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When plant tissue analysis won’t help

Plant tissue analysis is an extremely valuable tool for understanding nutrition problems and correcting imbalances to maximise yields. But there are times when it won’t help and may even give you the wrong advice.

Table 1 lists the six most common elements in the Earth’s crust. These are all present in high amounts in soils. Therefore, any soil adhering to a plant tissue sample, in particular one collected close to the ground, is likely to skew laboratory results of those elements. It is important to keep in mind that plant samples are never washed before analysis, because this can wash out highly soluble nutrients, such as potassium and boron, and can add others present in the water. So contaminated samples can give false readings.

Table 1. Most common elements in Earth’s crust.

We can ignore the most common element, oxygen.

Silicon is of interest to animal growers for its role in the development of bone and cartilage. Plants take up small amounts of silicon for use in cell wall structure, but as silicon occurs in most soil particles, even a speck of soil on a tissue sample will give a falsely elevated reading.

In soils with a low pH (that is, acidic soils), aluminium becomes mobile and can be taken up by plants, causing toxicities and signs of phosphorus deficiency. But aluminium’s high content in many soils again means that any contamination of tissue samples with soil will give a false positive.

Iron is an important plant and animal nutrient, but its ubiquity in Australian soils means that a high plant iron content is likely to indicate contamination by soil, not excess iron in plant tissues. On the other hand, a low iron content in plants is a good indication that there is no contamination by soil, and the tissue iron content really is low.

Calcium is another important nutrient, so contamination of samples can lead to another false high reading.

Sodium is not a plant nutrient (inasmuch as its ubiquity ensures that all organisms get what they need) and is not normally assayed in tissues.

Collect clean samples

So when you are collecting plant tissue samples for analysis, be careful to exclude any that could be contaminated with soil. This is more difficult for turf samples than for fruit tree samples. But don’t wash the samples before sending them to the laboratory: this will wash out some nutrients and possibly add others.

Don’t spray first!

We’ve seen extraordinarily high levels of copper, manganese and zinc in some plant tissue samples – high enough to kill the crops. And yet a soil analysis shows that these same nutrients are deficient in the soil. The reason has turned out to be fungicide sprays, such as copper oxychloride, mancozeb or zineb, applied just days before sampling. So keep in mind chemical or fertiliser treatments that may give false results.

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