Irrigation is much more than opening and closing valves and starting pumps. We really need to think about managing and measuring soil water.
Research trials in sandy loam-textured levee soils at the Frank Wise Institute established that maintaining soil tensiometer readings close to 15kPa resulted in higher yields and quality, with more extra large first grade fruit.
The average water applied for the 15kPa treatment was 23.2 megalitres per hectare for the 12 month period (not including rainfall).
Water application rates will vary between seasons and depend on many factors including seasonal conditions and crop development stage. A survey of grower practice since 1998 showed that growers applied 55 to 85 millimetres of water per hectare per week in summer, and 30 to 60mm in winter.
Corresponding rough crop factors thus range from 0.7 to 1.2 with an average hour of watering per day. The range in annual application of irrigation water in the ORIA is about 17 to 26ML/ha.
Scheduling irrigation through soil moisture monitoring, climate recording, and plant growth stage recording will assist in determining how much water is to be applied to replenish water in the root zone. By knowing the flow rate per outlet (sprinkler, dripper etc.) growers can calculate how long to irrigate.
Many scheduling methods and devices are available, ranging from checking soil colour and feel to sophisticated continuous measurement systems. The more common methods, together with advantages and disadvantages, are listed in the table below.
In the table Packsaddle loam is abbreviated as PsL, Ord loamy sand as Ols, Cockatoo sand as Cs, and Cununurra clay as Cc.
For sandy loam soils such as Ord sandy loam and Packsaddle sandy loam, full point is about 22% and refill point about 14%, leaving about 80mm of plant available water per 100cm soil.
Typical irrigation systems in the ORIA feature micro-sprinklers spaced at about 6m between laterals and about 5m along beds. This gives about 333 sprinklers per hectare and 4–6mm/hour average precipitation based on 95L/hour sprinklers.
| Soilspec system | Tensiometer | Gypsum blocks | Micro Gopher | Ferret | EnviroSCAN | Diviner 2000 | Buddy | |
|---|---|---|---|---|---|---|---|---|
| Measurement group | Water tension | Water tension | Water tension | Water content | Water content | Water content | Water content | Water content |
| Reading range | 0-80kPa | 0-80kPa | 60-500 and 10-200kPa | |||||
| Stated accuracy | 1kPa | 1kPa | 1kPa | 1% calibrated | Semi-quantitative | 0.1% calibrated | 0.5% calibrated | 1% calibrated |
| Operating principle | Capillary suction | Capillary suction | Electrical resistance | Soil capacitance | Soil capacitance | Soil capacitance | Soil capacitance | Soil capacitance |
| Measurement sphere | 10cm | 10cm | 10cm | 10cm | 5-10cm | 10cm | 10cm | 10cm |
| Installation details | Into augured hole | Into augured hole | PVC tubes and holes | PVC tube into augured hole, small diam tube | Probe inserted into soil to max 60cm | PVC tube into augured hole | PVC tube into augured hole | PVC tube into augured hole |
| Installation problems | Smearing of soil face | Smearing of soil face | Electrical contacts | Accuracy in installation, soil shrinkage air gaps around tubes | Contact with sensor probe may be problematic | Accuracy in installation, soil shrinkage air gaps around tubes | Accuracy in installation, soil shrinkage air gaps around tubes | Accuracy in installation, soil shrinkage air gaps around tubes |
| Logging capacity | Possible with transducer | Possible with transducer | Yes | Yes | No | Yes | Yes | Yes |
| Manual readings | Yes, with portable meter | Yes - gauge installed | Via logger | Yes, portable | Yes, portable | Yes, not portable | Yes, portable | Yes, not portable |
| Continuous readings | No | Possible | Yes | No | No | Yes | No | Yes |
| Soil suitability ORIA | PsL, Ols, Cs | PsL, Ols, Cs | All | PsL, Ols, Cs,Cc except when dry and cracking | PsL, Ols, Cs, Cc except when dry and cracking, dry soils difficult to penetrate | PsL, Ols, Cs,Cc except when dry and cracking | PsL, Ols, Cs,Cc except when dry and cracking | PsL, Ols, Cs,Cc except when dry and cracking |
| Farm size application | Small | Small | Small to medium | Small to medium | Small and nurseries, to check soil wetting depth | Large and intensive, high value crops, R&D | Medium to large, R&D | Medium to large, R&D |
| Expansion capacity | Limited only by time to service and read | Limited only by time to service and read | 64 sensors per logger | Limited only by capacity to manually collect data | NA | Combinations of access tubes and sensors to match max 32 per logger | No, but unlimited number of access tubes for manual recording | No, each unit stands alone with 1-8 sensors per logger, multiple loggers possible |
| Issues | Maintenance, air entry | Maintenance, air entry | Cabling required | Installation problems, cabling, one-off tube size, problems in shrinking soils | Installation problems, cabling, one-off tube size, problems in shrinking soils | Problems in installation and shrinking soils |
About 80% of banana roots are found in the upper 50cm of soil, therefore maintaining available water in this layer is essential.
Studies show that for maximum yield, bananas should be irrigated when one-third of the available soil water in this layer has been used. Soil moisture monitoring and effective irrigation are therefore essential. The easiest and cheapest method of soil moisture monitoring is to use tensiometers.