Fertilisers
Potatoes need an adequate supply of nutrients to maximise yield and quality. Most phosphorus (P) should be applied pre-plant but, to increase efficiency, more than 50% of nutrients such as nitrogen (N) and potassium (K) will be applied after planting in weekly to three-weekly applications, depending on soil type.
Phosphorus
Use pre-plant soil tests (0–15cm deep) to assess the P status of soil and apply P either in a single P (DSP, TSP) or compound (NPKS + trace elements) fertiliser according to Table 1.
| Soil test (mg/kg) | <5% clay (kg/ha of P) | >5% clay (kg/ha of P) |
|---|---|---|
| <11 | 160 | 250 |
| 11-20 | 145 | 225 |
| 21-30 | 75 | 200 |
| 31-40 | 75 | 180 |
| 41-50 | 50 | 150 |
| 51-60 | 40 | 120 |
| 61-70 | 30 | 85 |
| >70 | 30 | 30 |
Fertiliser requirements decrease from high rates at low soil phosphorus test to a maintenance rate of about 30kg/ha at high or target soil levels. Rates are higher on soils with higher P fixing capacity. Use the kg/ha values in the table to calculate fertiliser needs.
For example, if 160kg P/ha is needed, apply potato E plus at 1280kg/ha (160/0.125 as potato E plus has 12.5% P). This rate also applies 90kg/ha nitrogen, 108kg/ha potassium, 77kg/ha sulphur, 77kg/ha calcium, 1.7kg/ha copper and 2.6kg/ha zinc.
WA research has shown that phosphorus should be broadcast and incorporated prior to planting, not banded, on coarse sands (<5% clay). On heavier soils (>5% clay), banding is likely to be the better option.
Potassium, nitrogen, calcium and magnesium
A summary of guidelines for applications of pre-plant calcium, magnesium, nitrogen and potassium is provided in Table 2.
In particular pre-plant soil tests can also be used as part of potassium management. As a guide, if Colwell soil potassium (K) is greater than 120mg/kg, K can be reduced to maintenance rates of about 50kg/ha before planting plus 100kg/ha after planting.
Soil tests are not a reliable guide to fertiliser nitrogen (N) requirements but sands generally contain low N (<0.05-1.00% total N) which means they require moderate to high applications split before (50–100kg N/ha) and after planting (see below).
Following clover pasture, soil N may be considerably higher in heavier soil (>2% total) and rates can be reduced to 50kg/ha or less.
| Nutrient | Comments |
|---|---|
| Calcium | Adequate amounts usually applied in limes, amendments such as gypsum and compound fertilisers |
| Magnesium | Adequate amounts usually applied if dolomite lime used. Otherwise up to 40kg Mg/ha (400kg/ha as sulphate) usually sufficient |
| Nitrogen | At low soil N apply 50-100kg N/ha and less at high soil N |
| Potassium | If <120mg/kg soil test apply 50-80kg K/ha, and less at >120mg/kg soil test |
Soil tests are not always an accurate guide to calcium (Ca) and magnesium (Mg) needs. Liming to adjust soil pH usually corrects Ca deficiency but, due to low solubility, low tuber Ca can still occur even after high pre-plant applications. Use soluble forms for post-plant applications.
Even on sandy soils, usually no more than 40kg/ha Mg (400kg magnesium sulphate per hectare) is needed for maximum yield. Some limes such as dolomite provide magnesium in pre-plant applications which should provide adequate supplies for the crop.
Trace elements
Compound fertilisers may provide adequate amounts of some trace elements such as Cu, Zn and Mo as well as NPK. On sandy soils, manganese (Mn) and boron (B) may be needed in addition to copper (Cu), zinc (Zn) and molybdenum (Mo).
Mn, Cu and Zn are more available to plants at low soil pH and less at high soil pH, Do not overlime as this may induce deficiencies in plants.
Mo is less available at low pH and low availability may be corrected with liming. However, plant test first as only very low amounts of fertiliser Mo are required to correct deficiency. If too much Mo is applied it could cause problems in animals (molybdenosis) on pastures used for grazing after the potato crop.
Cu and B also are only required in low concentrations so don’t over-apply. For example, 500g/ha of borax (11% B) applied in 100L of water to the foliage twice after emergence should correct low plant B.