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).

MicronutrientTest methodSufficiency LevelSufficiency Level (ppm)Recommendation (lb/acre)
ClWater or dilute saltVery low0-50-150 (lb KCl/acre)
ClWater or dilute saltLow5-100-150 (lb KCl/acre)
ClWater or dilute saltMedium10-200-50 (lb KCl/acre)
ClWater or dilute saltHigh20-500
ClWater or dilute saltExcessive>500
BHot waterVery low<0.21-3 (lb B/acre)
BHot waterLow0.2-0.50-3 (lb B/acre)
BHot waterMedium0.5-10-1 (lb B/acre)
BHot waterHigh1-20
BHot waterExcessive>20
ZnDTPASufficient≥1.50
ZnDTPADeficient<1.55-15 (lb Zn/acre)
CuDTPASufficient≥0.6--
CuDTPADeficient<0.6--
MnDTPASufficient1-5--
MnDTPADeficientDeficient only occur when soil pH is >8.0--
FeSoil testing for Fe is not recommended because most test methods do not discern between forms of iron, and has little meaning for plant nutrition.------
MoSoil 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)

MicronutrientSoil test (ppm) (DTPA Test) (top 6 inch soil samplesFertilizer recommendation (lb micronutrient/acre)
B0-0.52
B0.5-11
B>10
Cu0-0.52
Cu>0.50
Fe
(inherently unreliable)
0-0.254
Fe
(inherently unreliable)
2.5-52
Fe
(inherently unreliable)
>5.00
Mn0-0.520
Mn0.5-110
Mn>10
Zn0-0.2510
Zn0.25-0.55
Zn>0.50

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

MicronutrientTypical sufficiency range (ppm)Deficiency (ppm)Toxicity (ppm)
Fe50-250<50>300
Zn25-150<10-20>400
Cu5-20<4
Mn20-500<15-20
B<206-18 in monocots
20-60 in dicots
Cl0.2-2.0% dry weight70-700>4% dry weight
Mo<1<0.2
Ni0.1-1
Co0.02-0.5
Source: Soil Fertility and Fertilizers, J.L. Havlin et al. 2005

Critical Nutrient Levels for Wheat Tissue Test (Whole Plant, Seedling before Jointing)

NutrientCritical Range (% Nutrient in Dry Matter)
N4-5%
P0.2-0.5%
K2.5-5.0%
Ca0.2-1.0%
Mg0.14-1.0%
S0.15-0.65%
Fe0.003-0.02% (30-200ppm)
Mn0.002-0.015% (20-150ppm)
Zn0.0018-0.007% (18-70ppm)
Cu0.00045-0.0015% (4.5-15ppm)
B0.00015-0.0004% (1.5-4ppm)
Mo0.00001-0.0002% (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

Critical Nutrient Levels for Winter/Spring Canola Tissue Test

NutrientCritical Range (% nutrient in Dry Matter)Peak Nutrient Level
N2.5-4.0%Emergence
P0.25-0.8%Emergence
K2.6-5.3%End of flowering
S0.9-1.81%Throughout season
Ca1.3-3.0%Emergence and peak flowering
Mg0.15-0.4%End of flowering
Cu0.0002-0.0007%Throughout season
Source: Nutrition of the Oilseed Rape Crop, M.R.J. Holmes. 1980