Soil & Tissue Tests for Micronutrients
Because micronutrients concentrations are very low in both soils and plant tissue, it is hard to judge sufficiency levels based on soil or plant tissue tests alone. The general recommendation is to test both soil and tissue from both “good and “problematic areas” where micronutrient deficiency is suspected. The comparison of the micronutrient concentrations of the samples will help determine if the problem is caused by micronutrient deficiency.
It is important to know the testing methods or extractions used by testing labs and the methods reported on the tables when evaluating sufficient levels of nutrients. Additionally, it is critical to observe the units to be sure that comparisons are equal.
Soil Tests
Soil test results should be compared with micronutrient fertilizer guidelines for your state. The following tables are micronutrients guidelines provided by Oregon State University Extension (Table 1) and Montana State University Extension (Table 2). Keep in mind that these are guidelines; decisions on micronutrient fertilization should be made based on knowledge of crop response to micronutrients in the local area. We recommended that growers conduct on-farm trials for >2 years to determine if micronutrient fertilization is beneficial.
Table 1
Extractable micronutrient soil test categories and suggested fertilizer rate recommendations by Oregon State University Extension (source: Soil Test Interpretation Guide, EC1478, 2011).
Micronutrient | Test method | Sufficiency Level | Sufficiency Level (ppm) | Recommendation (lb/acre) |
---|---|---|---|---|
Cl | Water or dilute salt | Very low | 0-5 | 0-150 (lb KCl/acre) |
Cl | Water or dilute salt | Low | 5-10 | 0-150 (lb KCl/acre) |
Cl | Water or dilute salt | Medium | 10-20 | 0-50 (lb KCl/acre) |
Cl | Water or dilute salt | High | 20-50 | 0 |
Cl | Water or dilute salt | Excessive | >50 | 0 |
B | Hot water | Very low | <0.2 | 1-3 (lb B/acre) |
B | Hot water | Low | 0.2-0.5 | 0-3 (lb B/acre) |
B | Hot water | Medium | 0.5-1 | 0-1 (lb B/acre) |
B | Hot water | High | 1-2 | 0 |
B | Hot water | Excessive | >2 | 0 |
Zn | DTPA | Sufficient | ≥1.5 | 0 |
Zn | DTPA | Deficient | <1.5 | 5-15 (lb Zn/acre) |
Cu | DTPA | Sufficient | ≥0.6 | -- |
Cu | DTPA | Deficient | <0.6 | -- |
Mn | DTPA | Sufficient | 1-5 | -- |
Mn | DTPA | Deficient | Deficient only occur when soil pH is >8.0 | -- |
Fe | Soil testing for Fe is not recommended because most test methods do not discern between forms of iron, and has little meaning for plant nutrition. | -- | -- | -- |
Mo | Soil testing for Mo is not recommended because soil Mo concentration are too low for most labs to evaluate | -- | -- | -- |
Table 2
Micronutrient fertilizer guidelines based on soil analysis by Montana State University Extension (source: Nutrient Management Module No. 7. Micronutrients: cycling, testing, and fertilizer recommendations, 4449-7, 2009)
Micronutrient | Soil test (ppm) (DTPA Test) (top 6 inch soil samples | Fertilizer recommendation (lb micronutrient/acre) |
---|---|---|
B | 0-0.5 | 2 |
B | 0.5-1 | 1 |
B | >1 | 0 |
Cu | 0-0.5 | 2 |
Cu | >0.5 | 0 |
Fe (inherently unreliable) | 0-0.25 | 4 |
Fe (inherently unreliable) | 2.5-5 | 2 |
Fe (inherently unreliable) | >5.0 | 0 |
Mn | 0-0.5 | 20 |
Mn | 0.5-1 | 10 |
Mn | >1 | 0 |
Zn | 0-0.25 | 10 |
Zn | 0.25-0.5 | 5 |
Zn | >0.5 | 0 |
Plant Tests
Similar to soil testing for micronutrients, plant tissue testing can help determine the nutrient status of the plant (or plant parts) at the time of sampling. Plant analysis may help confirm deficiencies suspected from visual deficiency symptoms. The following tables show example guidelines for sufficiency ranges in crop plants.
Critical Levels of Micronutrients in Dry Plant Tissue
Micronutrient | Typical sufficiency range (ppm) | Deficiency (ppm) | Toxicity (ppm) |
---|---|---|---|
Fe | 50-250 | <50 | >300 |
Zn | 25-150 | <10-20 | >400 |
Cu | 5-20 | <4 | – |
Mn | 20-500 | <15-20 | – |
B | <20 | 6-18 in monocots 20-60 in dicots | – |
Cl | 0.2-2.0% dry weight | 70-700 | >4% dry weight |
Mo | <1 | <0.2 | – |
Ni | 0.1-1 | – | – |
Co | 0.02-0.5 | – | – |
Critical Nutrient Levels for Wheat Tissue Test (Whole Plant, Seedling before Jointing)
Nutrient | Critical Range (% Nutrient in Dry Matter) |
---|---|
N | 4-5% |
P | 0.2-0.5% |
K | 2.5-5.0% |
Ca | 0.2-1.0% |
Mg | 0.14-1.0% |
S | 0.15-0.65% |
Fe | 0.003-0.02% (30-200ppm) |
Mn | 0.002-0.015% (20-150ppm) |
Zn | 0.0018-0.007% (18-70ppm) |
Cu | 0.00045-0.0015% (4.5-15ppm) |
B | 0.00015-0.0004% (1.5-4ppm) |
Mo | 0.00001-0.0002% (0.1-2.0ppm) |
Critical Nutrient Levels for Winter/Spring Canola Tissue Test
Nutrient | Critical Range (% nutrient in Dry Matter) | Peak Nutrient Level |
---|---|---|
N | 2.5-4.0% | Emergence |
P | 0.25-0.8% | Emergence |
K | 2.6-5.3% | End of flowering |
S | 0.9-1.81% | Throughout season |
Ca | 1.3-3.0% | Emergence and peak flowering |
Mg | 0.15-0.4% | End of flowering |
Cu | 0.0002-0.0007% | Throughout season |