Soil Nutrient Testing
A major reason for soil testing is to evaluate soil fertility (the ability of the soil to provide plants with nutrients).
Other reasons include evaluating physical properties (texture) or chemical properties (soil pH ), and many laboratories that offer soil fertility testing services also analyze these properties in addition to analyses for plants, manure, or water.
Soil fertility testing involves 3 steps:
1) Take soil samples and prepare them for analysis
2) Get the soil samples analyzed
3) Interpret soil analysis results (for fertilizer recommendations)
When the soil is sampled is critical to interpreting soil tests for N. For deciding what rates of N to apply, interpret the pre-sidedress nitrate test (PSNT, mainly corn) to make mid-season decisions about nitrogen fertilizer. Interpret the post-harvest nitrate test (PHNT, all crops) for feedback about nitrogen management in the year of sampling. Read more about these tests in the following:
- Record Keeping and Monitoring (PDF)
(excerpt from Nutrient Management Reference Guide)
Phosphorus (P) and Potassium (K)
Different laboratories use different methods to determine soil test phosphorus (P) and potassium (K). Convert soil test P and K values to “Kelowna method” values to compare results of different methods. Monitor and compare results over several years; samples should be taken at the same of the year to be comparable. Alternatively, use the information below to find soil test-based fertilizer recommendations that you can adjust as you monitor results:
- Soil Test Phosphorus and Potassium Converter Version 2, February 2015 (Downloadable XLS)
- Interpretations for Soil Test Phosphorus and Potassium: Guideline for Southern British Columbia (PDF)
Other Nutrients – All regions of B.C.
Phosphorous (P), Potassium (K), Magnesium (Mg), Sulphur (S), Boron (B), Copper (Cu), Zinc (Zn)
- Soil and Plant Tissue Testing Methods and Interpretations of Their Results for British Columbia Agricultural Soils (PDF, 1.4 MB)
Why is Nitrogen Different from Phosphorus and Potassium?
Whereas the conventional soil tests for P or K were designed to predict crop responses to fertilizer, the conventional soil test for nitrogen was not. The conventional N test taken at the end of the season (post-harvest) looks backward in time. Some laboratories report “estimated nitrogen release,” which is forward-looking, but they are rough estimates which may be unreliable. At this time, the only soil N test that reliably predicts N fertilizer requirements is the pre-sidedress nitrate test.
“The pre-sidedress nitrate test (PSNT) can be used for slower growing crops [like corn but not broccoli] because the soil inorganic N measured a short time after planting provides a measurement of the rate at which soil N is mineralized in time to apply sufficient N for crop but not too much that the N is wasted or becomes a pollutant.” – C.G. Kowalenko, 2004, The Parable of Fast and Slow Growing Crops in Advanced Silage Corn Management.