Tom Cook, Associate Professor
Department of Horticulture
Oregon State University
It seems like I spend a lot of my time doing autopsies on failed athletic fields. Many if not most of the sand base fields I look at are failures. Either they don’t drain or the grass won’t grow or the grass wears out faster than it should. I can honestly say I have never visited a field and had the caretaker say, “This is the best thing we ever did. The kids love it and it is so easy to take care of.” More often I hear, “This piece of s—t is the worse thing I have ever tried to maintain! I’d give anything to have our old soil field back.” So, what is going on here? Are sand fields a nightmare waiting to happen? Is it inevitable that all sand fields will fail? Are sand fields so difficult to work with that only the pros with unlimited budgets can possibly make them work?
I believe the real problems stem from a basic misunderstanding of why we build sand fields and a failure to see these fields as a special breed that requires a different style of management than the old soil fields. I guarantee that if you maintain your sand fields the same way you maintain your soil fields, they will fail every time. Further, if you use your sand fields the way you use your soil fields they will also fail every time.
What I want to do in this paper is discuss the ideas behind sand based fields, some of the critical factors in design and construction that make a difference, and the critical maintenance procedures that will maximize the chance that your field will drain and provide a playable surface for many years. I will also comment on the impact that use patterns have on short and long term field performance.
We build fields out of sand to take advantage of the natural drainage properties of sand. Sand fields are meant to perform when conditions are too wet for soil fields. We don’t use sand because it is easy to grow turf on (it’s not!). We don’t use sand because turf will hold up better under traffic (it doesn’t!). We do use sand because it can provide a firm surface even when the rain is pouring down.
During the dry times of the year soil fields are much easier to irrigate, they don’t require a lot of fertilizer, and they wear like iron. When the wet season arrives, soil fields fall apart real fast. The surface becomes mushy, water penetration drops almost to zero, and the result is a nearly impossible surface to play on. With sand based fields kids can play right through the wet season on firm tight surfaces (at least in theory).
What this all means is that in our quest for firm playable surfaces in the fall and winter we have to accept a new set of problems in the summer. Rule number one is: “Sand fields are built for drainage and require different maintenance strategies than soil fields.” Later in this paper we will look at just what these maintenance strategies are.
Avoiding Construction Failures
I honestly feel that relatively few field failures are due directly to construction screw ups. Don’t get me wrong, I do see my share of drainage failures due to poor construction. Poor construction is often the result of poor inspections by the company who designed the field. If we review the critical steps in building a field you will see the role inspection plays in the process.
I like to view a sand based field as a layer cake with three components. The first component is the drainage zone which consists of the sub-base, the drain trenches, and the drain pipes and gravel envelope used to surround the drain pipes. The second component is the rootzone mix (the sand base) for growing grass and managing water that has entered through the surface. At the top we have the third zone made up of the top 3 or 4″ of the rootzone mix and the actual turfgrass that covers the surface of the field. Each of these zones can cause problems so lets look at key mistakes to avoid for each one.
The drainage zone is intended to receive water that comes through the rootzone mixture. In other words it is the last zone to see applied water. The goal for the drainage zone is to move large quantities of water rapidly. In order to do this we install drain trenches with the base of the trench sloped downward towards an out fall. To insure unimpeded water movement we install drain pipes. To keep the drain pipes open we enclose them in a gravel envelope so that the pipe has gravel below it, on both sides of it, and on top of it all the way to the top of the drain trench. (Details for drain construction can be found in the extension publication PNW 0240. This publication can be obtained at county extension offices throughout Washington and Oregon.) To make sure drains have a chance to work consider the following list of Do’s and Don’ts.
Do these things and the drains will work:
- Dig trenches parallel to each other so they are 15-20 ft. apart.
- Clean the trenches of loose soil and construction debris before placing gravel in the trench.
- Unless you have used a laser guided trencher, put a bed of gravel in the bottom of the trench 2+ inches deep to enable you to adjust the grade of the trench.
- Lay the drain tubing on the gravel bed so that it is centered and has a constant grade of 1/2 to 1%.
- Place gravel around and over the drain pipe until it is covered up to the subgrade. Be extremely careful to avoid contaminating the gravel with soil as it is being placed in the trenches.
- Use uniform gravel that is free of fines (fine sand, silt, and clay). Small gravel in the range of 1/8 to 1/4 inch, 1/4 to 3/8 inch, or 1/4 to 1/2 inch will all work well if they are free of fines. Larger gravel will allow too much migration of the topmix down to the drain lines.
- Use either slotted rolled drain tubing 4″ in diameter or rigid drain pipe of the same diameter. If you use rigid pipe place it in the trench with the holes facing down.
- When placing the gravel over the drain pipe constantly check the grade to make sure the gravel does not flow under the pipe and raise it up.
- Have someone from the design office inspect the entire procedure from start to finish to make sure installation is done correctly.
Do not do the following:
- In the design phase do not stretch spacing of drain lines, specify drains smaller than 4″, or eliminate drain pipes completely.
- Do not lay drain pipes on the surface of the subgrade instead of in trenches. While this may be appealing because of reduced costs and fewer steps, it is a sure fire way to end up with crushed pipes that won’t drain.
- Do not use pipe covered with fabrics, do not cover pipes and gravel with filter fabrics, and do not encase the pipe and gravel in filter fabrics. The fear of fine particles migrating has caused many to cover pipes which may result in plugging of the filter fabric and consequently reduce flow of water through the pipes. Experience has shown that if drains will work with fabrics, they will work without them. Conversely, if drains won’t work without fabrics, they won’t work with them. Fabrics are an unnecessary expense in properly constructed drains.
- Do not allow trucks and other heavy equipment to drive across drain lines before, during, or after drain line and gravel installation. Once trenches are dug, vehicles should drive between the lines or straddle the lines.
- Do not assume that contractors who have been building roads all their lives understand the importance of building a sports field properly. It is the designers responsibility to make sure the contractor is aware of proper construction procedures and in fact builds the field in a manner that will allow the field to work. The designer or a person knowledgeable about the design should be onsite during the entire installation period and should make sure the work is done properly.
The top mix zone goes over the drainage zone. You have several things that are important to remember when working with top mix. The quality of sand is critical and the depth of the sand is very important. Details on rootzone mixes are presented in the extension publication PNW 0240 mentioned earlier in this paper. Here I want to focus again on what you should do and what you should not do with this zone.
Things to do to insure a functioning rootzone layer:
- Use the best sand you have available that meets the criteria in PNW 0240. Experience has taught me that rarely do we get to use sand that actually meets the specifications outlined in the bulletin. The key when using substandard sands is to err on the side of drainage. For example, reject concrete sand may be acceptable if it is free of fines (silt & clay). Remember, our goal is to move water through the profile and we can do that with sand that is coarser than desirable but not with sand that is finer than desirable. The trade off with coarser than desirable sands is they require more irrigation and dry out faster in hot weather. The trade off with finer sands is that they seal up and the surfaces become wet and mushy so we spend our time trying to keep them open and draining.
- Make the profile depth as deep as possible up to about 16 inches max. We always shoot for 12 to 16 inches of rootzone mix but since sand can be expensive there is a tendency to try to get by with less. When you go below 12 inches there may not be enough depth to insure that the field will drain freely in the winter. There is a tendency for shallow fields to remain too wet during winter and we are right back where we started with a mudhole.
- Check sand regularly as it arrives to make sure the sand quality stays constant. This is particularly important if the sand is coming from a pit rather than a screening plant. The only way to check the sand is to run it through a set of screens upon arrival at the site. A set of small hand held screens is a good investment.
- Rootzone mixes composed of sand and organic matter should be premixed away from the construction site. Materials should only be placed on site once uniform mixing has occurred.
- Trucks should straddle drain lines and dump their loads directly over the gravel covered drains. When all the rootzone mix is in place the site should look like a series of windrows of sand.
- Sand should be spread initially using a track vehicle rather than a wheeled vehicle to avoid crushing drain lines by driving over them during the spreading process.
- Final grading can produce a slight crown in the center of the field of no more than 6 to 12 inches or a flat field. Flat fields are utilitarian because you can situate fields anyway you like. Unfortunately it seems to be difficult for contractors to grade a level surface so we often end up with low areas scattered throughout the field. A slight crown is appealing because it makes the field look flat rather than concave. It also seems to be easier to grade for contractors. Beyond that, crowns serve no functional purpose on a sand field.
Do not do the following:
- Do not assume the contractor understands what you want. His job is to get in and out as fast as possible. As the saying goes, “Once its buried, everything looks the same.” Realistically you only get one chance to have your field built properly.
- Do not allow the contractor to cheat on the depth of rootzone mix. Specifications list a number and that is the depth the field should be when its finished.
- Do not make casual inspections. Nobody I know can tell depths or grades just by looking.
- Do not budge on sand quality. Being a nice person will leave you with a poorly drained field unless you are very lucky. Remember that to most people sand is just sand.
Maintaining a Free Draining Field
Now we come to the final layer of a sand field. The 3 to 4 inches where the grass is growing is the zone the turf manager is responsible for. Perfectly built fields will fail eventually unless this zone is properly managed. Poorly built fields will fail faster if this zone is not well managed. Once the field is constructed what you do to this surface zone will determine how long and how well the field will perform.
The drainage theory on sand based fields is simple. What we want to do is move water vertically into the surface, down through the rootzone mix, and out through the drains to an exit point. I think of it as three words, “INTO, THROUGH, & OUT.” Once the field is built we can really influence only the first step. Maintenance strategy number one is: Work the surface constantly to keep it open. Working the surface involves hollow tine coring 3 to 4 times per year, deep solid tine aerating (vertidraining) one to two times per year, and surface slicing weekly during each major use period. Allied practices such as slice seeding and surface scarification via a flail or similar machine help to insure a porous surface that will take water rapidly.
Lets look at a hypothetical soccer field and see how a maintenance schedule might work out to insure a healthy turf and good infiltration. Assume for this example that the field is located in a metropolitan area west of the Cascade mountains.
This field should be mowed at least once per week and preferably twice per week for most of the year. Peak mowing periods will run from March through November. December through February plan on mowing about every other week to every third week.
Total number of mowings will range between 45 and 75 depending on the year and labor resources. Clippings should be removed during the period between March and November to avoid accumulation of fresh organic debris that can be ground in by players cleats.
Sand base fields need regular fertilization with products containing both soluble and controlled release nitrogen. In an ongoing fertilization program plan on one pound of total N per 1000 sq ft of turf each month during March through October. A mid winter application of N from a controlled release source such as IBDU, Polyon, ESN, Poly-S, Tricote, or other polymer coated products at 2 lbs N/1000 sq ft should improve winter turf color and improve early spring growth. Total N rates will be as much as 1/3 to 1/2 less on fields where clippings are left after mowing.
Actual nitrogen rates can only be determined by observing turf performance. Consider the above numbers as a starting point but trust your own judgment and keep accurate records. Also keep in mind that sand is nutrient deficient and will require complete N-P-K fertilizers at each application. N-P-K ratios in the range of 5-1-2 to 5-1-4 will work fine. Apply N-P-K fertilizers plus complete micronutrients at least twice per year ( spring and fall), or as dictated by tissue analysis. Remember also that sand fields may need periodic applications of calcitic or dolomitic lime to provide adequate Ca and Mg or to raise pH to the 6.0+ level. Need for these elements are best determined by tissue analysis or soil testing.
Sand fields need to be irrigated frequently and kept slightly moist to prevent development of localized dry spots. The best advice is to apply water daily or every other day to match water use rates or evapotranspiration (ET). This is best accomplished by utilizing an onsite weather station that can communicate directly with the irrigation controller to determine daily run times. If that isn’t possible, use a soil probe to determine field water content and irrigate by your best guess. Plan on irrigating 3 to 4 times per week during the season and try not to apply more than 1/2 inch of water per application to avoid wasting applied water due to excess leaching. In fall, run the field as dry as possible prior to the onset of fall rains.
Core at the end of the fall season with either a conventional hollow tine machine with 3-4 inch tines or a vertidrain with solid shanks set at least 8 inches deep. Core again in March with either the standard hollow tine machine or the vertidrain. Around late May to early June Core with hollow tines one more time. Depending on the field conditions, core again in late August. Coring should be regular and consistent on sand fields. The vertidrain should be used at least once per year as noted above since it can penetrate deeper and break up the subsurface which will enhance water movement through the rootzone mix.
Slicing is a valuable tool for maintaining surface infiltration during the season when practices such as coring or vertidraining might be too disruptive. Slicing works well on sand fields and should be done at least weekly during the fall winter and spring use periods. Slicing effects are very short lived which is why this practice needs to be done consistently through the use season. I can’t emphasize how valuable this procedure is.
Thatch rarely forms on sand fields as we think of it on lawns. What we most commonly see is organic debris that gets ground in to the surface. If this material is not removed periodically, it will eventually plug the surface and impede water movement into the surface. Plan on dethatching at least every spring in association with coring or vertidraining. On heavily worn fields dethatching with a tractor mounted flail device may be needed to prepare the surface for reseeding.
Topdressing with sand identical to that used in construction will help maintain a uniform surface profile and keep a smooth grade for maintaining a safe surface for players. Topdressing should be done spring through summer to prepare the surface for fall and winter play. Late fall topdressing after the season is over is appropriate whenever reseeding is done. Plan on 4 to 5 topdressings per year at 4-5 week intervals during periods of active turf growth. Individual topdressings should never exceed 1/4 inch to avoid smothering of turf and turf thinning.
Sports fields are constantly worn and abused by players. Turf on these fields is always temporary so you need to replant regularly to keep the surface covered with desirable grasses. The slice seeder is an effective tool for overseeding because it is relatively none disruptive and plants the grass in grooves where they have a chance to germinate and establish. Overseeding is useful for thickening weak stands or filling in areas where turf has been destroyed completely. Target the late fall period for overseeding worn areas. Overseed again in mid spring and if needed again in early June. Don’t ever plan on being finished. Just plan on overseeding regularly for the rest of your working life.
Adjust these practices as needed to fit your budget and climate. Always remember that we build sand fields so they will drain. Your number one job is to do whatever you can to keep the surface open and free draining. The maintenance practices listed above will accomplish this goal and give you excellent turf as well. Its a winning combination that has been proven in the field and really does work. Plan for it and then carry out the plan. The result will be the best field you can achieve at your site.
What about use of the field?
The hardest part of maintaining fields is getting users to schedule the fields intelligently so there is a chance that you can keep grass on all of the field most of the time. I’ve proposed lots of different solutions to the perennial problem of overuse and abuse. Designing fields square to allow for field rotation, moving goals regularly to avoid excessive wear in goals, practicing in a manner to avoid wearing out field centers or goal areas etc.,etc.,etc. never seem to happen. What is the solution? There may not be one but if it exists it will end up being your responsibility. I have all but given up trying to enlist the aid of user groups. They just don’t see the connection with how they use the fields and how the fields look and hold up over time.
When it comes to sand fields I try to get clients to build a limited number of sand fields and as many soil fields as they can. My advice is always to use the soil fields in summer and as late into fall as possible before the fall rains turn them to mush. At that time switch to the sand fields and get the most mileage from them as possible during the time when soil fields are useless. A few people listen but most destroy the surface of the sand fields early on in the fall and end up playing out the fall or winter on fields void of turf.
Whatever approach you take on your sand fields remember that it will happen because you want it to. Don’t expect cooperation from users and don’t be disappointed if they refuse to help you out in your quest to give them better surfaces to play on. Just be persistent and don’t give up!