Effect of Sampling Time on Soil Test Potassium Levels. Lauren F. Vitko, Carrie A.M. Laboski, and Todd W. Andraski Dept. of Soil Science UW-Madison

Transcription

1 Effect of Sampling Time on Soil Test Potassium Levels Lauren F. Vitko, Carrie A.M. Laboski, and Todd W. Andraski Dept. of Soil Science UW-Madison

2 Outline Background Objectives Study Results Summary

3 Potash Fertilizer Prices (NASS-USDA, 2009)

4 Fluctuations in soil test K (STK) 1) Clay mineralogy Amount and type of clay minerals determine whether K is fixed (trapped between clay layers) or released 2) Environmental conditions Soil moisture (Leubs et al., 1956) Wetting and drying (Zeng and Brown, 2000) Freezing and thawing (Graham and Lopez, 1969)

5 Fluctuations in soil test K (STK) Other factors Plant uptake of K Possible leaching of K (sandy soils) K return through residue

6 Current recommendations Currently, UW recommendations suggest that it is best to sample consistently at same time of year But do not specify what time of the year to soil sample Are fluctuations in STK levels great enough in Wisconsin soils to have an impact?

7 Objectives 1) Determine how much STK levels fluctuate throughout the three-year study 2) Determine if STK differs significantly between fall and spring 3) Assess potassium buffer capacity (KBC) at each location (buildup and drawdown)

8 Study details Field sites Experimental design Soil collection and analysis

9 Field sites Location Arlington Hancock Lancaster Marshfield Fond du Lac Soil name Plano silt loam Plainfield sand Fayette silt loam Withee silt loam Kewaunee clay loam STK initial ppm 123 (H) 40 (VL) 70 (L) 111 (Opt) 95 (Opt)

10 Experimental Design Three year study: Split plot design, 4 replications Corn harvest management as the whole plot factor (grain and silage) Potassium fertilizer rate as the subplot factor (0, 67, 134, 201, 268, 335, 401 lb K 2 O/a) Corn in 2006 Soybeans in 2007 Corn in 2008

11 Spring 2006 Fall 2006 Rep1 Rep2 Rep3 Rep4 Silage 268 lb K 2 O/a Grain 67 lb K 2 O/a 134 lb K 2 O/a 0 lb K 2 O/a 201 lb K 2 O/a 401 lb K 2 O/a 35ft 335 lb K 2 O/a 20ft

12 Experimental Design Three year study: Split plot design, 4 replications Corn harvest management as the whole plot factor (grain and silage) Potassium fertilizer rate as the subplot factor (0, 67, 134, 201, 268, 335, 401 lb K 2 O/a) K fertilizer broadcast and incorporated in 2006 only All locations chisel plowed, except Hancock (moldboard) Corn in 2006 Soybeans in 2007 Corn in 2008

13 Soil collection/analysis Soil collection: 2006: before fertilizer application, 3 times mid-season, post-harvest 2007: before planting, 2 times midseason, post-harvest 2008: before planting, post-harvest Plots that were harvested for silage were only sampled pre-plant and post-harvest each year 6, 8" cores, composited from each plot Soils were oven-dried (95 C) and extracted with Bray-1 for K (current WI method)

14 Sampling Time and STK Marshfield p < 0.01 p= 0.07 p< 0.01 p= 0.03

15 Sampling Time and STK ΔSTK 3years = 31 ppm Marshfield p < 0.01 p= 0.07 p< 0.01 p= 0.03

16 Sampling Time and STK ΔSTK 3years = 11 ppm Marshfield p < 0.01 p= 0.07 p< 0.01 p= 0.03

17 Sampling Time and STK ΔSTK 3years = -6 ppm Marshfield p < 0.01 p= 0.07 p< 0.01 p= 0.03

18 Sampling Time and STK ΔSTK 3years = -20 ppm Marshfield p < 0.01 p= 0.07 p< 0.01 p= 0.03

19 Variability between replications ΔSTK 3years = -44 ppm Arlington p= 0.33

20 Slight change in STK ΔSTK 3years = -8 ppm Fond du Lac p= 0.15 p= 0.36

21 Slight change in STK ΔSTK 3years = -10 ppm Fond du Lac p= 0.15 p= 0.36

22 Fall versus spring sampling for all K rates Percentage of treatments per location that increased, decreased, or stayed the same in STK over-winter Location same + - same % % Arlington (44) Hancock 100 (71) 100 (36) Lancaster (10) 29 Marshfield Fond du Lac 100 (25) Value in parentheses is percent of those treatment that are statistically significant (P<0.05)

23 Inconsistencies between years and treatments ΔSTK= -29ppm* ΔSTK= 11ppm Arlington

24 Potassium Buffer Capacity (KBC) Amount of fertilizer needed to increase STK 1ppm KBCs currently used to determine fertilizer application rates for low and very low testing soils Soil Group Location in study KBC lb K 2 O/a per 1 ppm STK A Lancaster 7 B Arlington 7 C Fond du Lac 7 D Marshfield 6 E Hancock 6

25 KBC: buildup and drawdown of STK K2O rate (lb/a) Buildup KBC: Marshfield y = 4.9x R 2 = STK date2 -STK date1 (ppm) K2O rate - cum. K2O removed (lb/a) Drawdown KBC: Marshfield y = 10.7x R 2 = STK fall 08 -STK spring 06 (ppm) Buildup KBC: Slope of the K 2 O applied versus the change in STK Where the change in STK is: STK six weeks after fertilizer application- STK prior to fertilizer application Drawdown KBC: Slope of the net K 2 O removed versus the change in STK Where the change in STK is: STK post-harvest STK before fertilizer application in 2006

26 KBC results Location Harvest management system KBC buildup KBC drawdown lb K2O/a per 1 ppm soil test K Arlington Grain Silage 12.5 Hancock Grain Silage NS Lancaster Both NS 16.1* Marshfield Both * Fond du Lac Grain Silage NS KBC buildup was calculated for grain plots only. Data from grain and silage plots combined. *Indicates significantly (P<0.05) different than the value of 6 or 7 lb K 2 O/a per 1 ppm soil test K NS NS

27 Summary and Conclusions Effect of time on STK was generally significant throughout the three-year study However, STK remained fairly constant for some locations/treatments Changes between fall and spring STK inconsistent Best to sample same time of year KBC generally not different than current values KBC was higher when calculated as a drawdown

28 Thank you to sponsors! Wisconsin Fertilizer Research Program Potash Corp Walsh Soil Fertility Fellowship