Most people go to the Agrilife extension service in their county and they give them a sample bag and instructions on how to take the sample and where to send it (Texas A&M). However, all it will really tell you is the pH and how much N,P,K to apply. It would probably be better to tell the group what type of trees you are planting and the soil that you have(clay, sand, caliche, gumbo). Some trees sold at big box stores are not adapted to the areas they sell them in. Soil in the middle of yard could be too shallow or too wet. A soils test will not tell you this.

If they happened to be oaks, should also rule out wilt since it is unfortunately present in several parts of the state. Now is also the peak season for spread via beetles.