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.