The soil that makes a good track
The material you want under a motocross track is a sandy-clay loam. In practical terms that means roughly 30 to 40 percent clay, with the rest sand and silt. The clay is what binds a jump face and holds the shape of a berm. The sand and silt are what let water drain through and give the surface its traction, staying tacky rather than turning to slop. Enough clay to hold a face, enough sand to drain and stay tacky: that balance is the whole game.
If you want the engineering version, the target is a slightly plastic sand-clay-gravel with a plasticity index (PI) of about 5 to 15. The US Army earthwork manual, FM 5-410, calls a soil in that band the most satisfactory unsurfaced wearing surface there is. Its classification guidance is a useful yardstick for any fill you are considering: well-graded gravels and sands (GW, SW, GP) make the best base materials, fine silt and lean clay (ML, CL) are only fair to poor, fat clay (CH) is poor, and peat has to be removed entirely. You can read the source in US Army FM 5-410, Chapter 5.
How soil fails
Both extremes fail, and they fail in opposite ways. Pure clay bakes to a hard, slick hardpack when it dries out and turns greasy the moment it gets wet, so you are either riding concrete or riding grease. Too much sand stays loose and simply will not hold a jump face, no matter how well you shape it. The soil you want lives in between those two failures.
Build it in two layers
A durable track is not one uniform pile of dirt. It is two layers doing two different jobs.
- A structural base: a compacted, clay-rich layer that carries the load and holds the shape of your features. A roller seals this base so it sheds water instead of soaking it up.
- A wearing course: 6 to 12 inches of tilled loam on top. This is the surface that gives traction and gets groomed every day you ride.
Getting this two-layer structure right matters more than chasing a perfect number on a soil report. A good base with the wrong topsoil can be fixed with a few inches of the right material. A weak base cannot be groomed into strength.
Should you amend your dirt, or import better dirt?
This is where most landowners underestimate the job. Changing the texture of soil takes real volume, not a sprinkle. Adding clay to a sandy soil takes on the order of 20 percent by weight. Going the other way, adding sand to a clay soil, takes on the order of 50 percent by volume, and below that threshold you can actually make the clay worse, closer to concrete than to loam. (Those ratios come from agronomy and are directional for load-bearing fill, but the scale of them is the point.)
Because the numbers are that large, importing the right blend is often cheaper and more reliable than correcting a bad soil in place. It is worth pricing both before you commit a machine to the ground.
Moisture and compaction
Good dirt placed badly still fails, and compaction is where it usually goes wrong. Soil only compacts well inside a narrow moisture window, roughly within 2 to 3 percent of its optimum. Too dry and it will not knit together, too wet and it pumps.
The field test comes from the FIM course-preparation standard, and you do not need equipment for it: grab a handful and squeeze. It should ball up like bread dough. If it crumbles apart, it is too dry. If it smears, it is too wet. When the moisture is right, place your fill in thin lifts of 6 to 12 inches, and expect most of the density to arrive in about five passes of the roller. Thicker lifts simply will not compact all the way through.
Read your own dirt before you dig
You do not have to guess at any of this. Public USDA soil survey data already names the soil under any given parcel of land, whether that is a specific loam, a silty clay, or something else, and that name tells you most of what you need to know about how it will behave.
MotoTerra pulls the USDA soil data for your exact parcel and tells you whether to use it as-is or amend it, so you can plan the track around the dirt you actually have. It is a free way to answer the soil question before you spend anything on machinery or fill. If the survey says your ground is close to a sandy-clay loam already, you may need very little. If it says heavy clay or loose sand, you will know to budget for imported material. When you have your soil sorted, the full build guide takes it from there.