Mid West potatoes: irrigation, salinity and harvest management

Page last updated: Tuesday, 8 November 2016 - 11:59am

Please note: This content may be out of date and is currently under review.

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.

Contact information

Peter Dawson
+61 (0)8 9892 8461