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 micronutrients 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 used on the tables when evaluating sufficient levels of nutrients. Additionally, it is critical to observe the units to be sure that comparisons are equal.
The following tables are micronutrients guidelines provided by Oregon State University Extension and Montana State University Extension. However, the 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.
Tissue Tests
Critical levels of micronutrients in dry plant tissue (source: Soil Fertility and Fertilizers, J.L. Havlin et al. 2005)
| 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 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 |
Source: Nutrition of the Oilseed Rape Crop, M.R.J. Holmes. 1980
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) |
Source: Sampling Plant tissue for Nutrient Analysis, G.J. Schwab, C.D. Lee, and R. Pearce. University of Kentucky Cooperative Extension service. 2007
Soil Tests
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 (ppm) |
Recommendation (lb B/acre) |
|
|---|---|---|---|---|
| Cl | Water or dilute salt | Very low | 0-5 | 0-150 (lb KCl/acre) |
| Low | 5-10 | 0-150 (lb KCl/acre) | ||
| Medium | 10-20 | 0-50 (lb KCl/acre) | ||
| High | 20-50 | 0 | ||
| Excessive | >50 | 0 | ||
| B | Hot water | Very low | <0.2 | 1-3 (lb B/acre) |
| Low | 0.2-0.5 | 0-3 (lb B/acre) | ||
| Medium | 0.5-1 | 0-1 (lb B/acre) | ||
| High | 1-2 | 0 | ||
| Excessive | >2 | 0 | ||
| Zn | DTPA | Sufficient | ≥1.5 | 0 |
| Deficient | <1.5 | 5-15 (lb Zn/acre) | ||
| Cu | DTPA | Sufficient | ≥0.6 | |
| Deficient | <0.6 | |||
| Mn | DTPA | Sufficient | 1-5 | |
| 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 | |||
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)
Soil test results should be compared with micronutrient fertilizer guidelines for your state. Guidelines for Montana are shown in Table 2. Keep in mind that these are guidelines; decisions on micronutrient fertilization should depend on knowledge of growth responses to micronutrient fertilization in the local area.
Table 2
| Micronutrient | Soil test (ppm) (DTPA Test) (top 6 inch soil samples) |
Fertilizer recommendation (lb micronutrient/acre) |
|---|---|---|
| B | 0-0.5 | 2 |
| 0.5-1 | 1 | |
| >1 | 0 | |
| Cu | 0-0.5 | 2 |
| >0.5 | 0 | |
| Fe (inherently unreliable) |
0-0.25 | 4 |
| 2.5-5 | 2 | |
| >5.0 | 0 | |
| Mn | 0-0.5 | 20 |
| 0.5-1 | 10 | |
| >1 | 0 | |
| Zn | 0-0.25 | 10 |
| 0.25-0.5 | 5 | |
| >0.5 | 0 |