Irrigation and salinity
Schedule irrigation based on crop demand using the previous day’s pan evaporation value (Epan) from the nearest weather station. To determine irrigation requirements in millimetres per day, multiply the crop factors (%) for each crop stage (Table 1) by the daily Epan in millimetres.
Soil moisture monitoring can also be used to check the effectiveness of the program to ensure soil water depletion (% of AWC = available water content) doesn’t exceed guidelines.
| Crop stage | Name | CF (Epan) %* | AWC%** |
|---|---|---|---|
| 1 | Emergence | 90 | 30 |
| 2 | Vegetative | 100–110 | 25 |
| 3 | Tuber initiation | 110–120 | 30 |
| 4 | Tuber bulking | 110–120 | 20 |
| 5 | Maturation | 90 | 40 |
* CF = crop factor value to be multiplied by previous day’s pan evaporation (Epan)
** AWC = % depletion in available water content (AWC = field capacity – wilting point).
Under- or over-irrigation can lower tuber yield and quality due to reduced crop growth and distorted tubers due to pest, diseases and defects. Under-watering has been associated with increases in early blight, common scab and yield of misshapen tubers.
Over-watering at crucial growth stages such as tuber initiation can lead to an increase in powdery scab if present in the soil. Over-watering at planting can lead to seed piece breakdown or blackleg in the crop.
Salinity
Potatoes do not tolerate salinity, so high levels of salt in soil or irrigation water will reduce yield and quality. High salt in soil or irrigation water or fertilisers will also lower the dry matter (specific gravity) of potatoes. This is important in varieties used for processing as specific gravity value affects grower returns.
Salinity is displayed in several different units, most commonly as electroconductivity (EC) which is based on siemens (S). Some laboratories prefer to use millisiemens per metre (mS/m) while others use decisiemens per metre (dS/m). To convert mS/m to dS/m divide by 100.
Milligrams per litre (mg/L) is commonly used by commercial water consultants. Mg/L divided by 5.5 = mS/m.
Soil
Most Mid West soils are sands and appear to have low salt levels {EC (1:5 H2O) less than 0.10dS/m} prior to cropping, so don’t pose a risk for potato production. Potatoes can tolerate soil salt concentrations up to 0.12dS/m.
Irrigation water
Irrigation water quality varies with total soluble salt (TSS) concentrations from less than 480 to 640mg/L (EC1:5 75–100mS/m). Yields will fall if the salt in the irrigation water is above 650mg/L (>100mS/m). There is a risk of salt build-up even below these values, especially during periods of low rainfall.
Rainfall is low in salt (30mg/L) so it flushes salt down the soil profile and out of the root zone. To manage salt in the absence of rainfall, apply extra irrigation water, above crop demand, to flush the salt out of the root zone.
| Salt (mg/L) | Flushing ratio (% extra irrigation) |
|---|---|
| 500 | 7 |
| 750 | 11 |
| 1000 | 14 |
Plant
Chloride (Cl) concentration in petioles in years of normal rainfall in the Mid West is usually 3–4%, which is not reported to cause yield and quality loss. When rainfall is low, petiole chloride can increase to 8–10% with associated symptoms and yield loss. To avoid this, follow the irrigation instructions in Table 2.
Frost
The Mid West is not a high frost risk area for cereal production as this risk is based on the sensitivity to frost at anthesis (flowering) which usually occurs in spring. However, earlier frosts during winter (June to August) can damage potatoes. Dry winters – with cloudless skies – may require action to minimise crop damage.
Irrigation water is normally applied to minimise crop damage during frosts. With centre pivots, it is difficult to cover the whole crop area in sufficient time to minimise damage unless watering is started the night before the predicted frost.