Soils
Most soils in the Dandaragan area are classified as sand, varying from coarse sands with low clay (<4%) and silt (<0.05%) content to soils with higher clay (>8%), silt (>5%) and fine sand (>16%) content. Coarse soils have lower water-holding or field capacity and may require more frequent irrigation than finer textured soils.
Variation in soil texture may affect fertiliser requirements but soil testing is used to ascertain the nutrient status of the soil before planting. Use plant testing to evaluate the effectiveness of the fertiliser program on the growing crop.
Potatoes generally tolerate low soil pH but if pHw (in water) is less than 5.5 soil and plant aluminium levels may increase and limit yield. Soil pH should be adjusted with limes such as dolomite or lime sand. Soil pH can influence the level of potato scab diseases so if amendments are needed, manage them to prevent increase in scab risk. Only apply enough lime to increase soil pHw to 6.0. Soil pH measured in CaCl2 rather than water is on average 0.8 of a unit lower than the pHw values.
Paddock selection
Potatoes can be produced on a wide range of soils but avoid waterlogged, compacted, saline or rocky/gravelly soils. Rocky or gravelly soils are difficult to till and cropping these soils may result in bruising of tubers during harvest. The Mid West has large areas of well drained soils with low EC (1:5 H2O<13mS/m or 0.13dS/m) and no gravel so selecting suitable soils should not be a problem.
Herbicide residues from the previous crop can limit potato yield and quality. Of most concern are the sulfonyl ureas applied to cereals and imadiazolines used on oilseeds. Follow label directions to minimise the impact of herbicide residues applied to other crops on the potato crop.
Seed sites
Mid West seed potato crops are at higher risk of invasion by aphid virus vectors than summer crops grown in south coastal areas. Select sites that will be upwind of any earlier sown crops. These will be closest to the coast, assuming prevailing winds in winter will be south-westerly to westerly.
Crop rotation
Crop rotation or cropping sequence is important. The condition of the paddock and the practices used in the previous crop or pasture affect economic return and the economic, agronomic and environmental — soil structure — benefits of other crops in the rotation.
Although potato crops are often grown more frequently, a rotation of one potato crop every three or four years is usually better for soil management and gives best long-term control of pests and diseases.
Potatoes (or related solanaceous crops such as tomatoes or capsicum) should not be cropped on the same land two or more times or years in a row.
Continuous cropping often results in more weeds, pests and diseases and yield loss in subsequent crops. On sandy soils in new land areas, two crops of potatoes in a row may be accommodated without much yield impact in the second year.
Certified seed crops have special rotation requirements such that there is at least five years of non-potato or Solanaceous crops before low generation (G1 to G3) seed can be sown or at least three years for higher generations (G4 and G5).
Land preparation
A properly prepared seed bed is required. Sandy soils are easier to cultivate than heavier soils and may require less cultivation. Cultivation also provides opportunity to control weeds prior to planting by using irrigation and shallow cultivation to stimulate germination and then cultivation to kill the weeds when small.
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.
After planting fertilising
Post-plant, apply a total of 250–300kg/ha N and K if pre-plant soil N and K are low, depending on soil type, with higher rates for more sandy soils. On sandy soils, divide the total amount into weekly applications and on heavier soils into three-weekly applications.
On processing potatoes, apply K as sulphate or nitrate because muriate of potash has been shown to lower the specific gravity (dry matter %) of tubers compared with other forms of fertiliser potassium.
Apply some nitrogen as calcium nitrate to supply up to 75kg Ca/ha post-planting, in addition to any calcium applied before or at planting. For example, 394kg calcium nitrate per hectare (19% Ca and 15% N) applies 75kg Ca and 60kg N.
Seed crops often require less fertiliser N than non-seed crops, as they may grow for a shorter period, especially if the plan is to maximise the yield of small round seed. Tuber size in some varieties, such as Atlantic, is very sensitive to applied N and high rates may result in low yields of small round seed and high yields of oversize tubers. Excess N may also increase tuber disorders such as hollow heart in sensitive varieties.
Monitoring fertiliser program with plant testing
Monitor the effectiveness of your fertiliser program by using plant (petiole) tissue testing. For the trace elements Cu, Zn, Mo, Mn and B, petiole testing is much more accurate than soil testing as there are very few standards for deficiency or adequacy for WA soils.
Collect 20 to 30 petioles per test area of the crop and note the crop stage by measuring the length of the longest tuber in millimetres.
| Nutrient and symbol | % or mg/kg | At S2 (10mm tuber stage) |
|---|---|---|
| Nitrogen (N) | % | 4.5-5.5 |
| Nitrate-N (NO3-N | % | 2.6-3.5 |
| Phosphorus (P) | % | 0.8-1.0 |
| Potassium (K) | % | 10-16 |
| Calcium (Ca) | % | 0.5-1.5 |
| Magnesium (Mg) | % | 0.3-0.75 |
| Sulphur (S) | % | 0.2-0.35 |
| Boron (B) | mg/kg | 20-40 |
| Copper (Cu) | mg/kg | 5-16 |
| Iron (Fe) | mg/kg | 230 |
| Manganese (Mn) | mg/kg | 20-100 |
| Molybdenum (Mo) | mg/kg | 0.1-1.5 |
| Zinc (Zn) | mg/kg | 30-110 |
Use the results of the petiole test and other relevant agronomic information to adjust the fertiliser program to account for nutrient inadequacies or excesses if they arise.
Acknowledgements
Much information on this page is based on results from a number of potato research and development projects funded by Horticulture Australia Limited with voluntary contributions from APC Potato Producers Committee, matched by the Australian Government, and with in-kind contributions from and managed by the Department of Agriculture and Food, Western Australia.