Long-Term Soil P and K Trends inRelation to Nutrient Removal inCorn-Soybean Rotations Antonio Mallarino Iowa State University Joint Meeting NEC-17, NCERA-13, SERA-6June 22-25 2008
Basis of Soil P and K Testing Use • Soil test correlation and calibration • Interpretation: interpretation categories • Fertilizer recommendations • Response-based and often some buildup for the low-testing categories, those with high probability of response. • Maintenance of desirable soil-test levels, those with low probability of response. Based on nutrient removal and/or empirical data.
Removal-Based P and K Maintenance Example from Iowa Pounds per unit of yield Crop Unit of Yield P O K O 2 5 2 Corn bu 0.375 0.30 Corn silage bu grain equiv. 0.55 1.25 Corn silage 3.50 8.0 ton, 65% H2O Soybean bu 0.80 1.5 Oat and straw bu 0.40 1.0 Oat straw ton 5.0 33.0 Wheat bu 0.60 0.30 Alfalfa ton 12.5 40.0 Red clover ton 12.0 35.0 Pm-1688 pub, ISU
Some Relevant Questions • Are assumed buildup and maintenance rate appropriate? • Does maintenance make sense? What level should be maintained? • Does removal-based fertilization really maintain STP and STK levels? • How do yield, removal, and soil-test values in the short term and long term? • What are the impacts of wrong yield and nutrient concentration estimates?
Long Term Studies in Iowa • 5 trials for P, 12 years each, no-till • 5 trials for K, 10 years each, no-till • Corn/Soybean rotations, both crops each year on adjacent identical trials • 3 P rates: 0, 14, and 28 kg P/ha • 3 K rates: 0, 33, and 66 kg P/ha • Broadcast and band placement methods • Measured yield from all plots and soil tests for the control and the high rate
Grain K Concentration and Yield Avg = 1.2 lb K2O/bu Current (dry basis) = 1.7 lb K2O/bu
Grain K Concentration and Yield Avg = 0.22 lb K2O/bu Current (dry basis) = 0.35 lb K2O/bu
Mean = 0.57% Avg = 0.78 lb P2O5/bu Current (dry basis) = 0.92 lb P2O5/bu Grain P Concentration & Yield
Mean = 0.24% Avg = 0.31 lb P2O5/bu Current (dry basis) = 0.44 lb P2O5/bu Grain P Concentration & Yield
Cumulative P Removal & Soil P Averages of Non-Fertilized Plots Across Locations 47 lb P2O5/acre/year for 1 ppm/year decrease
P Removal & Soil P Increase Averages of Fertilized Plots Across Locations
Grain Yield and Concentrations • Crop yields vary greatly across field and years. We overestimated yield, as most farmers do with yield goals. • Grain P and K concentration also vary across field and years • Estimates of P and K concentration in grain were much higher than observed averages. • Increased up to a certain plateau with the fertilizer rate and soil-test values • Grain P and K concentrations were not correlated with yield level across sites.
Removal and Soil Test Values • Our overestimation of yield levels and nutrient concentrations resulted in a significant overestimation of removal. • Removal was well reflected on soil-test values in the long term, but very poorly or not at all in the short term • slow P recycling of residue P and K? • slow equilibria between soil P and K pools?
Implications for Management • Major errors at estimating P and K removal is caused by error in grain yield estimates. • Removal-based P and K maintenance based on good yield and concentration estimates maintain or only slightly increase soil-test P and K it over time. • Removal-based maintenance rates must be based on yield averages over time, not from year to year changes.