Crop, soil variables should be taken into account for irrigation

    According to University of Idaho Extension Water Management Engineer Howard Neibling, managing water levels in the soil on irrigated cropland is all about balance.
    “Capillary and surface tension forces hold the piece of water in the soil pore, and those capillary forces acting in all directions hold that water in place,” explains Neibling. “You also have osmotic or salt-related forces in the soil. Those two forces hold water in the soil, and irrigating crops efficiently is a balance between them.”
    Soil moisture begins with the particle sizes that make up the soil. The three basic soil types are sand, silt and clay. “Sand is defined as 2 mm down to .05 mm, or 50 microns,” says Neibling. “Particles from 50 microns down to 2 microns are silt, and anything below 2 microns is clay. They’re arbitrary lines, but they generally make good sense.”
    “Clay particles look like flagstone, and that shape is what gives them a lot of their properties,” he says. “Two microns is small, and not really visible at all. If you take a meter and whack it into millionths, that’s a micron.”
    If a clay particle is put in a glass of water and allowed to settle, it’ll take four to six hours to reach the bottom, he says.
    “If you have clay particles stacked together you can make a nice brick wall, of sorts, if they’re all oriented in the same direction. Normally they’re randomly scattered and oriented in different directions so you have pore space between them,” he adds, noting that’s where compaction and working wet soil comes into play.
    “Normally, if they’re stacked in all directions and you come across with a piece of equipment and it’s relatively dry there’s enough friction to hold the pieces where they are,” says Neibling. “If they’re wet, the water tends to lubricate those contact points and the particles will slide and consolidate into their most compact form before they hold the equipment up.”
    Neibling refers to an Ohio State University demonstration where a researcher took tractors and ran across a number of plots of wet high clay content soil. “For the next 10 years, whenever it rained he had a lake on those plots. It took 10 years of freezing and thawing to get rid of one compaction,” he says.
    Move the next particle size up to silt, Neibling explains they’re more three-dimensional in shape, similar to a football. “When you stack those you end up with genuine soil pores that are a little more stable, and they’re what holds the water for the crop.”
    In sandy soils, with large pores, he says there are small capillary forces relative to the weight of the water, so there’s rapid drainage and the plant has to exert little pressure to get the water out. “When my family and I first moved to Laramie the sod on our lawn was drying out so fast I got curious,” comments Neibling. He took a sample to the lab and found the sandy soil only held .3 inches of water per foot of sand. “That’s the lowest number I’d ever seen. Sand just doesn’t hold that much.”
    He says soils are made up of those three primary particles – clay, silt and sand – that can’t be broken down, and of soil aggregates, which are clods of all three. “The particles don’t have much in the way of soil pores to hold water, but the clods do, and that’s how you’re pulling and storing the water your crops use between irrigations.”
    “Silt/loam soils can hold 2.5 inches of available water per foot. From .3 in the sand to 2.5 in the silt/loam makes a big different in how we manage that soil,” he says, noting that not all water is truly available for plants to use. “Most plants can use half of the available water, and the silt/loam soils have the most usable water.”
    “On a good hot day, malting barley in the middle of the active growing season will use about a quarter inch of water per day. One inch of available water is gone in four days, and if the root zone is two feet deep you have maximum eight days of water before the crop starts to stress,” says Neibling.
    Neibling says most crops at optimum growth are able to use the top half of available water. Alfalfa at all stages uses 50 to 55 percent and field corn 50 percent. Cereal grains, except through flowering can use over half; from root through flowering they can only use 45 percent. Sugarbeets can use half and dry beans about 40 percent.
    Regarding center pivot systems, Neibling says every time the pivot goes around water is lost to evaporation. “If you’re running a ¼-inch revolution, you’re losing four times the amount of water to evaporation as you would running one one-inch application,” he explains.
    Neibling says growers should pay attention to a crop’s pattern of water use. “While grains are growing you can barely keep up, then all the sudden they shut off, and if you don’t see it and continue to water you get disease problems or you’re pumping water you could use elsewhere,” he explains.
    Also, different crops use water from different areas of the root zone. Sugarbeets use water down to four feet, while alfalfa reaches to six feet or deeper. “Different types of roots utilize different depths,” he says.  
    Surface sealing should also be taken into account when running sprinkler irrigation, especially on high silt soil, he says. “If your drop sizes are too large or the soil surface has been worked a few too many times and the aggregates are broken down, all the sudden you’ve got a lot of runoff.”
    He says that’s where a thin layer of crop residue will help, giving the soil some sort of protection so it doesn’t fall apart and is able to absorb more water.
    “You want to really work toward maintaining a deeper rooting system if you can,” he says of irrigation management.
    Howard Neibling presented information on irrigation to the Wyoming Ag Business Association’s annual meeting in Sheridan in early August. Christy Hemken is assistant editor of the Wyoming Livestock Roundup and can be reached at christy@wylr.net.