Guideline Index

Chapter 8: Assessing Soil Nutrients

8.4 Plant tissue testing

Plant tissue testing is the preferred method for diagnosing micronutrient (trace element) toxicities, deficiencies, and imbalances for plants.

Plant tissue testing:

  • Checks on fertiliser management recommended from soil testing.
  • Checks if nutrients not applied as fertiliser (such as calcium, magnesium, or trace elements) remain adequate.
  • Can help to determine nutrient deficiencies in animals, if taken as mixed herbage sample -see Section 8.4.2).
  • Is helpful in diagnosing nutrient levels in pasture or crop diets offered to animals.

Many field experiments have been used to verify the results of laboratory testing of soils and of plant tissue. Research has shown that using soil tests to indicate trace-element deficiencies can be less accurate, especially on acid soils.

The same guidelines apply for plant tissue testing as for soil test sampling when doing simple problem diagnosis:

  • Take representative plant samples from the area of interest. In the case of a problem diagnosis, make sure to take samples from both the affected and normal areas.
  • Use transects.
  • Take the samples in accordance with the physiological growth stage of the plant, when the plants are not stressed and at the same time of each day; generally in the morning between 6 – 10 am.
  • Make sure that sufficient quantity of material is collected for testing. The number of tillers or plant parts stipulated has to account for high moisture content when dried in the lab before sampling.
  • Do not take samples until about 8 weeks after the last fertiliser application.

If plant tissue samples are being collected on an ongoing monitoring programme, collecting the sample from the right plant part and at the right time will be more crucial. This is due to the change in nutrient concentrations in plant parts as the season progresses. The results will be useful only if these sampling guidelines are followed carefully.

8.4.1 Testing for plant nutritional deficiency

Plants have different demands for nutrients, even for the same nutrient. If the availability of a nutrient becomes scarce in the soil not all plants display visible symptoms at the same time, i.e. different crop and pasture species have varying sensitivity to nutrient deficiencies. When sampling for plant nutritional deficiencies taking the sample from the legume component of a pasture (e.g. clover or lucerne) provides more accurate results than a sample from the grass component. This is more pronounced in the case of boron and potassium deficiencies. If no legumes are present however, the grass component can be sampled (e.g. ryegrass).

When clover is sampled it is best to select the most dominant species in the paddock. This may be white clover, sub clover, or strawberry clover. A mixture of clover species is not recommended because the various clover species have slightly different adequate levels for each nutrient and will be at different stages of maturity.

Collect the leaves and petioles (stems) from about 60 white clovers or 60 strawberry clovers or 80 to 100 sub clovers (around 2 hands cupped together and filled once with clovers).

Sample the youngest fully grown leaves and their petioles of the same species of clover – see Figure 8.4. Post the samples early in the week so that they are not left to deteriorate in a post office over the weekends, especially during the summer months. Always put samples into paper bags and avoid leaving the sample for too long before posting. Do not leave the sample in a hot tractor cabin, on the ute dashboard, etc.; and refrigerate the sample if there is a delay in sending. Samples can also be oven (preferably not a microwave) or air dried before sending to the laboratory. This is recommended for extended delivery periods, or to avoid the possibility of mould establishing on high moisture plant samples.

Figure 8.4   Section of clover to sample for plant tissue analysis
Figure 8.4 Section of clover to sample for plant tissue analysis

 

When taking tissue samples, it is vital to include as much information as possible. Describing the visual symptoms, noting which leaf is affected (youngest or oldest), and noting whether or not there is an effect on the edge (margin) of the leaf or between the veins are important. You should also outline the paddock history in terms of pasture type, soil type and previous nutrient applications.

When taking tissue samples, you should also ensure that the plant symptoms are not the result of other stresses such as, differing soil textures, seasonal changes, root disease, leaf disease, waterlogging, severe frost, insect attack or recent chemical application.

Tissue samples should not be taken when the plant is under a major stress, such as lack of moisture, waterlogging, frost or recent herbicide application.

Another key point is to note the amount of dry matter the plant is producing relative to what is considered adequate. In other words, is the plant growing slowly and producing very little dry matter, or is it producing close to maximum production. This will help the person interpreting the results.

Sampling the correct plant part is also vital, as described above. However, no matter which plant part is taken, ensure you tell the laboratory which plant part you have taken. In most cases, the youngest fully open leaf is the best plant part to take, regardless of the plant type, and the sample should be taken preflowering if possible. If there is a good and bad section in a paddock or farm, take a sample from each area as this is very useful for comparison.

Supplying the species of plant, plant part and stage of growth will allow the person interpreting the results to put the correct nutrient standards in for your sample. Actual adequate levels for any nutrient vary depending on species, plant part and stage of growth, so this information is critical.

For more information on soil, plant and water sampling refer to the link: http://www.incitecpivot.com.au/sampling_instructions.cfm

8.4.2 Testing for animal health problems

When sampling for animal health issues, collect a mixture of the plants that the stock are eating. Include weeds if relevant. Collect at least 20 handfuls across the pasture, using scissors to cut the sample off so as to avoid soil contamination from pulling. This is referred to as a mixed herbage sample.