March 26, 2013
Mapping soils to improve crop irrigation
What a difference a month can make? In March 2012 many growers changed their cropping plans to focus the water resources they believed they would have to work with by irrigating a smaller area of vulnerable crops, writes Nick Winmill, Potato and Irrigation Specialist for Agrii.
Faced with half empty irrigation reservoirs (at best), low river flows and inevitable restrictions on summer abstraction wholesale changes were made as we basked in temperatures of 20oC. We moved on into April and the weather pattern changed with a vengeance leading to above average rainfall thereafter and the consequences on yield and quality that resulted.
Despite the issues last year we came close to irrigating crops in some areas in mid to late August whilst a handful of growers did irrigate to facilitate lifting! With reservoirs topped up and ground water reserves in a more healthy state concerns over irrigation resource are less than 12 months ago but we still need to manage irrigation well to maximise yield and quality of crops. Although the need may seem less of an issue today, the weather pattern can change to dry this season just as quickly as it changed to wet last year.
For growers targeting crop supply towards the main retailers there will be a focus on managing the risk of skin blemishing due to common scab. Potato Council sponsored work over the period 2007-2010 demonstrated that the most reliable means to control scab was through well managed irrigation through the phase when developing tubers are most susceptible to infection. Tubers remain susceptible until the lenticels on the newly developing tubers become suberized. In practice this may require growers to maintain low soil moisture deficits for typically a 3 to 4 week period from the onset of tuber initiation (TI) but may be longer according to factors including variety and soil type.
Over-irrigating during the critical period for scab control can lead to severe tuber cracking and may have effects on the processing quality of the mature crop such as fry colour and dry matter. Working to moisture deficits as low as 9 to 17 mm to affect scab control also increases the potential risk of nitrogen leaching beyond the rooting zone with attendant environmental and early senescence concerns.
There are various methods of assessing crop water requirement and irrigation need. Some growers rely on a basic daily balance sheet approach making use of local evapo-transpiration information to calculate crop and soil water losses and provide an estimate of water need. This approach can be time-consuming, though some bureau systems provide this service based on a simple spreadsheet calculation. Taking this a stage further some systems calculate a soil water balance by modelling crop growth and demand for water, which when linked to a weather forecast can give a useful ‘look-ahead’ to plan irrigation requirement. Equipment giving a direct measurement of soil water whether through the use of tensiometers or neutron probes has also been popular relying on often weekly visits to take readings and relay them to growers. However growers of all sizes are turning increasingly to using remote telemetry systems to access soil moisture, rainfall and temperature data.
The 24/7 availability of this information in near real-time can be accessed through laptops and portable devices such as Smartphone’s and iPad’s giving growers a greater insight into soil water dynamics. When coupled with a local weather report farmers can often plan their irrigation in a more efficient manner.
Agrii’s IrriQuest soil moisture monitoring service is using ‘Precision Agronomy’ techniques to ensure that probes are positioned and installed in the most representative areas of fields. Electrical conductivity (EC) scans at 30 and 90 cm are used to produce maps in effect showing the variability of soils within grower’s fields. The scans from SoilQuest are usually used to plan subsequent GPS referenced sampling and analysis to support variable rate fertiliser application and seed rates as required.
However researchers at Cambridge University are also making use of these EC scan maps as part of a Potato Council Ltd (PCL) sponsored project to investigate how growers can make their cultivations more cost-effective without affecting crop income. Agrii’s SoilQuest is collaborating with the research team at Cambridge to provide EC scan maps, which not only reflect changes in soil texture and/or degree of compaction but also serve as a ‘water-holding capacity’ map
This soil water mapping approach is now being used to guide on the most appropriate locations for installation of soil moisture probes within fields.
Agrii typically uses 60 cm Adcon SM-1 probes to provide soil moisture information at 10cm increments through the profile, although in some instances for deeper rooting crops 90 cm probes are installed. The direct reading of soil moisture status through the profile allows growers to manage their irrigation to ensure sufficient water is applied to keep the root zone supplied, whilst minimising losses to drainage, which may otherwise lead to loss of nutrients. Sufficient water must be available to the crop at key periods to preserve overall crop quality and yield. Thankfully the PCL is continuing to fund the Cambridge University team to examine the fine balance of irrigating for quality and yield whilst investigating investigating the influence of irrigation regimes on other aspects of tuber quality and N leaching.
For further information on Adcon soil moisture probes or SoilQuest please contact your local Agrii Agronomist.