Potatoes - Irrigation

Note Number: AG0321
Published: February 2000
Updated: Febraury 2010

Potatoes are shallow-rooted plants with finely branched root systems, and are therefore sensitive to even small deficiencies of water in the root zone. Whenever the plant is subjected to small moisture deficiencies the growth rate is reduced and the moisture stress on the plant will affect both the quality and yield of tubers.

The crop

To grow a potato crop free from moisture stress requires considerable skill. Irrigation water is applied to replace soil moisture used by the plant in the growth process. The grower has to estimate how quickly the soil moisture is being used by the plant, and what quantity has been used, so that they can replace this moisture with irrigation. Further more, if a cyclical watering system is being used, the grower actually has to predict this water consumption by the crop, so that irrigation equipment will arrive at the right part of the crop at the right time. These key questions of 'how much water, how often' are dealt with in this Agriculture Note.

The climate

The rate of moisture use by the crop depends very much on the weather. On a hot summer day with a shade temperature higher than 40°C and a strong dry northerly wind, moisture losses from a crop in full leaf cover can be as high as 12 mm in 24 hours. At the other extreme, overcast weather with little wind and very high humidity can result in transpiration losses lower than 1 mm. All parts of Victoria experience these extremes of weather. Much of the state's potato crop is grown in the Victorian Highlands, where the daily transpiration losses are around 3.5 mm through the summer months. In coastal areas such as those around Warrnambool, Koo Wee Rup and the Bellarine Peninsula the losses are a bit higher. These average daily transpiration losses for a potato crop in full leaf cover are listed in Table 1.

Table 1. Estimated average daily transpiration losses from a potato crop in full leaf cover

* *Figures in the Table are derived from Bureau of Meteorology data using the formula ET = F1 (fECA) where: ET is evaportranspiration, F1 is the crop factor which for potatoes in full leaf cover has been taken to be 1.0, F is the conversion constant between evaporation from a class A pan and evaporation from a free water surface. (Value = 0.7), ECA is evaporation from a class A pan

The soils

One of the many things the potato soil must do is to hold enough moisture in the root zone between falls of rain or irrigation applications so that little or no moisture stress occurs. Soils vary in their capacity to do this, depending on their texture, organic matter content and cultivation management.

The available water capacity of a soil is a measure of the quantity of water available for plant growth between its state when it has drained after a soaking by rain or irrigation (field capacity) and its state when permanent wilting and death of the crop begin to occur (wilting point).

Because potatoes have a fine root system a slight moisture stress and yield depression begin to occur when only 50% of the available water has been removed by the crop. Estimates of the amount of stored soil water allowing optimum potato growth in some potato soils are given in Table 2.

Table 2. Estimates of available water and stored soil water allowing optimum potato growth

* Source: K. Olsson, Irrigation Research Institute, Tatura, Victoria
** Derived by assuming that yield reductions will occur once 50% of the available water has been removed and that the effective rooting depth of potatoes is 600 mm for sand and 450 mm for all other soils types.

Using a water budget

Table 1 shows the average rate of water consumption by a potato crop in full leaf cover in some of our potato districts. Table 2 shows how much of the water present in the soil can be used up before yield reductions begin to occur. These two sets of figures can be used to estimate the average frequency of application and amount of water that should be applied to a potato crop.

For example, the Ballarat soils (clay loam) hold 37 mm of water in the root zone which can be used before yield reductions occur (Table 2). In January, in average weather, this amount will be transpired in 10 or 11 days (3.5 mm x 11 days = 38.5 mm) (Table 1).

Koo Wee Rup soils hold about 60 mm of water for optimum growth in the root zone (note, though, that recent information indicates considerable variation from site to site in the district). For this average soil in an average season, the interval between waterings will be 13 days in January and 15 days in February, providing that the full 60 mm is replaced at each watering (4.5 mm × 13 days = 58.5 mm).
Rainfall interrupts the irrigation schedule. Small falls of less than 12 mm of rain do make contributions to plant growth, but are generally not enough to warrant interrupting an irrigation cycle. Heavier falls, however, may require a delay in the irrigation cycle for three or more days, depending on the amount of rain received. For example, a fall of 18 mm in Thorpdale (clay loam) in December would mean a delay of six days in the normal irrigation cycle.

The water consumption figures in Table 1 are for a potato crop in full leaf cover. When the crop is just emerging, it uses water much more slowly. The water use of a young crop is roughly proportional to the percentage of the ground surface covered by the foliage. This percentage increases every day. To simplify the calculations, water use in the early part of the season (before the rows have closed over) is best estimated as 50% of the consumption of a crop in full leaf cover.

These calculations of water requirements by a crop are for an average soil type during an average number of days in an average season. How often do these average conditions occur? During the 1981-82 season, estimated transpiration rates of potato crops at Koo Wee Rup varied from less than 1 mm per day on 25 January to 10 mm per day on 20 January. The five days from 20 January to 24 January had a cumulative transpiration loss of 37 mm, which would have fully depleted moisture reserves on some of the lighter soils. Total moisture loss for January, 1982 was 172 mm, which is 63 mm above the long-term average. Thus the average figures cannot be used for accurate water budgeting. They can give the grower a guide to how much water to apply and how often, but this schedule should be revised by growers each week or even daily according to local weather conditions and their knowledge of their own soil type.

Critical watering periods

Keeping a potato crop at optimum moisture levels for the duration of the season requires a high degree of management. There are certain stages of growth where water management is more critical:

1. Emergence. Avoid watering to 'get the crop up'. If the soil is dry before planting, pre-irrigation should always be considered. If the crop is watered either just before or just after emergence, seed piece breakdown and fungal wilts are usually promoted.

2. Tuber set. Victorian potato varieties do not naturally set large numbers of tubers. In order to encourage tuber set, it is important not to allow soil temperatures to rise much above 25°C. At higher temperatures, many of the small newly initiated tubers are re-absorbed and the plant may be left with only two or three tubers. Irrigation water applied at this stage will keep soil temperatures down.

3. Bulking up. Slight moisture stress at this stage will depress yield and this will occur well before any signs such as a darkening of crop colour or wilting of plants can be seen.
More severe stresses at this stage will affect tuber shape. Stress early in the bulking up stage will cause some tubers to be pointed like a pear at the stolon end. Stress late in bulking up can cause a point at the rose end. Growth cracks develop if plants are stressed until the time when skins start to become firm late in the bulking up phase, then receive plentiful moisture so that the tubers begin to enlarge again.

4. Final crop watering. While tubers are bulking up, their final size can be regulated by controlling soil moisture. If the crop is watered right out until the death of the tops, large tubers will usually result and these are preferred by the French fry trade.
If tubers of a high specific gravity are desired, it is better to finish the crop dry. This is normal practice in crops grown for potato crisps. However, this can be overdone, particularly if soils become so dry that they heat up rapidly and cause a deterioration of the cooking quality of the tubers. Some varieties, such as Sebago, develop tuber disorders if allowed to finish too dry.

5. Pre-harvest irrigation. Harvesting in dry soil conditions greatly increases the level of tuber damage. Much can be gained by applying a light irrigation just one or two days ahead of harvesting to soften the clods and improve conditions for both the crop and the harvester crew.

Summing up

Potatoes are very sensitive to irrigation. Correct irrigation gives yield responses of around 0.2 tonnes/ha/mm water, which make it highly profitable. Irrigation can also be managed to minimise or prevent some diseases and to produce tubers of the desired quality
and size required for a specific use.

Acknowledgements

This Agnote was developed by Andrew Henderson, Plant Standards of Victoria in February 2000.
It was reviewed by Neville Fernando, Farm Services Victoria in February 2010.