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Plant tissue analysis – what’s your plant telling you?
We’re trained to diagnose nutritional problems from soil analyses.
When we have a problem with plant health, we often have visual signs of nutrient deficiency or excess; but at other times we need to go a step further and collect a plant tissue sample to acquire more information. Plant tissue analysis is a vastly underutilised means of understanding the nutrient status of a plant and how the plant interacts with the soil and responds to management inputs such as fertiliser. With correct interpretation, plant tissue analysis can be a very useful management tool that takes much of the guesswork out of nutrient management for optimum growth and yield.
Nutrient sufficiency ranges
The sufficiency ranges below are guidelines: actual values can be expected to change with growth stage, plant part and weather. Plant nutrient contents can be influenced by several factors, including crop injury, drought, soil saturation, time of sample collection and fertiliser history. Remember that the actual plant nutrient content is the only accurate indication of plant nutrient status: a soil test will not tell you how much of each nutrient the plant has taken up.
|
Crop |
N % |
S % |
P % |
K % |
Ca % |
Mg % |
|
Maize |
2.5–4.0 |
0.2–0.6 |
0.3–0.5 |
1.8–2.6 |
0.4–1.0 |
0.2–0.6 |
|
Soybean |
4.5–5.5 |
0.3–0.6 |
0.3–0.5 |
1.8–3.0 |
0.4–2.0 |
0.3–1.5 |
|
Oats, barley |
1.3–3.0 |
0.1–0.4 |
0.2–0.4 |
1.0–3.0 |
0.2–0.5 |
0.1 –0.4 |
|
Wheat |
2.7–3.5 |
0.1–0.4 |
0.2–0.4 |
1.5–3.0 |
0.2–1.0 |
0.2–0.5 |
|
Lucerne |
2.5–5.0 |
0.2–0.5 |
0.2–0.5 |
1.0–2.0 |
1.0–2.0 |
0.2–0.5 |
|
Grasses |
1.8–3.0 |
0.2–0.4 |
0.3–0.5 |
2.0–3.0 |
0.3–0.6 |
0.2–0.4 |
|
Crop |
Zn ppm |
Fe ppm |
Mn ppm |
Cu ppm |
B ppm |
|
Maize |
20–70 |
30–200 |
20–150 |
7–20 |
5–25 |
|
Soybean |
20–50 |
30–200 |
20–100 |
10–40 |
20–40 |
|
Oats, barley |
12–40 |
25–200 |
25–150 |
5–20 |
5–15 |
|
Wheat |
20–60 |
25–200 |
25–150 |
5–25 |
4–30 |
|
Lucerne |
15–40 |
30–150 |
30–100 |
7–15 |
20–60 |
|
Grasses |
20–50 |
50–200 |
30–150 |
7–15 |
5–15 |
The lower value in a nutrient sufficiency range is the content below which a 5% to 10% yield reduction can be measured. The upper value is the content above which toxicity symptoms begin to appear.
Plant nutrient ratios
The following plant nutrient ratios can also be used to help determine both deficient and excessive nutrient levels:
Optimum N:S ratio is about 12:1 for most crops (range 10–15:1).
Optimum N:K ratio is about 1.7:1 for most crops (range of 1.2–2.2:1).
Optimum Fe:Mn ratio should be >1.
The limits to plant nutrient testing
Plants, like animals, have a strong ability to maintain certain nutrient ratios in their tissues regardless of whether one nutrient may be in limiting supply. Thus, plant tissue analysis may not pick up “hidden deficiency” symptoms, where plants look normal but simply don’t grow well. It is very effective, however, at diagnosing acute deficiency symptoms, where the nutrient supply is so deficient that symptoms show up. Examples include iron or other trace element deficiency (yellowing between the veins) and acute nitrogen deficiency (overall paleness).
Plant nutrient testing can be used to recommend fertiliser requirements even in the hidden deficiency situation where nutrients may be below some target level in a plant species for which normal ranges are well researched and understood (e.g. the common crop plants).
The appearance and measurement of nutrient deficiency in plant foliage gives no information about why these deficiencies occur. It is often useful to combine tissue testing with soil testing to answer this information and to improve and the soil.
Further reading
Plant analysis – what does it mean for turf growers?
Trace Elements for Pastures and Animals in Victoria
Acknowledgment
This article has been adapted for Australian conditions from the newsletter of MVTL Laboratories, USA, with the kind permission of the author, Duane Schlieman.