How do you test soil for zinc deficiency?
Written by
Nguyen Minh
Reviewed by
Prof. Samuel Fitzgerald, Ph.D.Professional soil testing for zinc deficiency includes careful sampling and laboratory analysis to evaluate nutrient availability. The most accurate results require that the collection and extraction techniques be performed properly. Soil tests report zinc levels and other related parameters, e.g., pH level and phosphorus, that may affect zinc uptake. Soil testing is diagnostic- not a recipe; thus, applied nutrients will not be wasted or misapplied.
Sample Collection
- Collect 10-15 subsamples per acre
- Use plastic tools to prevent contamination
- Take samples from 6 inch (15 cm) depth
Preparation Method
- Air-dry samples at room temperature
- Remove stones and plant debris
- Mix thoroughly before sending to lab
Lab Submission
- Request DTPA extraction specifically
- Include pH and phosphorus analysis
- Provide crop type information
The DTPA extraction method has long been recognized to provide the highest quality zinc test. As a chelate-based solution, it mimics the process by which plant roots absorb metals, resulting in the most accurate available zinc measurement. Labs report back results in mg/kg, with anything below 0.5 indicating a deficiency. Always request this method, as other methods will yield values that underestimate zinc availability.
Consider the results with the pH & phosphorus data. At a pH above 7.0, zinc becomes unavailable, regardless of the test result. Phosphorus concentrations exceeding 30 ppm Olsen-P will also inhibit zinc uptake. This comprehensive analysis determines whether zinc application or pH remediation is prioritized.
The timing of soil testing is crucial for achieving an accurate diagnosis. Soils should be tested every 2-3 years for maintenance purposes and at least once per year in areas where deficiencies are identified. Soil testing should be done preferably during the spring or fall when temperatures are more temperate. Testing should generally not be done immediately after a zinc application or during periods of drought for best results.
Utilize soil tests in conjunction with tissue analysis for confirmation. Collect young, actively photosynthetic leaves 4-6 weeks after applying foliar products. For the majority of crops, a target tissue concentration of 20-100 ppm zinc is considered sufficient. This combined analysis allows one to ensure that the soil test is accurate and the treatment is effective.
Read the full article: Zinc Deficiency Plants: Symptoms and Solutions
