1. Long-Term Soil P and K Trends in Relation to Nutrient Removal in Corn-Soybean Rotations Antonio Mallarino Iowa State University Joint Meeting NEC-17, NCERA-13, SERA-6 June 22-25 2008

2. 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.

3. Removal-Based P and K Maintenance CropUnit of Yield P 2 O 5 K 2 O Cornbu0.3750.30 Corn silagebu grain equiv.0.551.25 Corn silage ton, 65% H 2 O 3.508.0 Soybeanbu0.801.5 Oat and strawbu0.401.0 Oat strawton5.033.0 Wheatbu0.600.30 Alfalfaton12.540.0 Red cloverton12.035.0 Pounds per unit of yield Pm-1688 pub, ISU Example from Iowa

4. 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?

5. 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

6. Soils and Properties

7. POTASSIUM

8. Soybean Long-Term Trends

9. Corn Long-Term Trends

10. Average Relative Responses

11. Soil-Test K Long-Term Trends

12. K Removal & Post-Harvest STK

13. Grain K Concentration and Yield Avg = 1.2 lb K 2 O/bu Current (dry basis) = 1.7 lb K 2 O/bu

14. Grain K Concentration and Yield Avg = 0.22 lb K 2 O/bu Current (dry basis) = 0.35 lb K 2 O/bu

15. K Removal and Yield

17. PHOSPHORUS

18. P Soybean: Yield Trends

19. P Corn: Yield Trends

20. P Average Relative Responses

21. Grain P Concentration & Yield Avg = 0.78 lb P 2 O 5 /bu Current (dry basis) = 0.92 lb P 2 O 5 /bu Mean = 0.57%

22. Grain P Concentration & Yield Avg = 0.31 lb P 2 O 5 /bu Current (dry basis) = 0.44 lb P 2 O 5 /bu Mean = 0.24%

23. Grain P Removal and Yield

25. Soil-Test P Trends Over Time

26. P Removal & Post-Harvest STP

27. Cumulative P Removal & Soil P 47 lb P 2 O 5 /acre/year for 1 ppm/year decrease Averages of Non-Fertilized Plots Across Locations

28. P Removal & Soil P Increase Averages of Fertilized Plots Across Locations

29. 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.

30. 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?

31. 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.