Soil & Plant Sample Testing & 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 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.

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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	Not applicable
Cu	DTPA	Deficient	<0.6	Not applicable
Mn	DTPA	Sufficient	1-5	Not applicable
Mn	DTPA	Deficient	Deficient only occur when soil pH is >8.0	Not applicable
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.	Not applicable	Not applicable	Not applicable
Mo	Soil testing for Mo is not recommended because soil Mo concentration are too low for most labs to evaluate	Not applicable	Not applicable	Not applicable

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	Not applicable
Mn	20-500	<15-20	Not applicable
B	<20	6-18 in monocots 20-60 in dicots	Not applicable
Cl	0.2-2.0% dry weight	70-700	>4% dry weight
Mo	<1	<0.2	Not applicable
Ni	0.1-1	Not applicable	Not applicable
Co	0.02-0.5	Not applicable	Not applicable

Source: Soil Fertility and Fertilizers, J.L. Havlin et al. 2005

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

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