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Water Management on Greens with Soil Moisture Sensors

After last week’s posts, I got several follow-up questions by phone and email on how to use moisture meters. Dr. Bremer is our microclimate guru, so I asked him to provide some pointers. – Megan Kennelly

 

How much water is too much or too little for your greens?

 

By Dale Bremer

As Dr. Jack Fry discussed in his recent article on this blog Good Water Management Will Help Get Greens Through Summer Stress, too much or too little water can be detrimental. I thought I would follow up with a few practical tips on how to determine the correct amount of water for your green using a soil moisture sensor that determines volumetric water content. The most common soil moisture sensor is probably a TDR (time domain reflectometry) such as the one shown in Figure 1. Keep in mind that different depths (lengths) of probes are available, and that you are primarily interested in measuring soil moisture in the root zone; any moisture that is below the root zone is unavailable to the plant. The depth of the root zone may vary during the summer (shallower in midsummer, deeper in spring and fall), but a good compromise would probably be the 3 inch probes.

Fig1

(Figure 1)

 

Ideally we should avoid constantly saturating soils with water. Instead, allow them to dry down to a predetermined level of soil water content just before the onset of drought stress symptoms. By definition, soils are saturated when 100% of the soil pore volume is filled with water. After irrigation, the soil will eventually reach field capacity, which is the amount of water remaining in the soil after free drainage has ceased. At field capacity, soils have good aeration but also have sufficient water for plant use. General guidelines for the volumetric water content at field capacity are 15-20% in sandy soils, 35-45% for loam soils, and 45-55% for clay soils (1).

On the other end of the scale, permanent wilting point is the soil water content when plants wilt and don’t recover when the soil is rewetted. Obviously this should be avoided in a green! Textbook values for volumetric water content at the permanent wilting point are 5-10% for sandy soils, 10-15% for loam soils, and 15-20% in clay soils.

However, for a number of reasons soil water content values at field capacity and permanent wilting point may vary from the textbook values for your green. This could be caused by differences in sand particle size, organic matter content, age of a green, etc.

A simple way to determine how much water to apply to your greens is to calibrate your soil moisture probe to your soils with the following steps:

  1. Irrigate the turf thoroughly, then take readings with your soil moisture sensor one hour later. Measure in several spots around your green, perhaps even in a grid pattern as you see fit.
  2. Take readings twice daily and note visual stress symptoms of the turf.
  3. Continue taking readings until turf shows symptoms of drought stress.
  4. Once these levels have been determined, use them help guide future irrigation events.
  5. Calibrate for each soil type.

My colleagues at the University of Arkansas (Doug Karcher and Mike Richardson) (2) used this method on native soils and on 1-year old and 10-year old USGA greens and came up with the dry down curves in Figure 2.

graph

Although soil moisture levels in the 10-year old green are higher than the 1-year old green, it is likely that drought symptoms begin at a higher soil moisture content in the 10-year old green; the same is likely for the native soil. This illustrates why it is a good idea to calibrate separately for different soils and for greens that may differ in age. It’s also important to note that drought threshold levels may change through the year, as the root system changes (as alluded to above). For example, a shallow root system in midsummer may require that irrigation be applied at a higher soil water threshold because roots are not able to “mine” water deeper in the soil as they may have earlier in the growing season.

References:

  1. http://nrcca.cals.cornell.edu/soil/CA2/CA0212.1-3.php
  2. http://www.stma.org/sites/stma/files/Conference/2012_Conference/Karcher.pdf

Good Water Management will Help Get Greens Through Midsummer Stress

by Jack Fry, KSU

It’s like watching a toddler play with an open staircase nearby. You’re in a good mood and things are going reasonably well, but if you turn your head away for an instant, disaster could strike. It’s no different than managing water on putting greens during midsummer. Minor flaws in greens construction or management may go unnoticed until now; when temperatures are 85 degrees or lower, bentgrass is able to tolerate it. But, an extended stretch of 100+ degree days highs, along with high night temperatures, can bring out the weaknesses in construction or the superintendent’s management program. A rootzone that remains wetter longer can exacerbate problems. Maybe you’re dealing with push-up greens that have been topdressed for years with sand; better than nothing, but not as good as a well-constructed, well-drained profile. Maybe your greens were constructed to “almost” – USGA specifications. For example, maybe sand particle size wasn’t evaluated by a testing lab, pea gravel doesn’t meet specifications, or the rootzone is 8 inches deep on some parts of the green and 15 inches deep in others. Perhaps your topdressing sand particle size distribution, or frequency of application, are different from the previous superintendent. All of these factors can contribute to the rootzone holding too much water, or not enough.

A rootzone that stays wet too long will have limited oxygen, and also be hotter than one that drains well (water helps retain heat); neither situation bodes well for bentgrass roots. Optimum bentgrass root growth occurs at 50 to 65 F. The top two inches of most putting surfaces in full sun during mid-day during July in Kansas will be 90 degrees or higher. Up until now, we have had excessive rainfall, and roots of bentgrass and annual bluegrass on many greens are no deeper than a couple of inches, particularly if the rootzone retains more water than desired. Furthermore, the roots that are there may not be functioning at an optimum level with the ongoing heat. The plant’s water-absorbing capability has been severely limited (it’s like being really thirsty with a cold glass of ice water in front you, but you’re not able to swallow). The “deep and less frequent” strategy for irrigation is not going to be effective when roots are shallow or not effective at taking up water. Instead, match frequency of irrigation to rooting depth, which could mean irrigating at least once a day.

Frequent scouting of greens will help identify areas that are experiencing stress first – the purple/blue color is a good indicator. Lightly watering these areas by hand will help make up for deficiencies in water distribution by the irrigation system and differences in the rate the turf uses water across the surface of the green. Pay particular attention to sloped areas that dry out faster. Hydrophobic localized dry spots will continue to exhibit stress symptoms unless a wetting agent is used in combination with probing the areas to encourage water penetration. The ability to hand water correctly is not something we’re born with – train your best people how to effectively scout and to apply the right amount in the right places. Hand-held soil moisture meters are becoming commonplace for determining volumetric water content of greens (Fig. 1). If you don’t have one, put it on your wish list. By using the probe, you or your employees will be able to identify areas of the greens that are drying faster and need water, or those that are moist enough that watering should be avoided.

 

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Fig. 1. Soil moisture meter for measuring uniformity of rootzone water content across the putting green.

 

Syringing, supplying a light mist on the surface of the leaves, can be used to help cool the leaf’s surface. However, it’s most effective when the humidity is lower and/or if there is air movement to help the water evaporate. Thin out or remove trees, or install fans, before relying on syringing to get creeping bentgrass through periods of heat stress.

Managing water on greens in midsummer is tricky business. Pay attention, put up a gate, do whatever it takes to keep the toddler away from the staircase.