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

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 CropUnit of Yield P 2 O 5 K 2 O Cornbu Corn silagebu grain equiv Corn silage ton, 65% H 2 O Soybeanbu Oat and strawbu Oat strawton Wheatbu Alfalfaton Red cloverton Pounds per unit of yield Pm-1688 pub, ISU Example from Iowa

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

Soils and Properties

POTASSIUM

Soybean Long-Term Trends

Corn Long-Term Trends

Average Relative Responses

Soil-Test K Long-Term Trends

K Removal & Post-Harvest STK

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

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

K Removal and Yield

PHOSPHORUS

P Soybean: Yield Trends

P Corn: Yield Trends

P Average Relative Responses

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%

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%

Grain P Removal and Yield

Soil-Test P Trends Over Time

P Removal & Post-Harvest STP

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

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.