WSU CAHNRS

CAHNRS and WSU Extension

Wheat and Small Grains

Soil & Plant Sample Testing and Interpretation

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. 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 be helpful to 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 Oregan 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
PMM
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

 

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
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