Tuber Talk with Lukie Pieterse
Key Elements of Successful Irrigation Scheduling
Irrigation scheduling can be described as both a science and an art. Scientists have spent many years studying the variables that come into play when irrigation scheduling is on the table, publishing myriads of papers dealing with an understanding of these variables, and often advising irrigators how they can work with, or at times work around, these variables in an effort to obtain optimal irrigation scheduling. On the other hand, most growers often rely on “gut feelings,” which includes knowledge of their crop, the soil type of a particular land and common sense.
Achieving successful and efficient soil moisture application management boils down to paying close attention to detail—sometimes at a micromanagement level. The quantity of water that actually reaches potato plants when irrigating can vary widely. Often this is dependent upon a host of variables, including type of irrigation, soil type, variability in weather conditions and so forth.
Under-irrigation of a potato crop inevitably leads not only to a loss in total yield, but also to a loss in market grade, tuber quality and contract price. Over-irrigation, on the other hand, leads to a loss of water, a waste of electricity for pumping, leaching of nitrogen from the root zone, increased fertilizer costs and a waste of valuable time. According to Alexander Pavlista, an extension potato specialist at the University Of Nebraska Panhandle Research and Extension Center, over-irrigation can cause disease, specifically toward the end of the season with tuber wet rots (soft rot, leak and pink rot). Most of these losses can be avoided if growers pay close attention to accurate irrigation scheduling.
Most experts and growers alike would agree that successful scheduling hinges on the following essential pieces of information:
• knowledge of the soil types in the field,
• water-holding capacity of the particular soil type,
• depth and thickness of soil layers in the field,
• amount of rain received on the field prior to irrigation application,
• amount of water the irrigation system can apply during a certain period of time, and
• amount of water the crop is using during the growing season.
Water usage by a crop is further dependent on the stage of growth, availability of water in the soil, air temperature, solar radiation (amount of sunlight) and relative humidity as well as other weather variables. Pavlista notes that wind is often a forgotten factor. High winds dramatically increase transpiration, especially if it is hot with low relative humidity. The amount of moisture a potato crop needs for optimal growth varies from day to day, depending upon weather conditions and the available moisture in the soil. These are factors that need to be taken into account when scheduling irrigation applications during the growing season.
Thus, it is important to remember that irrigation criteria will vary with climate and soil type as well as location. Sandy soils need more frequent irrigation applications, while heavier soils allow for more time between irrigations because of the water-holding capacity of heavier soil types. Areas that receive a large proportion of their precipitation from rainfall are more forgiving because risks of overwatering can be reduced; whereas, in drier areas, a heavy rainfall can cause soil to become too wet.
There are many different methods and tools that growers can and do use for measuring moisture at the potato root zone when deciding upon an irrigation schedule. Whether growers use one or more of the technological tools available to measure soil moisture—ranging from simple soil moisture blocks using electrical resistance, to time-domain reflectrometry methods and electrical capacitance-based products—all have to be calibrated to the individual soil type.
Pavlista says many producers prefer “hands-on” methods to measure soil moisture. One effective test some farmers apply is to simply dig a few potatoes out of the hills often during the growing season. Hills are meant to get the “feet” of potatoes out of water, but research shows that sometimes water doesn’t get to the roots effectively, but runs off the sides to the bottoms of the hills instead. In general, the area eight to ten inches below the top of the hill will be the driest. Growers can use that area as an important gauge to estimate the moisture content of the rest of the hill and below. By simply feeling the soil, growers are doing two things—checking the quality and size of potatoes and the soil moisture at the same time.
This “feel and appearance” method of measuring soil moisture is practiced by many potato growers. Below, Pavlista outlines in practical detail how growers can judge the appearance and feel of the soil to determine what percentage of water is present in a soil sample.
Growers also need to understand water use and water needs of potatoes as they grow and develop through different stages, to the end of the growing season. As is the case for every annual, potatoes start out with low water use. After flowering, water usage increases until hitting a peak, then leveling off somewhat followed by another increase when tuber bulking occurs. The tuber bulking phase is the period when demand for water is highest in the plants and they are most sensitive to deficits. Below, Pavlista provides a summary of problems resulting from water deficit or excess soil moisture based on production period.
Although growers do not have control over the amount of rainfall during the growing season, potential moisture problems develop as a result of inaccurate irrigation scheduling. The importance of managing irrigation systems at all times with the utmost care cannot be overstated. Irrigation scheduling should always be done in accordance with the growth phases of the plants—if done successfully, growers will prevent unwanted problems in the field, save money and ensure the harvest of a quality crop.