How often should soil testing be done?
Written by
Nguyen Minh
Reviewed by
Prof. Charles Hartman, Ph.D.Checking the soil every few years will reduce phosphorus imbalances and reduce vegetative feed. Soil testing should occur at least annually before the planting season to see the levels of nutrients you may need. However, calcareous soils should be tested twice a year, given their ability to fix phosphorus at rapid rates. *Keep records for at least three years* so you can manage quantities and replicate.
Standard Soils
- Test annually before spring planting
- Sample 6-8 inches deep across entire area
- Focus on phosphorus, potassium, pH and organic matter
- Compare results to previous 3 years
Problematic Soils
- Calcareous soils: Test fall and spring
- Sandy soils: Test after heavy rains or irrigation
- High-yield fields: Post-harvest and pre-plant tests
- Newly amended soils: Test every 3 months initially
Special Situations
- Container plants: Test potting mix every 2 months
- Hydroponics: Monitor phosphorus weekly
- After extreme weather: Test immediately
- When changing crops: Pre-rotation testing
Calcareous soils require additional testing consideration due to their higher pH, which causes phosphorus to become quickly fixed in insoluble forms. I recommend testing every 6 months to monitor phosphorus depletion. Additionally, tests conducted in the fall will reflect the effects of summer crops. In contrast, tests performed in the spring will aid in seasonally relevant applications during the planting season. To summarize, never skip these tests.
Maintain thorough records for trend analysis. A minimum of 3 years of historical soil test results should be retained. Track your application rates and yield for each crop season. This information illustrates the shifting levels of phosphorus. Your records are informative for adjusting future phosphorus application rates, thereby reducing the risk of either waste or deficiencies.
Utilizing the recommended sampling approaches can provide you with correct results. Take 10-15 subsamples per acre and mix them well. Avoid any unusual spots, such as fence lines or wet areas. Samples should dry before sending them to the laboratory. Label it clearly with the crop history and any previous treatments received.
To accurately interpret the results, it is essential to understand the optimal ranges. Most crops need 15-30 ppm of phosphorus in the soil. Soil levels below 15 ppm indicate a deficiency, necessitating corrective action. Levels above 50 ppm can cause toxicity and lockup. Whenever possible, tailor your recommendation to the specific needs of the crop species.
Testing is required for special circumstances. Following a hefty yield, test for nutrient depletion. Following a major weather event, check for nutrient leaching. When switching crop types, check the new crop requirements first. These tests prevent unexpected nutrient deficiencies during critical growth stages.
Read the full article: Phosphorus Deficiency Plants: Signs and Solutions
