The Effect of Four P & K Rates on the Drawdown and Buildup of Soil Test Levels

Illinois Fertilizer Conference Proceedings
January 23-25, 1995

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The Effect of Four P & K Rates on the Drawdown and Buildup of Soil Test Levels

Michael J. Mainz and Glenn A. Raines¹

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Introduction

Crop yield response to added phosphate or potash fertilizer has been minimal most years since these studies were originally started in 1980 and 1982 at the Orr (Perry) and Northwest Research Centers (Monmouth), respectively. In some years, soil tests values have not correlated well with observed yield responses. This revised study was started in 1990 to a) monitor drawdown and buildup of pH, P₁, P₂ and K soil test levels at the 0-8, 8-16 and 16-24 inch depths, when 4 rates of phosphorus (P) and potassium (K) are applied annually to dark prairie and moderately dark forest soils that have received varying rates of P or K biannually and annually, for 8 and 10 years at Monmouth and Perry, respectively, and then annually since 1990; b) correlate soil test changes at the 3 depths with fertilizer applications and removal in the grain; c) monitor movement of P & K in the 0-24 inch soil profile; and d) monitor the effects of P & K rates on grain nutrient analysis, grain yields and crop growth for corn, soybeans and wheat (wheat at Perry only).

Materials and Methods

Separate P and K rate studies were started in 1980 and 1982 at the Orr and Northwest Agricultural Research Centers, respectively. Annual applications of 0, 60, 120 and 1801bs P₂O₅ and 0, 100, 200 and 300 lbs K₂O/acre were made to the respective plots in each study. These rates were compared on corn, soybean and wheat each year at Perry. Because of high initial soil test levels at Monmouth, P₂O₅ and K₂O rates of 0, 40, 80 and 120 were applied every other year rather than annually in an attempt to lower the soil test levels. Starting in 1990 all of these large plots were subdivided into 4 subplots for the split or "New" rates. For example: every "Old" 0, 40, 80 and 120 lb/acre plot now receive 0, 40, 80 and 120 lb P₂O₅ or K₂O/acre annually. Instead of the original 4 treatments there are now 16 combinations of the old and new P or K rates.

Other changes made in 1990 include: a) additional soil sampling depths of 8-16 and 16-24 inches are now taken annually and analyzed for pH, Pl, P2 and K, b) leaf tissue samples were taken at early pollination for corn and soybeans and analyzed for 11 elements (these samples were discontinued in 1993), c) grain samples were collected and analyzed for 11 elements, and d) surface soil samples are now analyzed for organic matter (OM) and cation exchange capacity (CEC).

Uniform nitrogen (N) applications are made annually to the wheat and corn. Ag lime and P or K are applied to maintain soil tests at recommended levels. Herbicides and/or mechanical tillage are used to control weeds. Insecticides are applied when needed. Hybrid/variety selection is maintained in 3 year cycles. Tillage practices were changed at Monmouth in 1985 and again in 1990. Prior to 1985 and starting again in 1990, corn stalks were chisel plowed, generally in the fall, soybean ground was left untouched until prepared for planting with secondary tillage implements such as the disk or DynaDrive. Following harvest at Perry all ground was chisel plowed with the exception of when the moldboard plow was used in 3 year cycles following wheat.

Results and Discussion

Phosphorus soil test values (Fall 1993 sampling) at Perry range from very low (12) to very high (246) while those at Monmouth range from medium (46) to very high (182). At Perry, there doesn't appear to be a trend or correlation between P soil test and/or P fertilizer application and corn yield response in 1994 (Table 1). Corn yields ranged 18 bu/a at Monmouth, with all soil P tests above 45 lb/a. Neither soil test level nor fertilizer application consistently affected corn or soybean yields at Monmouth (Table 1 & 2). Generally, soybean yields were highest at Perry when fertilizer P was applied in 1994, except in those plots that had soil tests above 45 lb/a.

Wheat yields in 1994 were affected more by soil type and environmental conditions than either fertility level or fertilizer P application. Table 6 shows the plot plan for the wheat portion of the P Rate Study at Perry. The blank rows between the blocks/reps represent the blocks for the other crops. Wheat yields above 66 bu/a are all printed in BOLD. With few exceptions, all of the higher yields were on Herrick silt loam soil.

Potassium fertility levels at Perry range from 138 to 700 lbs/a (Tables 3, 4, and 5). Wheat yields did not appear to be affected by K soil test level or fertilizer rate (Table 3). At soil test levels less than 200 lb K/acre, yields were increased by the application of K at Perry. At Monmouth K levels ranged from 238-448 and didn't significantly affect corn yields. Soybean yields at Monmouth were generally the lowest at K tests below 250 lbs/a (Table 5). At Perry the lowest soybean yields were on those plots receiving 3001bs/a of K₂O every year for 5 years.

Summary

More data that was collected during the past year than could be included in this report. The focus here is on Fall 1993 soil samples and their affect on 1994 grain yields. Again this year there has been very little response by grain yields to either soil test levels or added fertilizer. These results are very similar to those seen in the past, especially at Monmouth. Fertilizer application rates are too high at Perry to allow us to accurately predict crop responses. For example: the 300/300 plots in the K rate study have received 4500 pounds per acre of K₂O over the past 15 years. Potassium soil tests continue to increase at Perry, but not at anticipated rates. Previously we have discussed the variability of soil test results on an annual basis. This and previous work suggests strongly that P-1 and K test values of 50 and 300 respectively are sufficient to provide maximum yield of corn and soybean, and P and K fertilizer applications should be limited to maintenance rates.