Make Time For A Tune Up

Soil Types Impact Water-Storage Ability

Soil texture, whether it’s clay loam, sandy loam or sandy, will impact its ability to store water. Here are some examples of soil feel and appearance from Texas A&M and other sources:

  • For a clay loam at 50%-75% moisture content, there will be a moist appearance. It will form a small ball with defined finger marks. Soil has a ribbon effect when squeezed between the thumb and forefinger.
  • For sandy clay loam at 50%-75%, soil appears and feels moist and pliable. Its color is dark and it forms and ball and ribbon when squeezed.
  • Sandy loam at 50%-75% moisture appears and feels moist, is dark in color, forms a cast or ball with finger marks and leaves a smear or stain and wet outline on the hand.
  • Fine sand or loamy fine sand at 50%-75% appears and feels moist, is darkened in color, forms a weak cast or ball and leaves a wet outline or slight smear on the hand.

Well capacity can vary a little from year to year or during the crop season, depending on rainfall and demand on the system. More than one High Plains well has run dry at peak irrigation times in mid-summer.
“Weak capacity may prevent growers from delaying pre-watering,” says Texas A&M’s Dr. Dana Porter. “Limited capacity in some systems will mean that more time is needed to provide the desired quantity of water to the root zone.”
For example, a well capacity of one gallon per minute (GPM) can deliver 0.053 inches of water per day, or 0.37 inches per week; 3 GPM can apply 0.16 inches per day or 1.11 inches per week, and 6 GPM per acre can apply 0.32 inches per day, or 2.23 inches per week.
The root zone for most Texas cotton is 2.6-5 feet. Like other field crops, cotton will take up 70%-85% of its water requirement from the top 1-2 feet of soil. The root zone depth, the soil profile and the well’s GPM rating will determine how long it will take to put down a specific amount of water in a pre-irrigation program.
More information on off-season maintenance and preplant irrigation is available on the Texas A&M website: http://lubbock.tamu.edu/irrigate/
Soil monitoring instruments, such as gypsum blocks or tensiometers, can provide a good estimate of soil moisture content. But veteran growers are also familiar with hand-sampling methods of gauging moisture content.
“You can estimate a soil’s moisture condition by observing its feel and appearance,” says Porter. “Use a soil probe, auger or spade to extract a small soil sample within each foot of root zone depth. Gently squeeze the sample in your hand to determine whether the soil will form a ball or cast, and whether it leaves a film of water and/or soil in your palm.
“Press a portion of the sample between your thumb and forefinger to observe whether the soil will form a ribbon. Compare your sample with the guidelines for your particular soil type.”

With irrigation likely the most costly input for over half of West Texas cotton production, having a system properly primed for 2008 should be at the top of a grower’s to-do list.

And knowing the amount of preplant irrigation needed can help growers get the best jump on a healthy stand next spring, says Dr. Dana Porter, Texas A&M University agricultural engineer who specializes in irrigation at A&M’s Lubbock Research and Extension Center.

Porter was among those just short of spellbound over the amount of rainfall the Texas High Plains received in late spring and early summer. Soil profiles were overflowing for many. And cool weather made much cotton 2-3 weeks late. That meant center-pivot LESA (low elevation spray application), LEPA (low energy precision application) and other irrigation circles weren’t used nearly as much in the spring. Nor were subsurface drip irrigation (SDI) systems, and furrow systems using both poly pipe and other types.

Even though cotton was being pressed to obtain enough heat units, growers weren’t complaining about the reduced natural gas, electric and diesel bills which resulted. Still, good managers were aware of the need to maintain an efficient system, because 2008 could mean the return of dry weather that is all too common for the region that averages barely 18 inches of rainfall per year.

A good maintenance program should involve checking older systems for efficiency and how they will be expected to function, says Porter, given the water availability and degree of yields expected.

“The first question to ask is if the system is performing according to its specifications,” she says. “Does it need to be repaired or upgraded, or does the center pivot or linear system need to be re-nozzled?”

Another question should be whether the nozzle package has “drifted” over time, or if broken or lost nozzles may have been temporarily replaced with the wrong size nozzle. “Such quick fixes repeated too often over time can lead to poor distribution uniformity,” says Porter, noting that growers should consider the last time they conducted a distribution uniformity test, calibrated the pivot, or just made sure the nozzles on the pivot match the design nozzle package printout.

For SDI systems, which now water more than 250,000 acres in Texas, make sure the filtration and maintenance program are adequate. Pressure gauges and flow meters should be used, and can help in troubleshooting system efficiency, as they can indicate problems such as leaks or plugging in the system, says Porter.

Match the System

New LESA, LEPA, SDI and furrow-irrigation systems are needed from time to time to replace worn-out systems, to replace less efficient systems with more efficient ones, or to add new irrigated acres. Porter encourages growers to select the right system and specifications for the task at hand.

Cheaper May Not Be Better

Cutting costs is on everyone’s minds in any farming venture, but “cheaper may not be better” in a new or refurbished irrigation system, says Dr. Dana Porter. Here are her suggestions for judging whether a new system design is adequate for the job requirements:

  • Is there adequate pressure/vacuum relief?
  • Does the design feature flexibility to accommodate crop rotations and well capacity fluctuations?
  • Does it have ease of maintenance?
  • Does it feature appropriately sized underground pipelines (consider friction losses, especially in longer pipeline runs)?
  • For center pivots, determine whether pressure regulators are needed (they’re more likely to be justified in sloping fields).
  • For SDI systems, adequately address filtration and other maintenance, including acid injection, fertigation, etc., as needed.
  • And, of course, install the system correctly by following design specifications.

Texas A&M research at the Halfway station, located north of Lubbock, indicates that in limited irrigation situations, SDI and LEPA systems can lead to higher yields. In situations where full irrigation is available, LESA systems can provide as good or better yields. For any system, good management is key to good performance and efficiency.

Selecting the right system for your situation should begin with a good system design. “That will likely require working with a certified irrigation designer or professional engineer,” says Porter. “Design the system for realistic well capacities. Consider whether the water delivery is likely to decrease during the season.”

Prepare for Pre-Watering

A preplant irrigation is often needed to generate the stand that will lead to a good crop. But growers can refrain from applying too much water by knowing their soil profile before they crank up engines, motors and pumps.
The recommended soil moisture storage is about 75% of full field capacity. “This will allow for room to store water from timely rains,” says Porter. “If the goal is to apply water to moisten the root zone to a target level, it’s essential to know how much water the soil will hold at field capacity, and how much water is already in the soil.”

Captions:

Dr. Dana Porter
LEPA irrigation saves energy and conserves water.

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