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01 November 2013

Pa Soil Test Optimums for Soil Test interpretation

Soil Test Rules of Thumb

Here are some guidelines from an older Agronomy Guide for assessing soil test levels. Dr. Beegle has certainly changed some items but it is a good start.
It is extremely important that if you have surpassed your yield estimates yearly that you account for the high yield depleting your soils over time. For instance if you figured a 150bu/acre corn yield removing 45 lbs of K and then end up getting 200 bu /acre actual or 60lbs a difference of 15 lbs over a period of three years you would have depleted the soil buy about 45lbs or one full crop year.  So keep this in mind as we improve yields and try to maintain soil test levels. Penn State reports now in ppm but in case you are still in the lbs/acre you need to convert.
Conversions
Remember to convert from lbs per acre to ppm. 
  • ppm to lbs/ acre multiply by 2. 
  • K to K20 multiply by 1.2.
  • P to P205 multiply by 2.3
Average Rules of them to maintain between
CEC 8 about average for Pa.
Organic Matter 2% should try to increase.
Saturations
K 2-5%
MG 5-15%
CA 60-90%
pH Changes
Acidity level multiplied by
  • 750lbs of calcium carbonate equivalent to go  pH of 6
  • 840lbs to get to pH  6.5
  • 1000lbs to get to pH 7
Dr. Beegle had an informal response to this concept.
There is nothing wrong with this but we determined through our calibration research that it was unnecessary complication especially considering the variation we see in estimating CEC.  What we gain by using % saturation K to adjust for the CEC we lose in the CEC variation.  For the normal range in CEC ~9-12 meq/100g  we just went to using the K test result ie.  100 ppm is on average in our soils is about 15% above 2% K sat.  If you are at the extremes in CEC ie. Real low or real high this critical levels in this table are probably better.  We could probably just use 60 ppm for low CEC soils and 120 ppm as the critical level for high CEC soils and do just as well.  This is one of those cases where something makes sense conceptually but turns out to not be worth the bother in the real world so in those cases I try to go with the simplest approach.  We still do report the % saturation  on the soil test report.  So with a PSU soil test report, you do not need this table because you can just look at the % saturation directly.  See below.  This soil is 4.5% K Sat and 175 ppm K.  Both give you the same information about the K level in the soil.


All this table did is just convert the 2-3.3% saturation range to lb of extractable K2O/ acre.  Also, we quit reporting lb/A extractable  and just report ppm instead because people seem to think that since this was given in lb/A units you can just subtract these numbers from the recommendation which is absolutely not true. Also, the % saturation approach, while useful, has been greatly abused over the years so this change separated us from that issue.


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