Guide for Interpreting CO2 Burst Results
When carbon is released from the soil as CO2 due to microbial metabolism, the nitrogen that was associated with it may become available to support crop yield. Because of this, test results for respiration may theoretically be used to estimate Potential Mineralizable Nitrogen (PMN), with some caveats. The calculator shown below estimates the amount of nitrogen associated stoichiometrically with carbon turnover, assuming soil steady-state conditions. Soils not in steady-state are unrealiably assessed by this method.
For many reasons a soil not be off steady-state making estimating N-potential uncertain. Recent organic matter additions would be an example. Is the respiration from the soil system or from a recent additive? Was a cover crop just incorporated? Perhaps ironically, the healthier a soil is, the more it functions outside of steady-state and is more difficult to predict. Examples are constant infusions of carbon from growing plants, manures and crop residues. One very significant modifier of steady-state is the C:N of plant debris or added compost deviating significantly from normal. Finally, any large fluctuations in wetness and dryness of soils over periods of time will reduce predictability. Unfortunately these postive features are often misunderstood and applied to criticize predictions based on them.
To allow for some non steady-state conditions, this calculator has a special function for altered soil C:N ratios and for seasonal anomalies (too dry or too wet). These forces act to significantly modify – and often severely reduce – the PMN.
The calculator also assumes you have qualified CO2-Burst results employing the latest methods (Solvita-2019). You may consult the Solvita soil lab map for the QAQC ID of each lab performing Solvita tests to determine the method used in the most recent Solvita Reference program. If you are using the Field Solvita method, please consult the Field Calculator which provides a more accurate means to obtain detailed seasonal adjuster factors.
|SOLVITA CO2 Burst (ppm) from Test (1)|
|Available N by Soil Test (lbs/acre)(2)|
|Do you have soil C:N data or Soil Health Tool (SHT) results? (3)||Yes No|
|Total-C or WEOC (a)|
|Total-N or WEON (b)|
|Microbial Biomass (ppm) (4)|
|Solvita Potentially Mineralizable Nitrogen (PMN) (lbs/acre)|
|Total Usable N (lbs/acre)|
|Organic C:N (c)|
|Soil Health Calculation (0-50 Scale)(d)|
|Choose crop from drop-down menu (5)|
|Target Yield (bushels/acre)(6)|
|Cost of Nitrogen Fertilizer ($/lb)(7)|
|Total Crop Required N for Yield Goal (lbs/acre)|
|Gross Cost of N-Fertilizer ($/acre)|
|Actual Required N (lbs/acre)|
|Actual Cost ($/acre)|
|Savings per Acre|
Notes to Calculator:
*This calculator uses a stoichiometric relationship to establish available N which includes C:N. When no C:N ratio is provided, a conservative C:N of 14 is used. For more accurate info please consult your Soil Test User Manual.
(1) Enter the soil lab result for the Solvita CO2-Burst. A valid lab result is obtained by using the latest Solvita-2019 SOP. Values less than 20 mg/kg in soil for CO2 Burst are not recognized as useful for PMN predictions.
(2) This value is for soluble nitrate as NO3-N and Ammonium-N (if available) in lbs/acre. (note ppm * 2 = lbs/a); European Units (kg/ha) use kg/ha * 0.89 or to get to kg/ha from lb/acre use 1.12.
(3) Labs performing a Soil Health Tool validated by Woods End or alternatively any test for soil C:N ratio can fulfill this condition. If you are unsure, choose No.
(4) To convert microbial biomass (ppm) to lb/a, multiply by 2, and to go to kg/ha multiply additionally by 1.12
(5) For commercial vegetables (at heavy feeding level) use the estimate for Tomatoes; Calculate yield as bushels where a bushel tomatoes = 53 lbs (24kg)
(6) To assure target yields are realistic for your state and county, consult your local advisors or try the SNAP tool. Put the Solvita PMN in as Soil Test Results: “Nitrogen lb/a”. If Soil Nutrient Test results are available also input the P2O5 and the K20 from your report into the SNAP tool.
(7) Cost of fertilizer calculated from USDA ERS (mos trecent) data (March 2014) is $0.63/lb.
(a) Water soluble organic carbon is a test offered by Woods End and is used in conjunction with the Solvita 1-day CO2
(b) WEON (water extractable organic nitrogen) is a test some labs offer by taking the difference in total-N versus soluble-N. Due to the inaccuracies of nitrate-N tests, this results in a highly variable and unreliable “water soluble C:N ratio”. Use this value with caution. On the Woods End Labs soil fertility report, soluble carbon is simply divided by actual tested level of available-N for a gross C:N ratio estimate.
(c) This ratio is used to adjust the Solvita N-min to avoid over-estimations that can occur from a high C:N ratio.
(d) This calculation uses various factors and Solvita 1-day CO2 to provide an assessment of overall soil health. Ignore the value if only soil C:N data is being used from normal analysis
Note: Climate factors. The laboratory version of this PMN tool provided with each Woods End SOIL REPORT accounts for your climate location.
Note: Woods End Laboratories presents no implied warranty that calculations rendered from user-inputed data are necessarily correct or accurate for N-min estimates. For supporting data and complementary analyses please consult the Woods End Laboratories soil health test program.