Soil testing
The sandy soils discussed here include the grey-white (Bassendean/Joel), yellow (Karrakatta and Tuart) and red-orange (Spearwood) sands of Western Australia. Apart from the Joel sands, all others have reasonable phosphorus (P) retention capacity and soil testing of residual P can be used as part of management.
In order to determine how much phosphorus to apply, a pre-planting soil test from the top 15cm is required. The soil must be analysed using the Colwell test and the extracted P expressed as mg/kg or parts per million (ppm).
It is important to collect the soil samples correctly. Take about 20 cores from the area to be cropped to 15cm deep (using a 2cm diameter corer) in a zigzag pattern, and put them in a plastic bag. Take one or two of these samples for a cropping area that has the same paddock history (i.e. fertiliser application) and soil type.
Take separate samples for different soil types or areas that have different paddock histories. Avoid unrepresentative areas such as near sprinkler lines, fencelines and sheds that may have had higher rates of fertilisers through spillage, traffic etc. than the main area of the paddock.
Choice of fertiliser
The recommendations below show the rate of phosphorus to apply as double super (17.5% P) to reach maximum yield at any given soil test for two soil types. For the Joel sands use the yellow sand recommendation, but these soils will probably require lower rates.
Although the recommendations are for double superphosphate they can easily be converted to other fertilisers from the phosphorus content (%). For example, the rate of triple superphosphate to apply is 17.5/20.5kg/ha x 926 = 790kg/ha at <11ppm on a yellow sand.
| Soil test (mg/kg) 0-15cm | Yellow (Karrakatta) or grey sands (kg DSP/ha) | Red-orange (Spearwood) or yellow (Tuart) sands (kg DSP/ha) |
|---|---|---|
| <11 | 926 | 1629 |
| 11-20 | 840 | 1589 |
| 21-30 | 634 | 1495 |
| 31-40 | 428 | 1286 |
| 41-50 | 228 | 1080 |
| 51-60 | 156 | 880 |
| 61-70 | 150 | 680 |
| 71-80 | 150 | 486 |
| 81-90 | 150 | 274 |
| >90 | 150 | 150 |
Cadmium
It is also important to use phosphorus fertilisers that are low in cadmium to reduce risk of contamination of tubers. The quality assurance program for the potato industry sets the limit on cadmium (Cd) in phosphatic fertilisers at 150mg/kg of phosphorus (that is, ppm Cd on a P basis).
Most fertiliser companies are aware of the problem and have a number of products with less than 100mg Cd/kg P. Select these products when available. There are now superphosphates with less than 100mg Cd/kg P when previously this was a high Cd fertiliser.
Method of application
Phosphatic fertilisers should be broadcast before planting on the coastal sands as banding at planting reduces yield. Applying 75% of the phosphorus after planting rather than all before planting may improve efficiency of use on grey (Joel) sands.
Plant analysis
Collect samples of petioles (20 per time) after planting to monitor the phosphorus status of the crop. The phosphorus concentration for maximum yield ranges from 0.8—0.9% (10mm diameter tubers) to 0.2—0.25% at 120 to 130 days after sowing.