The importance of drainage for salt reduction

By Ron Hostick CSFM, CGM

In 2011 we were drying our bermudagrass outfield down to eliminate overseeded ryegrass from the winter season and noticed large areas of turf had declined. Many of the areas were virtually void of turf showing a whitish cast to the soil. Not surprisingly when tested the soil our salinity was extremely high. The field built, in 1999, is 12 to 18 inches of medium fine sand over clay native soil with fabric covered perforated plastic drains on 40-foot centers. We had been lobbying for an improved drainage system for several years, discussing and tracking the increasing salinity with administration. This year showed we couldn’t wait any longer. It took 2 years to get everything in place but in the summer of 2013 we installed a complete drainage system. It consisted of a 2-inch drain line every 10 feet oriented parallel to the slope and 2-inch slits perpendicular to the slope every 20 inches both filled with a slightly coarser sand than the existing field’s soil.

The process started with a request for proposal (RFP) where we stated what we were looking to install, the qualifications any installer would be expected to meet, and the University’s requirements a company must meet to perform work on State property. As is usual, we were not given the go-ahead soon enough to get all the bidding completed in time to meet our schedule and the contract was performed as a job order contract (JOC). As a change order to the original contract we decided to include drainage for the infield as well.

Once the contract was awarded delivery of sand and pea gravel began, and our contractor used a local company to reduce transportation costs. Then identification of elevation and location at the point of connection to our existing storm drain, behind first and third, which dictated the depth of the entire system. Digging of the 6-inch ABS perforated main collection drain was completed around the arc of the skin and down both foul lines; our outfield drains toward the infield, so this was the most disruptive portion of the process.

A 24-inch wide, 36-inch deep trench was completed with backhoes while working around existing irrigation and other utilities. The contractor then started the trenching process for the 2-inch perforated drain pipe covered by pea gravel then about 1 foot of coarse sand to the existing grade of the field, once compacted. All of the existing soil was removed to an offsite location and replaced by the new pea gravel and sand.

Once the 2-inch drains were connected to the main trunk line, the process began of installing the 2-inch slit drains perpendicular to the newly installed perforated lines and the field’s natural slope, consisting of 2-inch slits, 8 inches deep on 20-inch centers, filled with the same coarse sand as covering the pipe. All back filling of sand was completed in lifts and the addition of water while compacting was a key element in reducing settling after construction.

Once installation was completed we ran irrigation for several days to flush the system, then soil tests from the outfield were compared to tests from a little more than 2 years earlier. Key factors of soil chemistry and nutrient availability are ECe (a direct measure of salinity through electrical conductivity), levels of sodium in parts per million and Cat ion percent saturation.

As a result of being able to flush the soil water system into drainage, even with what by potable standards is high sodium irrigation water:

  • The relative salinity was reduced by 88%.
  • Sodium is high in our water and has three detrimental effects. One being it dominates the Cat ion sites so aggressively other nutrients aren’t available to the plant; second it can cause the soil to lose a desirable structure effecting water movement resulting in poor root structure; and third its dissolution into the water reduces root uptake of moisture. Sodium concentrations after one summer of flushing were reduced by 78%.
  • Finally, Cat ion saturation percentage measures plant nutrients with similar charges relative to one another and it requires a healthy balance of nutrients. Cations such as potassium, magnesium and calcium are necessary for healthy plant growth and sodium competes for the same soil nutrient holding sites. The sodium percentage relative to 2 years ago was reduced by 50%.

In aggregate, very positive signs the soil chemistry system is moving in a beneficial direction as a result of leaching the undesirable elements from the soil into the new drainage system.

It would preferable to have good drainage installed at installation and here are key thoughts on drainage construction: Have all soil partials reviewed to make sure they work well together so the sand doesn’t infiltrate into the gravel or the gravel doesn’t clog the perforated pipe. Sock, a fabric covering for perforated pipe, is not recommended. The distance between pipe and sizing of main drain pipe should be well thought out; if you are going to go to this expense and field down time don’t try to save a few dollars by stretching the distances between lateral drains or under sizing pipe.

If you must wait for drainage or are thinking of installing into an existing field, plan on 3 months of having the field out of commission and know that the field likely will need seed or sod, so take that into consideration during the budgeting process. This was very disruptive and the loss of field use was painful for our program but due to years of intensive fertilizer use, poor quality irrigation water due to evapotranspiration left behind increasing levels of sodium, a poorly functioning drainage system, and virtually no percolation into the underlying native soil our challenges in poor physical and chemical properties of the soil prevented us from growing a healthy turf.

Ron Hostick CSFM, CGM is the Manager of Landscape Services at California Polytechnic State University San Luis Obispo. The project described in this article was completed while he was the lead groundsworker for athletic areas at San Diego State University, on Smith Field at Tony Gwynn Stadium. The installation was completed during June 2013; the contractor was Green One Industries.