Keeping on top of water use efficiency – British Potato Review

Soil is the main reservoir for water, so managing it is key to matching crap demand with available supply, and with rivers being low at the time of going to press, the threat of irrigation restriction was very real.

“Judicious use of water is more crucial than ever,” Andrew said. “Good soil conditions can reduce the need for irrigation and well-consolidated soils are the best at dealing with irrigation and/or rainfall. If soil is compacted or too sandy, it holds less water and this affects how much water is available to the crop, therefore good soil management that avoids compaction, slumping or capping can have a large impact on infiltration rates and water use efficiency.”

Understanding how much water to apply at a particular time to best drive yield and quality is challenging because crops grow at different rates, and may vary according to field location, soil profile and topography, crop development, variety and local microclimate, amongst other factors.

Andrew said: “A crop of ware needs 350mm of water/ha, which is 3,500t/ha, and 50% of this is likely to be soil water, whether the crop is irrigated or not. Moisture also helps enhance organic activity in the soil, increasing nutrient availability in the soil.

“Soil structure plays a major role in water efficiency, but to a certain degree it depends on your soil type and organic matter (OM)- just 1% OM can make a difference to holding capacity. This can create an available reservoir around the root zone of the plant.”

Soil type and texture determine water holding capacity because sand, silt, and clay particles have different pore sizes, he explains, noting that soils with higher amounts of clay have smaller pores than sandy soils, helping them to retain more water.

Understanding the soils in the field being used for growing potatoes remains key, and while most growers know the soils on their own farms well, and the areas which tend to become saturated, sometimes rented land can bring surprises.

“Getting the soil right after wet harvests can take several years, and is also a challenge for those growing potatoes on rented land. Nonetheless, addition of bulky organic manure and incorporating chopped straw can help improve short-term water retention,” said Andrew.

“If you are planting in a sandy soil, it is worth considering growing in beds made for two or three rows with no furrow between them, or with a shallower valley, as this can help water stay in the all-important root zone”

Moving on to talk about irrigation, he explained the importance of monitoring the soil moisture deficit before making decisions on whether to apply water.

“When the soil is too dry, the plant closes the stomata to prevent water loss, but of course this also affects growth as the plant shuts down and waits for better conditions,” he said.

“There is a myth that if the soil is dry, the root system will expand to seek out water, but if the crop does not have sufficient moisture and the corresponding access to nutrients, its roots are unable to grow well.”

Early drought restricts canopy expansion and light interception, so the crop may set fewer tubers, hitting yield and quality, he pointed out.

“If there is a drought early in the season and you need to irrigate, you will be wetting a lot of soil, which is inefficient use of water, but by tuber initiation you need the top of the soil to be at field capacity.

“If irrigation is available, then you need make the most of it, and that includes measuring SMD, losses in transpiration and using this information for irrigation scheduling.

“This is because good scheduling that keeps soils just below field capacity can help keep run-off to a minimum. For example, if the soil moisture deficit (SMI)) is just 15mm, there is little point in irrigating more than this amount (eg, 20mm) because the soil will not be able to hold on to it.”

Water sources for irrigation include bore holes, on-farm reservoirs, river and water courses.

Andrew said it is a good idea to get irrigation water tested for quality, pointing out that some companies require a water analysis as part of the auditing process. Accurate irrigation scheduling improves crop quality and yields, and therefore impacts on the growers’ profit margins, he added.

The next steps in water scheduling include deciding on the equipment; water availability and water-use efficiency, what is suitable for particular fields, and the necessary budget.

“When choosing the field, it is a good idea to think at the same time about your preferred irrigation method, any effects of topography and whether you have nearby water storage areas, boreholes and rivers.”

When it comes to irrigation tools, he points out that booms, rain-guns and drip systems all have advantages and disadvantages.

“Drip irrigation is accurate and efficient, uses less energy and makes the most efficient use of water, as it can be applied accurately with sensors, and this method is not affected by wind. It works better than the other methods in hot, dry seasons as you do not lose so much moisture to evaporation.

“Also, because you are applying the water onto the soil, you keep the leaves dry and so there is a lower risk of creating an environment for disease spread. In addition, you lose less water to evaporation so your water use efficiency is higher.”

As water is applied little and often, room can be left for rainfall when it is forecast, he adds. In the future, he sees its potential to work as a conduit for applying nutrition and biologicals to the crop, and trials are underway to evaluate this.

“When setting up drip irrigation, the equipment needs correct placement with the tape either at the top or the ridge or in the ridge”.

When it comes to irrigation tools, he points out that booms, rain-guns and drip systems all have advantages and disadvantages.

“Drip irrigation is accurate and efficient, uses less energy and makes the most efficient use of water, as it can be applied accurately with sensors, and this method is not affected by wind. It works better than the other methods in hot, dry seasons as you do not lose so much moisture to evaporation.

“Also, because you are applying the water onto the soil, you keep the leaves dry and so there is a lower risk of creating an environment for disease spread. In addition, you lose less water to evaporation so your water use efficiency is higher.

As water is applied little and often, room can be left for rainfall when it is forecast, he adds. In the future, he sees its potential to work as a conduit for applying nutrition and biologicals to the crop, and trials are underway to evaluate this.

“When setting up drip irrigation, the equipment needs correct placement with the tape either at the top or the ridge or in the ridge” Although costs of initial set-up of drip irrigation are considerable and time consuming, less labour is needed throughout the season than for the other irrigation tools.

“Drip irrigation is an ongoing process that still needs managing, and it is not suitable for rectifying a high SMD problem as large amounts of water cannot be applied quickly Rain-guns are cheaper to operate and can be used as more of a “fire-brigade’ approach whichAlthough costs of initial set-up of drip irrigation are considerable and time consuming, less labour is needed throughout the season than for the other irrigation tools.

“Drip irrigation is an ongoing process that still needs managing, and it is not suitable for rectifying a high SMD problem as large amounts of water cannot be applied quickly Rain-guns are cheaper to operate and can be used as more of a “fire-brigade’ approach which can be useful, said Andrew. However, because of the speed of application, 25mm can flatten the haulm, and it can lead to run-off. It is also Loo easy to apply more water than is necessary. and wind can make them inaccurate.

“If soils are smeared, compacted or capped, infiltration is compromised and the water does not reach down to the root zone, so the water can be lost. If it is very dry, the ‘fire-brigade’ approach can sometimes lead to too much water going on, particularly if it is followed by heavy rain,” Andrew said. “Attention to detail needs to be taken as too much water can lead to growth cracks and the spread of powdery scah”

These growth cracks can occur when the skin is unable to stand the internal pressure created by over-hydrated tubers.

“Although rain-gun control has improved as they can now be controlled remotely using apps on your phone, rain guns still use more man-power throughout the season.”

Booms are better at imitating rain, observed Andrew, noting that water rates can be adjusted when they are used, facilitating a more uniform application of water than with rain-guns.

Nevertheless, they can still create run-off if the field is on a slope.

“Using a boom can also be more difficult if there are trees and poles to contend with”

Andrew also cautions about trying to do too much. “If you have limited capacity for irrigating, you are better to manage what you can do properly rather than stretching your capacity and compromising yield, quality and profit,” he said.

Following weather forecasts remains important to decisions about irrigating, particularly when summer storms threaten in July and August, because they are likely to bring heavy rain. “This is when we see the benefit of a well-structured soil which is able to use water more efficiently.

“In our trials, we have noted that soils with a reasonable level of organic matter (OM) are better able to hold on to water. Nonetheless, if heavy rain is forecast it is better to hold off with the irrigation.”

He goes on to note that tools and techniques that disrupt wheelings and tramlines, such as Wonder Wheel and the use of valleys, can help retain water and therefore reduce run-off.

“Wonder Wheel has angled tines to lead water down and under the growing crop.

The machine creates a convex domed profile in the ridge bases to channel water towards the tine zones,” he said.

“Whatever irrigation method you opt for, you still need to measure and manage to ensure that there is value for money from irrigating in terms of increased yield and/or quality. If you have the irrigator reel on the headland, and there is usually 10-15m in front of the reel that has not been irrigated, this can be used for comparison with further up the field as an easy in-field assessment.”

Later irrigation and rainfall can affect senescence and dry matter (DM) of the crop. If dry matter is too low, stopping irrigation and initiating senescence will help increase them, he said.

In addition, in certain varieties, overwatering later in the season can cause raised lenticels, which are open cells and therefore can allow an entry for infection, Andrew warned.

When it comes to lifting, even small changes in tissue elasticity and turgor influence bruising susceptibility, so Andrew notes that when soils are dry, irrigation prior to lifting can help to keep the soil on the web for longer and reduce mechanical damage and subsequent bruising.

Of course, using any sort of irrigation will raise the carbon footprint of the crop because of the energy used to pump the water. For every litre of fuel, there is a 2.5 litre carbon penalty. “However, on the plus side, potatoes are good at soaking up carbon in the atmosphere, particularly as they are still green in July and August when other crops are senescing. Water is a valuable, finite resource and we need to make the most of this limited resource.”

Field capacity (SMD-0) is the amount of water the soil can hold – a guide to this is when soil held in a hand is squeezed and just holds together.

Soil Moisture Deficit (SMI) is the amount of water needed to bring the soil moisture content back to field capacity. This should be measured in the immediate vicinity of the plants, and measurements taken at different depths.

SMD varies throughout the season according to rooting and surface area of the crop. At 10% emergence, SMD should be around 12-15mm, at bulking 40mm and senescence 50mm. Ways of measuring SMD include:

• Neutron probes. These are usually used in collaboration with a contractor, and the instrument measures the neutrons colliding with hydrogen atoms (which are present in water).
• These mimic what the plant roots are doing in the soil to measure the tension and the drier the soil the soil, the greater the tension. Accuracy depends on location, and these should be placed both in the ridge and also lower down.
• Capitance probes. These are measurement devices that are placed in the soil in protective tubes. The bottom end of the probe on the ground sets up an electric field and the change in electrical capacitance is proportional to the moisture content of the surrounding material (soil).
• Water balance sheet. Estimations of rainfall, irrigation and evapotranspiration are used to evaluate the water in the field, but as it uses assumptions, it may be inaccurate.
• Digging a hole and examining the soil.
• Weather forecasts should also be followed to determine potential rainfall, observes Andrew.

• Pay attention to detail in monitoring soil moisture to assess crop water requirements
• Ensure irrigation pressure is correct – if pressure is too low it creates big droplets which are more prone to run off; too high pressure causes small droplets, which can result in misting and drift
• Check rain guns are at the correct angle to avoid drift and ensure even application – no under- or over-lapping
• Maintain boom nozzles to ensure correct water pattern and even application
• Measure and record water abstraction rates