How to test soil for magnesium deficiency?
Written by
Liu Xiaohui
Reviewed by
Prof. Martin Thorne, Ph.D.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
