Taking petiole samples
Commercial plant test kits should contain instructions for collecting samples for analysis. In general, for each planting collect the youngest mature potato leaf from each of 20 to 30 plants in a systematic pattern across the crop area to be tested.
Systematic may mean taking the samples in a zigzag or grid pattern such that the whole crop area is sampled comprehensively. In brief, one sample is required for every 200 square metres if 20 samples are collected for each 0.4 hectare.
The youngest mature leaf is the fourth or fifth from the top of the plant. The leaves should be collected with the petioles (leaf stalks) intact. Break off each leaf where it joins the main stem of the potato plant. Remove the leaflets and place the remaining 20 to 30 petioles in a labelled sample bag.
Content is measured on a dry weight basis — note that milligrams per kilogram (mg/kg) is equivalent to parts per million (ppm). To determine S2 crop stage, measure the length of the longest tuber under two or three plants. Note the number of weeks from the S2 stage (when largest tubers are 10mm long) to determine growth stage.
| Nutrient | Content (% or mg/kg) | Crop stage S2 +0 weeks | Crop stage S2 +3 weeks | Crop stage S2 +5 weeks | Crop stage S2 +7 weeks |
|---|---|---|---|---|---|
| Nitrogen | % | 4.5–5.5 | 3.6–4.4 | 3.5–3.9 | 3.0–3.4 |
| Nitrate–N | % | 2.6–3.5 | 2.5–3.0 | 1.8–2.2 | 1.5–1.7 |
| Phosphorus | % | 0.80–1.1 | 0.7–0.9 | 0.6–0.8 | 0.4–0.6 |
| Potassium | % | 10–16 | 10–16 | 10–16 | 7–14 |
| Calcium | % | 0.5–1.5 | 0.5–1.5 | 0.5–1.5 | 0.6–1.8 |
| Magnesium | % | 0.3–0.8 | 0.3–0.8 | 0.3–0.8 | 0.6–1.6 |
| Sulphur | % | 0.2–0.4 | 0.2–0.4 | 0.2–0.4 | 0.2–0.3 |
| Boron | mg/kg | 20–40 | 20–40 | 20–40 | 20–40 |
| Copper | mg/kg | 5–16 | 5–16 | 5–16 | 3–10 |
| Iron | mg/kg | 230 | 230 | 230 | 230 |
| Manganese | mg/kg | 20–100 | 20–100 | 20–100 | 30–225 |
| Molybdenum | mg/kg | 0.5–1.5 | 0.1–1.5 | 0.1–1.5 | 0.1–1.5 |
| Zinc | mg/kg | 30–110 | 30–110 | 30–110 | 20–80 |
For general crop monitoring a ‘planting’ can be considered as an area of crop that has a uniform history, both agronomically (e.g. fertiliser history) and environmentally (e.g. soil type).
A planting may be one or a number of bays of one hectare or less and it is unlikely even for the largest cropping enterprises to be more than 4 hectares. Smaller areas may be selected if a diagnosis of a nutrient disorder is required. In this case, samples should be taken from both the affected or 'unhealthy' area of crop as well as an area of unaffected or 'healthy' crop for comparison.
Crop stage
It is important to define the growth stage of the crop by measuring the size of the largest tuber and by recording the days from planting and emergence.
The S2 growth stage is commonly used as a point of reference in potato crop agronomy and is when the length of the longest tuber is 10mm. Collect the first petiole sample at the S2 stage and repeat the sampling every two to three weeks later, noting crop stage, as necessary for up to four samples.
Handling samples
Petiole samples should be submitted to an accredited plant testing service for analysis as soon as possible after collection. Samples should be kept as cool as possible and not left in the sun or in a vehicle.
If there is a delay between time of collecting and submitting of samples, keep them in a cool room at 4ºC in the meantime.
Using the table
Compare the results from the analysis service with Table 1 to see if the concentrations of nutrients in the petioles fall within the range expected for adequately fertilised, high yielding crops.
If the result for a particular nutrient is below the range in Table 1, then for most varieties the nutrient is likely to be inadequate and more should be applied if practical. That is, extra nutrients should only be applied if it is cost effective, and it is possible to apply the fertiliser either via machinery or the irrigation system and it is not too late in the life of the crop.
If a nutrient concentration is too high, reduce the rate of application of fertiliser containing the nutrient to avoid toxicity in the current crop.
Different standards
Some of the concentrations listed in Table 1 may be different to published standards from other parts of Australia or other countries. Some of these differences may be due to different varieties, or other factors such as soil type, planting time or method of fertiliser application.
For example, in the early growth stages the concentration of phosphorus (P) in the petioles needed for maximum growth of potatoes on the coastal sands is higher than for potatoes grown on heavier soil types, even for the same variety.
The concentration of nutrients in the petioles may also be higher during early crop growth, when all of a fertiliser is applied before planting compared with crops where most of the fertiliser is applied after planting.
Acknowledgements
Information on this page is based on work carried out in Project PT213 on fertiliser and irrigation management of potatoes, funded by Horticulture Innovation Australia, the WA potato industry and DAFWA.