Topdressing is the luxury car of sports field cultural practices. It’s nice to have, but it may be the first thing to go when the budget gets tightened. Topdressing is when one material is applied to the surface of another. Sometimes you will hear it referred to in baseball field management as you are “topdressing” the infield conditioner on the surface of the infield skin. Here we will be referring to topdressing on grass sports fields. This mainly consists of spreading a type of soil (i.e. sand, topsoil, organic material, etc.) onto your grass.
To understand topdressing, you must understand organic matter (OM). Organic matter for sports fields is usually thought of as peat. Although rice hulls, sawdust, compost, and yard trimmings are also significant sources of OM. Many newly constructed fields have a soil profile that is a specific ratio of sand to OM. Professional-level fields frequently use a 90% sand / 10% peat. With such little OM, the soil profile will drain and stabilize great but will have a harder time growing grass. OM is what holds on to water, nutrients, and oxygen down in the soil, allowing the grass roots to consume it. When you don’t have enough OM, the soil will quickly leach those water, nutrients, and oxygen before the roots can get them. Too much OM results in, you guessed it, too much water, nutrients, and oxygen in the soil.
With this theme in mind, the main benefit, or goal, you are trying to accomplish with topdressing sports fields is to elevate or dilute the OM in your soil profile. You trigger the rise or fall of OM % by the soil media you choose. If you are trying to raise OM then you would need to make sure the soil you are choosing to topdress with has a significant percentage of OM, like peat or compost. If you are trying to lower the OM of your soil, then you would use a very low OM percentage material, like sand. Being able to control the OM % in your soil is the biggest benefit to topdressing.
Increasing the percolation rate of your field’s soil would be a close second. It piggybacks off the previous benefit because as you reduce OM by raising sand content, you open up more pore space and allow for the water to move freely within the soil. This is very important for irrigation purposes, but this primarily addresses rainout concerns. When a field gets saturated, or very wet, it begins a cascade effect of potential issues. Wet fields get over-compacted very quickly when they eventually dry out and muddy fields can increase injuries substantially among players.
The last main benefit of topdressing is to control the thatch in your field. Through natural processes, OM is made in the soil by cutting grass decaying, microbial activity in the soil, and other factors. This creates a layer at the surface where the soil and grass plant meet. This layer is called “thatch” and will eventually (in several hundred/thousand years) will turn into soil, but right now it’s acting like a sponge keeping the surface wet, soft, and susceptible. As you topdress sand over your field, it interacts with the thatch and allows the OM to break down quickly and naturally and eventually, turn into soil. By topdressing frequently, it keeps that from becoming a detrimental factor on your field.
What material is being used for topdressing is the primary concern. The immediate secondary concern is how often you plan to topdress. This will influence the depth at which you spread your sand or other topdressing material. If your goal is to topdress .5” annually, but you can only topdress one time, that now becomes a pretty significant project to topdress and drag in that much sand (~160 tons over one 80k ft2 football field).
This also creates an issue with the plant’s ability to “grow through” the material you topdress. If you mow your field at 1.5” and you topdress .5”/year one time, then you cover 33% of the grass plant when you topdress. This can influence the softness and stability of the field, as well as the health of the plant for an extended period. If you have the capability to close a field for 2-3 weeks when you topdress this heavy, the depth of topdressing is not as detrimental.
Now, if you were to take that same .5” of sand we plan to put out this year and split it into 2 applications of topdressing per year, then you spread .25” of sand (~80 tons) twice a year. This is much more manageable regarding labor hours, budget, grass health, and field use. Using the example above, topdressing .25” of sand would only cover 17% of the exposed leaf. This allows the grass plant to stay rigid and not sacrifice the playability of the field. This also prevents the field manager from closing the field for 2-3 weeks. After .25”, you could be back on the field 2-3 days later.
I hope this illustrates how important the frequency at which you topdress is, but also shows why topdressing can sometimes be perceived as a less-important cultural practice because it is time-consuming, has a very delayed gratification, and we didn’t even get into the cost or logistical challenges that come with topdressing.
In most of these discussions, I usually mention “there’s no wrong answer, as long as you do it”, but I don’t know if that’s the case with topdressing. If you can topdress on a consistent and frequent basis, then it’s an extremely important cultural practice that will keep your sports field draining well and playing safe for years. If you can only topdress once every 5 years and it takes 30% of your budget, then that money may be better spent on aeration, fertility improvement, staff, etc. It still doesn’t mean once every 5 years is “wrong” but the benefit is reduced to negligible, and your ROI can be much higher elsewhere.