Long-Term P and K Applications: II. Effect on Corn and Soybean Yields and Plant P and K Concentrations

Increasing emphasis on environmentally sound nutrient management requires an improved understanding of fertilizer P and K needs for profitable crop production. A 20-yr study was conducted on two glacial till soils to determine: (i) the effect of various P and K rates on corn (Zea mays L.) and soybean (Glycine max (L.) Merr.] yields and profitability during a 12-yr application phase and an 8-yr residual phase, and (ii) whether triennial applications of P and K produce crop yields equal to those from annual applications. Initial Bray P₁ and exchangeable K soil tests were 22 and 150 ppm, respectively, for the Webster clay loam (tine-loamy, mixed, superactive, mesic mic Endoaquolls) and 10 and 228 ppm, respectively, for the Aastad clay loam (fine-loamy, mixed Pachic Udic Haploboroll). Phosphorus and K were applied annually for 12 yr at rates of 0, 50, and 100 lb P₂O₅/acre and 0, 50, and 100 lb K₂O/acre while rates of 150 lb P₂O₅/acre and 150 lb K₂O/acre were applied every third year. Yields were improved by the 50- lb P₂O₅ rate in 6 of 12 yr when the soil test P (STP) was <22 ppm for the Webster soil and in 8 of 12 yr when the STP was ≤10 ppm for the Aastad soil. Yields were not improved further by the 100-lb P₂O₅ rate on either soil. Yields from triennial P application were similar to those from annual P application in 23 of 24 siteyr. Annual application of fertilizer K increased yield only in 3 of 24 site-yr. During the 8-yr residual period, yields were increased above the control in all 15 site-yr by carryover from the 541b P₂O₅ rate. Corn yields were optimized at leaf K concentrations ranging from 1.14 to 2.29%, indicating the importance of knowing nutrient uptake characteristics of hybrids when assessing the K status of corn. Economic return was generally highest with the 150-lb P₂O₅ rate applied every third year. Results from this study indicate that most profitable long-term returns can be obtained with broadcast applications of 150 lb P₂O₅/acre on a triennial basis and by monitoring STP and soil test K (STK) to take advantage of residual availability. Research QuestionEconomically profitable and environmentally sound crop production requires a better understanding of long-term fertilizer P and K applications for improved nutrient management. The objectives of this study were to determine the effects of P and K fertilizer rates on corn and soybean yields and profitability during a 12-yr application phase and a subsequent 8-yr period to measure the residual effect, and to compare triennial applications of P and K with annual applications of the same total quantity of nutrients. Literature SummarySoil testing lab summaries indicate that soil test P (STP) and K (STK) frequently have been built to High and Very High levels by fertilization during the past 30 to 40 yr. Studies reported during the past 10 yr, however, indicate little, if any, yield response by corn or soybean to fertilizer P and K applications when STP or STK are in the Mediuim or higher range. Some studies have recommended that profits could be improved by not applying P and/or K to soils testing above the Mediuim range. Study DescriptionLong-term field studies were conducted from 1973 through 1993 on a Webster clay loam at Waseca in south-central Minnesota and on an Aastad clay loam at Moms in west-central Minnesota. Phosphorus and K were applied annually for 12-yr (1973–984) at rates of 0, 50, and 100 lb P₂O₅/acre and 0, 50, and 100 lb K₂O/acre. Rates of 150 lb P₂O₅/acre and 150 lb K₂O/acre were applied every third year to additional plots. Corn was grown in 14 yr and soybean in 6 yr. Corn ear leaf samples were collected at silking and analyzed for P and K concentration. Soil samples were collected from the 0- to 7-in. depth each September before treatment application. Corn and soybean grain yields were taken by combine harvesting each October. Applied QuestionsDid corn and soybean yields respond to annual applications of both P and K? In the 12-yr application phase of this study, yields were improved 6 yr on the Webster soil with an initial STP of 20 ppm and 8 yr on the Aastad site with an initial STP of 10 pprn (Bray P₁). During the residual phase, yields were increased by the previously applied 50-lb P₂O₅ rate in 8 of 8 yr on the Webster soil and 7 of 7 nonstress years on the Aastad. Yields were not improved further by the 100-lb P₂O₅ rate. Fertilizer K increased yield only in 3 of 24 site-yr during the application phase at these two initially high STK sites. Were crop yields higher with annual application of 50 lb P₂O₅/acre or triennial application of 150 lb P₂O₅/acre? Yields from triennial P applications were similar to annual P applications in 23 of 24 site-yr. Ten-year corn yields averaged 150.1 and 150.0 bdacre for the Webster soil and 110.1 and 112.0 bdacre for the Aastad soil for the annual and triennial P treatments, respectively. Two-year soybean yields averaged 52 and 53 bdacre on the Webster soil for the annual and triennial applications, respectively, and 49 bdacre for both treatments on the Aastad soil. What were the optimum ear leaf P and K concentrations at silking? Optimum leaf P concentrations varied considerably among the 20 site-yr, but yields did not appear to be reduced in any site-year when leaf P was >0.30%. Because of very high year-to-year variability in leaf K, which appeared to be related to corn hybrid, and the lack of yield response to fertilizer K, a critical leaf K concentration could not be determined. In the 3 yr when yield was increased with fertilizer K, yields were optimized at leaf K concentrations of 2.29, 1.69, and 1.14%. Thus, corn hybrid should be considered when comparing leaf K concentrations to standard published critical levels. Were net economic returns affected by the rate and frequency of P and K application? Highest economic return during the 12-yr application phase was obtained at both sites with triennial applications of 150 lb P205/acre (Table 1). When the 8- yr residual period was included highest return occurred with the annual 50-lb P205 rate for the Webster soil and the 100-lb P205 rate for the lower testing Aastad soil. Economic return to K was minimal due to the lack of yield response. RecommendationsBecause corn and soybean yield response is so closely related to STP and STK, we recommend that fertilizer P and K rates be based on soil test for optimum, long-term profitability. When past fertilizer applications have exceeded crop needs and soil tests have been built to High to Very High levels, residual P should be used for increased profitability. Economic return can be increased by maintaining STP and STK at optimum devels with triennial applications due to reduced application costs. Hybrid nutrient accumulation characteristics should be considered when interpreting leaf K concentrations. Table 1Cumulative economic return to annual and triennial applications of fertilizer P and K (including cost of application) during the application (12-yr) and application + residual (20-yr) phases on the two soils†. WebsterAastadNutrientApplication rate12-Yr20-Yr12-Yr20-Yrlb/acre per yr---------------- $/acre ---------------P₂O₅50136.50641.20242.40662.60100−92.40381.40210.90797.30150 (triennial)170.70--312.00--K₂O5068.30--−48.10--100−128.50--−124.90--150 (triennial)32.60--−70.60--†P₂O₅ = $0.22/lb; K₂O = $0.12/lb; Application = $3.00/acre/application; Corn = $2.40/bu; Soybean = $6.30/bu.