Mid West potatoes: irrigation, salinity and harvest management

Introduction

Out of season or winter production of fresh, crisp and seed potatoes can be increased north of Perth between Lancelin and Dongara in Western Australia. This will increase the value of the potato industry and the period of the year when additional supplies are available. It will also create opportunities for industry to access interstate and export markets counter to the main cropping season in WA.

The focus of production is currently near Dandaragan (140km north of Perth and 75km south-east of Jurien Bay) where 160ha were sown by three growers in 2012. Department of Agriculture and Food (DAFWA) data indicates adequate arable land and water of suitable quality to increase production.

DAFWA assessed factors limiting yield by monitoring insects and diseases in crops and providing support for agronomic decisions on three commercial properties.

While seed quality was an important limitation, improvements in agronomic management were also important.

This page looks at best management practices for irrigation, salinity management and harvesting.

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.

* 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.

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.

Harvest

Temperature, tuber condition (hydration, temperature, specific gravity) and harvester operation must be considered if potatoes are to be harvested with minimal damage.

Cells are damaged when the skin is bruised and tubers deteriorate because entry of pathogens becomes easier and water loss increases. This damage can have severe consequences.

Machinery maintenance and operation

The most important factor influencing bruising of tubers is the ratio of ground speed to conveyor speed. Bruising is minimised when conveyors are kept full of potatoes. Higher ground speed can result in faster harvest, can increase and smooth out the flow of tubers, and reduce damage. Individual conveyors are often at the wrong speed relative to the preceding or following conveyor. Manage equipment properly to reduce bruising.

Level of tuber impact

The greater the drop height, the more bruising that occurs. This also leads to a shift in damage from blackspot to shatter bruise. Long impact durations and low velocities result in blackspot bruising and internal crushing (for example, small bumps along the conveyor), while short impacts and high velocities result in internal shattering (for example, drops from the elevator to the bunker).

An integrated approach is required to maximise the percentage of bruise-free potatoes. Harvest as close to ideal temperature and soil moisture conditions as possible, matching the volume of material flowing through the harvester to its capacity.

Soil and tuber conditions

Soil moisture, temperature and soil type influence the severity of bruising, as do tuber hydration, temperature and specific gravity.

Soil condition at harvest determines how easily potatoes are separated from the soil. Medium to light, loose, moist soils separate easily while heavy, compact, wet soils are difficult to separate from tubers. Dry sandy soil will separate too rapidly on the primary conveyor, reducing the total load of soil and tubers on the conveyor, which can increase tuber damage. Rocky soils will also damage potatoes at harvest.

Soil temperature

Optimum soil temperature for harvest is 12–18ºC. Harvest temperatures above 25ºC or below 5ºC can lead to bruising.

Soil temperature is primarily controlled by air temperature and solar radiation but can be reduced by more frequent irrigation. Evaporative cooling is important. The drier the soil, the closer the soil temperature will be to air temperature.

Soil temperature and moisture can be managed by applying irrigation before harvest if soil is too dry or tuber temperature is above 18ºC. Apply enough water to cool the soil sufficiently but not saturate it as this leads to increased risk of bacterial rots. During summer, harvest at night or when soil temperatures are lower.

Soil moisture

Irrigation prior to harvesting is very important in increasing the soil moisture content in clay soils. This reduces the number of clods and provides a layer of wet soil between the tubers and machine components, reducing impacts and consequent damage. On sandy soils, manage moisture so soil will not separate from the conveyor, thereby increasing tuber damage. If soil is too wet, delay harvesting. Soil moisture for harvest should be between 60 and 80% of field capacity.

Skin maturity

Allow two to four weeks after top removal for skins to harden. Maintain soil moisture so tubers do not dehydrate underground while the skin is hardening.

Tuber temperature

Tuber temperatures between 12 and 18ºC are considered ideal for harvesting as this is when less bruising occurs and microbial growth is not too fast.

Tubers warmer than 18–20°C and under drought stress are susceptible to blackspot bruising. Harvesting when tuber pulp temperature exceeds 18°C or soil temperature is greater than 25°C increases the risk of microbial rots, especially in damaged tubers.

Tuber temperatures should also be kept within the optimum range during washpacking.

Reducing damage

Reducing drop heights from conveyors will improve potato quality.

Considerable bruising can occur where drops are greater than 40cm, particularly when the drop area is on belting supported by a hard metal roller or plate.

The impact surface affects the amount of damage. Cushioning materials considerably reduce impact. Tubers are less likely to be damaged if mechanical components are covered in rubber.

Acknowledgement

Much information here 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 with in-kind contributions from and managed by DAFWA.