Potato Review

www.potatoreview.com POTATO REVIEW MARCH/APRIL 2023 33 IRRIGATION “Using a boom can also be more di cult if there are trees and poles to contend with.” Andrew notes that using any sort of irrigation will raise the carbon footprint of the crop because of the energy used to pump the water. For every 1l of fuel, there is a 2.5l 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.” Following weather forecasts remains important, particularly when summer storms threaten in July and August, because they are likely to bring heavy rain. “ is is when we see the bene t of a well-structured soil which is able to use water more e ciently. In our trials, we have noted that soils with a reasonable level of organic matter (OM) are able to hold on to water so there is a bigger reservoir in the rooting zone. “Nonetheless, if heavy rain is forecast it is better to hold o with the irrigation.” Knowing the soils in the eld 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. Soil type and texture determine water holding capacity because sand, silt, and clay particles have di erent pore sizes. Soils with higher amounts of clay have more small pores than sandy soils, helping them to retain more water. “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, as this can help water stay in the all- important root zone. “Gentle rainfall is the best for our industry. On one hand if the soil in the ridges has capped, quite often gentle rain can help break it, but also it can help to reduce greening as it lls up cracks which have appeared in the ridges.” What are Field Capacity and Soil Moisture Deficit? 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 does not fall apart. Soil Moisture De cit (SMD) is the amount of water needed to bring the soil moisture content back to eld capacity. Top tips for eective use of water • 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 o ; too high 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 Controlling common scab – is there more than just water management? Soil type, irrigation and varietal susceptibility all play key roles in reducing the incidence of common scab (Streptomyces scabiei), says Andrew. e standard scab control regime is to irrigate for 28 days from tuber initiation (TI). is is the time when the crop is susceptible because new tissue is being formed by new cells, and is reduced once the tuber reaches the size of a golf ball, he explains. Tuber initiation takes place 15-40 days after emergence but in some varieties, depending on weather conditions, it can occur in two bursts. He recommends that the soil in the ridges should be moist during this time, and so irrigation needs to start in good time. In addition, if irrigation is stopped too early, it can create an even better environment for the pathogen to proliferate, resulting in higher scab levels. “If emergence is uneven, it is best to keep the crop moist until all the tubers have had a chance to grow out of the susceptible phase, which is when the tuber has stopped producing new cells.” Irrigation helps boost the populations of good microbes that compete for the same nutrients as the scab-causing Streptomyces, and resulting in fewer pathogens. However, he notes, extra irrigation does not give greater protection as there are no further increases in bene cial microbe numbers and may even lead to infection with powdery scab. Irrigation, nevertheless, is one of the key strategies to reduce incidence of common scab, yet it is not the whole story. Correct planting depth for even emergence, avoiding overcultivation and compaction plus a good understanding of soil type are also critical. “Soil health is also important as soils with good structure and organic matter (OM) with healthy numbers of microbes show lower incidence of common scab,” said Andrew. ➜ Water rates can be adjusted when using booms, which facilitates a more uniform application of water than when rain-guns are used, however, they can still create run-o if the field is on a slope.