Soil test phosphorus variability in .33 acre grids

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Soil sampling is an essential practice to maximizing yields and economic returns while protecting the environment in grain crop production systems. To that end, Grid Soil Sampling (GSS) has been widely utilized by many of Ohio farmers with the purpose of gathering soil test information on small areas of a field to facilitate better nutrient management. Soil samples taken are geo-referenced thus permitting varying amounts of fertilizer or lime to be applied per soil test result in designated smaller areas as opposed to blanket applications across a field. Further by overlaying soil test results, yield maps, soil type maps and topographic maps, better associations of the factors influencing yield (and profit) can be made along with the appropriate management actions.  

Traditional agricultural crop soil test results (2.5 acre grids or field samples of 10 acres in size or larger) are often quite variable. To examine soil test variability over smaller field areas; 0.33 acre square grids were geo-referenced and soil tests taken from the same spot for 5 years in north central Ohio. An analysis of 6 randomly selected 0.33 acre grids from a total of 15 grids was conducted to examine the stability of soil test P over time, fertilizer applications and crop removal. The soil tests were taken in November of each year. There were not any crops grown in year one of initial soil testing.

Phosphorus removed per unit of yield  over the time period (180 bu/A corn 2 crop years; 48 bu/A soybean for one crop year, and  one crop year of 93 bu/A wheat ), was calculated at 230 lbs/acre P2O5. The total amount of fertilizer applied in the four year period was 309 lbs P2O5.  Thus, P soil test levels would be expected to go up and that occurred in 5 out of the 6 randomly selected grids from year 1 soil test values to year 5 soil test result.  Average soil test level P over the 6 grids went from 17 to 28 ppm P.  

If the following relationship were to be used to measure soil test P buildup (soil test P increase by 1 ppm for every 20 pounds P2O5/ applied over crop removal); the expected soil test P increase would be 4 ppm (309-230/20). The increase in P (11 ppm) is not totally explained by the fertilizer applied, crop removal budget or relationship used to calculate the increase in soil test P. However, research done in Kentucky by Thom and Dollarhide in 2002 found initial soil test P to be a major factor influencing how much P2O5 is needed to increase soil test P level.  Thus, many factors such as: soil type, initial soil test level, soil pH, soil test method, type of phosphorus applied, weather, time of year of the soil testing, soil test location, soil test P laboratory calibration, etc. may have impact on the final soil test P result. In conclusion, even though variability in soil test P can be expected, soil testing is an essential practice to estimating plant available P needed to  protect and preserve crop yields, manage fertilizer applications, and  protect the environment.

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Nick Tharp    
Liberty, IN  |  November, 28, 2011 at 03:43 PM

Still a lot we don't know about interactions between crops, nutrients applied, weather and everything else going on out there in the field.

Nick Tharp    
Liberty, IN  |  November, 28, 2011 at 03:43 PM

Still a lot we don't know about interactions between crops, nutrients applied, weather and everything else going on out there in the field.

Tim Smith    
Monticello, IL.  |  December, 01, 2011 at 10:35 AM

The build up and drawdown numbers should not be the same for all soils. It is easier to buildup soils than draw them down. We also are only seeing part of the picture when we sample only to 6-8 inches. the plant draw nutrients from deeper in the soil and leave residue on the soil surface that has nutrient that have been drawn from deeper in the soil.

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