Correct soil sampling practices can detect plants deficient in magnesium long before any physical symptoms can be diagnosed. Sampling is conducted at a depth of 6-8 inches, taking samples from several locations, with the aim of selecting a sample that represents the field or garden. Avoid sampling if you have recently applied fertilizer, as this may result in incorrect readings. My practice is to take a sample of 10-15 subsamples across an acre and to mix them in a plastic bucket. Early detection of hidden deficiencies is essential.
Sampling Technique
- Use stainless steel probe at 6-8 inch depth
- Collect 10-15 subsamples per acre or 500 sq ft
- Avoid fertilizer bands or compost piles
- Mix samples thoroughly in plastic container
Lab Analysis Methods
- Request ammonium acetate extraction test
- Measures exchangeable magnesium cations
- Includes soil pH and cation exchange capacity
- Provides base saturation percentage
Timing Considerations
- Test 4-6 weeks before planting season
- Retest every 2-3 years for established crops
- Test during flowering for annual plants
- Avoid testing during drought conditions
Grasping how to interpret laboratory assessments can prevent misdiagnosing nutrient deficiencies based on analytical results. Magnesium levels below 25 parts per million (ppm) indicate a critical deficiency and indicate a need for magnesium. Between 25-50 ppm is a marginal magnesium status and will need to be monitored, while the optimal range varies by crop (generally between 50-120 ppm). I value the results of laboratory analyses for complete nutrient status by comparing with tissue tests.
Choose reputable laboratories for the most accurate analyses. Look for laboratories that use Mehlich-3 or ammonium acetate extraction procedures. When submitting samples, include basic information about the crop and its growth stage. I would also include a brief history of the previous years' crop to help interpret the results. You can typically receive your results back within 7-10 days.
Make corrections based on test results. For magnesium levels below 25 ppm, apply Epsom salts (20-30 lb/acre) immediately. For moderate deficiencies, use dolomitic lime (500-1000 pounds per acre) before planting. Retest the soil 6 weeks after amendments are applied, to verify if the levels have improved. Your systematic approach will ensure a balanced and healthy intake of magnesium.
Read the full article: 7 Critical Signs of Magnesium Deficiency Plants
