Bill Basford

Open day report - better irrigation

11/12/2018

Crumbly, friable soils with a loose surface texture are essential if plants are to gain full benefit from irrigation. Timely irrigation, with the right volume of water applied in the best way is equally important if yields and quality are to the maximized under all weather conditions, according to independent mechanisation specialist Bill Basford and Peter White of Soil Moisture Sense, a company specialising in moisture sensor technologies.

During seminars at the Briggs Irrigation open days in Corby, Peter White asked farmers whether they should water all their crops badly or do some very well and the rest not at all. 

“Do you know which would make you more profit?” he asked, before suggesting that many growers would increase profitability by doing less better.

“In 2018 yields of many crops varied significantly, depending on local rainfall and irrigation. Onion yields, for example, fell by 40% on average but the range was huge, with between seven and 100 t/ha recorded. Effective irrigation had a major part to play in achieving yields at the upper end of the scale,” he pointed out.

As yield is proportional to water use, for many growers this meant applying 30mm of water twice a week. But if plants are to use this water effectively, application and infiltration rates must match. This means ensuring water can penetrate the soil and is available to the plant in its active root zone at a rate it can consume.

In practice, smooth surfaced ridges and beds, often seen on good quality silt soils, shed water which may then run off the land along compacted wheelings or down the furrow. Capped soils present a similar problem. Water logging following heavy applications of water, or when rain follows, also results in stress, causing a temporary slowdown in growth and subsequent reduction in yield and quality.

In a worst case scenario the ridge can be dry, again leading to drought stress, restricted growth and minimal scab control while the furrow is saturated, potentially leading to leaching of nutrients. As the canopy of plants such as potatoes grows it can disperse the droplets and reduce the problem but by then much of the benefit of irrigation can be lost.

One way of avoiding these problems is to ‘pre-water’ with a light application before the main irrigation pass, Peter suggested. For example, in a very dry potato ridge the infiltration rate may be too slow to take a 30mm boom application in one go. Paired applications may therefore be more effective. Wetting agents could also be useful and he recounted good experiences using the brand H2Flow.

In all situations, monitoring soil moisture is essential, he argued. Modern probes can help record the impact of irrigation at different depths. With the aid of accurate data it is possible to balance application rates with infiltration speed. Cultivation techniques can also be compared, along with the impact of changes to irrigation techniques such as trying different nozzles or wetting agents.

Methods of getting water onto the soil without damaging crops or the soil itself and without waste was examined in greater detail by Bill Basford.

Booms are capable of applying larger volumes more quickly than rainguns, peaking at around 30mm/hr compared to 20 mm for rainguns, he pointed out. When both are operating at optimum pressure a boom also delivers much smaller droplets, 90% of which are likely to be below 2mm in diameter compared with 90% above this size for the raingun. As doubling the droplet size increases its weight eightfold, the impact on the soil and plant can be significant. Water from a raingun also has greater velocity, in part because it leaves the gun at around 75 mph.

Bill Basford also highlighted the importance of soil structure, pointing to the huge differences in the rate of water infiltration caused by soil compaction from multiple tractor passes. Slopes must also be taken into account, he pointed out, with a slope of just 5% probably reducing infiltration by 50%.

Practical means of reducing run off are already available, he noted. These include the Briggs tied ridger which creates small dams between beds. These should be removed before harvest, perhaps with a front mounted tine. A spiked roller such as the Aqueel can be used to indent the surface of the bed, making it more receptive to water. There are also simple machines to break up the wheeling, such as BYE Engineering’s Wonderwheel.

It is also important to remember basic principles of irrigation, he stressed:

“Optimum operation of all irrigation irrigation equipment depends on maintaining sufficient pressure throughout the system. Rainguns require the highest pressure and if this falls, droplet size increases, which may cause damage or capping, uniformity is lost as there are fewer fine drops to ‘fill in’, while on exposed sites it is harder to combat wind drift. Our trials have shown that areas of the crop will therefore receive insufficient water.”

Ensuring there is adequate pressure is dependent on good design for the whole system, he stated.  For example, in a ‘typical’ system, increasing the diameter of the pipes from the ring main to the spurs in the field by 25% could save around 3.5 bar in pressure loss, improving uniformity and optimising droplet size.

Energy losses through the turbine should also be taken into account.

“In modern hosereels the pressure loss is 0.4-0.8 bar, depending on the speed at which the hose is being retracted, whereas for older designs the loss is 1.5-3.0 bar. Updating equipment can therefore be cost effective.” 

The pros and cons of using boom irrigators are well established, he suggested. Advantages include the ability to manage droplet size for different crops and various stages of growth. Crop and soil damage can be minimised and uniformity is excellent. Booms also require less energy than rainguns as they can usually operate at just 2-4 bar.

Conversely booms may require a little more labour to set up, although they are still relatively easy to handle. They also cost more and growers need to be aware of the effect of placing high volumes of water on their particular soils over a short period of time.

Booms costings

 

•          £16,500 - 5 years                                            p.a.

•          Depreciation to £3000 -                                £2700

•          Repairs and spares @ 2%                               £330

•          Interest on half cap. @ 2%                             £165

•          Extra time – Labour

6 pulls @ 0.5 hr @ £15                                                   £45

                                                                               £3240

 

20 ha use =3240/20 =£162/ha (£65.60/ac)

 

Automation may also be worth considering, purely on the basis of time saved. For example, saving one hour a day, plus mileage can easily equal £18 per day overall. If 120 days are spent irrigating, the saving is thus £2160. A one-off cost of £1330 for an interactive telephone control system is thus quickly recouped.

The ability to monitor, control and record using systems such as Brigg’s Raindancer adds further benefits and enables improved management across the farm.