertilizer is surface broadcast with a Gandy dry fertilizer spreader.
Plots have been chisel plowed at Perry, with the exception that the entire area is moldboard plowed once every three years. At Monmouth, corn stalks were chisel plowed and soybean residue received a shallow tillage with field cultivator or disk from 1982-84. In 1985, fields at Monmouth were changed to a ridge-till system, and have remained in that system until the spring of 1990.
In the spring of 1990, all of the plots at both locations were divided into four subplots. Each of the respective rates are now annually applied to each of the four old rate plots. Example: The old Zero rate plots were subdivided to include the four rates (0, 40, 80 and 120 lbs/a). This is done for all crops at both locations.
Prior to 1990, soil samples were generally taken from every plot and/or subplot, annually at both locations for both the P & K studies. Sampling depth has always been from the surface 8 inches at Monmouth. Originally the sampling depth was from the surface 12 inches (0-6 and 6-12) at Perry. But, starting this spring, soil samples were taken in 8-inch increments to a 24-inch depth in all plots at both locations. The additional depths were taken in an effort to help explain why these soils weren't responding to P or K fertilizer as expected.
Corn yields were generally not significantly different at either location. Lowest yields were found in the 0-0 plots at both locations. But, at Pi tests of 20 and 29, Orr and NW Centers, respectively, a yield response has occasionally occurred. Corn yield averages, 166.2, 173.7, 185.4 and 185.3, showed a good response to the old P fertilizer rates, or 0, 60, 120 and 180 lbs/a, respectively. They indicate sufficient P fertility when 120 lbs/a P205 have been applied annually. Soil P levels have not been drawn-down sufficiently at Monmouth to produce a yield response every year.
Generally, the P rates did not produce significant responses in grain moisture, test weight or plant population at either location. There appears to be a trend toward lower grain moistures at the higher P rates. Grain test weights were generally the lowest in the old zero rate plots and increased with P fertilizer. At Perry, test weights were increased 1.5 lbs/bu at the new 120 lb average compared to the new 0 and 60 lb averages.
Soybean yields were not significantly different for either the old or new P rates or their interactions at either location. Yield averages varied less than 2.5 bushels per acre for both the old and new rates at both locations. At Perry, grain moistures were the driest at the 180 lb/a rate (old rate) and significantly different from the other three rates. A similar response was observed in soybean plant height with the shortest plants at the 180 lb/a (old)rate. At Monmouth, there were very few differences between any of the plant parameters measured in response to P fertilizer rates.
Wheat yields at Perry were generally not significantly different for either the old or new P rates or their interactions. However, there was a trend toward lower yields at the highest P rates. The highest wheat yield was at the 0-0 (old-new) rate plot. Grain moisture was the highest at the 0 rate and then decreased with increased P fertilizer. Test weight was not affected by P rate.
At the ORR Center, P, and P2 soil tests at the 8-16 inch depth generally followed the pattern of the 0-8 inch depth except at a much lower level. That is the lowest tests were in the old zero rate plots and the highest in the 180 lb/a plots (see soil test graphs). Both Pi and P2 soil tests are significantly different for all three crops at both the 0-8 and 8-16 inch depths. P soil tests at the 16-24 inch depth are. extremely low with P, and P2 tests only a few units apart.
At the NW Research Center, there was no significant difference between the old rate plots for the-8-16 or 16-24 inch deep samples for either Pi or P2 tests. There appeared to be a trend toward higher P soil tests at the 8-16 inch depth for one field. There was a good pattern in response to the 4 P rates at the 0-8 inch depth (see graphs).
Corn yields were significantly different in their response to added potassium (K) at both locations, but only to the new rate averages. New rate yield averages, when averaged across the old rates for Perry (163.0, 174.7, 175.4 and 171.7) and Monmouth (177.6, 180.9, 184.1 and 184.3) for the 0, 100, 200, 300 and 0, 40, 80 and 120 lb/a rates for Perry and Monmouth, respectively. Yields were' more responsive in 1990 to the new K rates than they were to the old rates. Generally yields didn't respond much to K fertilizer.
Test weights and grain moistures were not significantly affected by K fertilizer rate, except that at Monmouth, the lowest grain moistures were found in the old zero rate plots. Corn populations at Perry were the highest at the new 300 lb rate for all the old K rate plots. Neither populations nor lodging was affected at Monmouth.
Soybean yields had a significant response to the old K rates At Monmouth. Yields were as follows: 54.7, 56.5, 56.9 and 57.3 for the 0, 40, 80 and 120 lb/a plots, respectively. At both locations, the zero-zero plot (old-new rate) had the lowest soybean yields. At the old zero rate, yields steadily increased with each addition unit of K fertilizer. At the Orr Center, there was a trend towards higher yields with both the old and new K rates, although not significant.
Soybean grain moistures were significantly affected by K rates at Perry but not at Monmouth. Grain moisture was the highest at the zero rate and decreased with each additional unit of fertilizer. Soybean populations, lodging score and test weights were not significantly affected at either location.
Wheat yields were erratic in their response to K fertilizer at the Orr Center. There appears to be a replication interaction that may be interfering with the responses. A soil type change in the K rate study appears to be more significant than do the fertilizer rates.
Potassium soil tests in the K rate study at the Orr Centet change greatly with depth. The surface 8-inch depth responds very nicely to each added increment of K fertilizer. Potassium tests at the 8-16 inch depth are much lower at the two highest rates, but occasionally higher than the 0-8 inch depth at the lower rates. At the 16-24 inch depth, the K soil tests were generally 150+ lbs/a of available K higher than any of the 8-16 inch depth and the lower rates at the 0-8 inch depth. It would'- appear that there is sufficient K for crop growth in the 16-24 inch layer, but the plants never reach it. The low readings in the 8-16 inch deep layer might indicate that the plants are utilizing it during some years, but that it isn't being recharged by the surface applications. If this is a general pattern on these forest soils, it might be useful to make deep applications of potassium.
The biennial applications of potash at the NW Center have not been sufficient to maintain the K soil tests. But, the crop yields would indicate that there still is enough available to produce optimum yields. Potassium soil test levels at the 8-16 and 16-24 inch depths don't appear to be responding to the surface fertilizer applications. There is very little difference between the two depths at any rate. Switching to annual K applications should help buildup the soil test values; although they haven't responded according to the text book in the past.
Yield responses to the new fertilizer rates were not as great as had been expected, especially on the old zero rate plots. At both locations the soil fertility levels are below those recommended in the Illinois Agronomy Handbook. Close examination of the soil test graphs and their corresponding yield graphs would indicate larger yield response. Corn and wheat generally responded more to the phosphorus rates than did the soybeans. The soybeans responded more to the potassium rates than did the corn and wheat, but there were always exceptions. Plant populations were not generally affected by any of the fertilizer treatments. Grain moistures were usually the highest at the zero rates, but not always.
We have always known that plants get water and nutrients from depths other than the surface 8-inches. But, soil sampling is not generally taken any deeper than the plow layer unless testing is done for nitrates. It is not generally expected that P or K fertilizers move much in the soil, unless moved by tillage. Since the moldboard plow is only used occasionally in the fields at Perry and never at Monmouth, it may be that the P fertilizer is moving more than was once expected. The extremely low K soil tests in the 8-16 inch depth at Perry may indicate a mining of this soil type. It would also appear that there is an abundance of available K only a few inches deeper in the profile. But for some reason, the crop hasn't been able to utilize it. There are still more questions than answers in these rate studies. Continuation of this study should give a clearer picture of what is happening in the soil profile.
Leaf tissue, grain and. additional soil samples have been taken during the 1990 growing season. Researchers are currently waiting for the results from these samples. Soil fertility levels will continue to be monitored in these studies and others on both research centers.
Phosphorous Rate Study, NWRC 1990. P1 soil tests. Soybean
Phosphorous Rate Study, NWRC 1990. Soybean Yields.
Phosphorous Rate Study, ORC 1990. P1 soil tests. Soybean
Phosphorous Rate Study, ORC 1990. Soybean Yields
Potassium Rate Study, NWRC 1990. K soil tests. Soybean
Potassium Rate Study, NWRC 1990. Soybean Yields.
Potassium Rate Study, ORC 1990. K soil tests (Spring 90). Soybean
Potassium Rate Study, ORC 1990. Soybean Yields
