I have sent the makers of Roundup (Scotts) an email concerning the use of Roundup in my sandy soil, this is their reply.....Hope is helps someone, it cleared up some things for me.

You can use Roundup safely around the base of mature trees. The brown bark of the tree will prevent the product from being absorbed into the trunk and killing the tree. If your tree has exposed, damaged roots or wounds in the bark, avoid getting Roundup on those areas. Do not allow product to drift to the leaves of the trees as that can damage or destroy them. 

Sandy soil is porous and does not hold moisture. Water applied to sandy soils disappears rapidly. It is necessary to water more often (every other day during summer drought). When watering, it is best to water heavily, to a depth of 6 - 8 inches by applying 1/2" of water. Since most of the grass roots are located within this area, applications in excess of 1/2 inch are wasteful. The excess water simply leaches away through the soil.

The best way to improve a light sandy soil is to add organic matter - not just a little, but a lot. The addition of organic matter, such as compost, peat moss, composted manure, redwood sawdust, finely ground pine or fir bark, and composted rice hulls will help the soil hold moisture and nutrients in the root zone. The more organic matter you add to a sandy soil, the more you increase its moisture-holding capacity. Spread a layer of Scotts Turf Builder Seeding Soil or Miracle-Gro Enriched Sphagnum Peat Moss evenly over the sand. It is important to work any amendments into the soil as thoroughly and evenly as possible, so that grass will benefit from it consistently. If you are in the process of establishing a new lawn, this is the ideal time to amend the soil. Apply 4 inches of topsoil and work into the top 4-6 inches of soil. Mix in the amendments with a shovel or preferably a roto tiller. You can begin planting as soon as you have mixed the amendments into the soil.

Again, thank you for taking the time to contact us and for your interest in Scotts. Please feel free to contact our company anytime we may be of assistance.

Kathleen Shreve
Consumer Response Representative
The Scotts Company and Subsidiaries
14111 Scottslawn Road
Marysville, OH 43041
888-768-6387

Something else about Roundup, when the air is 90 degrees or above, Roundup and other herbicides can "drift" and cause damage to plants not even being sprayed. A neighbor taught me this after spraying his own yard. I called my extension agent to try and figure out what had happened to my plants then checked with the neighbor and he had indeed sprayed his yard on a hot day.

Cecil, what exactly did it clear up for you? 

Hi ejp3