If you are struggling with a very sandy soil in your flower beds, you are having trouble keeping your plants watered since the sandy soil does not retain much water.
In the long run, as you add organic material to your sandy soil each year, the soil will be able to retain much much more water. In the short term however you may want to consider using some water absorbing polymers to help retain water for your plants.
Polymer Gels are synthetic polymers in the form of tiny beads or crystals, again looking like sugar. They have an enormous capacity to absorb water, expanding up to 40 or 50 percent of their size when mixed with water. The polymer material cannot be absorbed by the plant. However, plant roots are attracted to it, gradually migrating toward it and eventually growing right through individual water-filled crystals. These crystals will help your sandy soil retain water for about 5 years when they decompose. At that time, your organic amendments should be sufficient to retain all the water your plants need.
You do not want to put too much of the polymer crystals into your soil; it is best to be conservative. Figure about one teaspoon of dry crystals per square foot of garden. Dig the crystals down into the soil to at least a 3 to 4 inch depth. On approach is to use the crystals every time you place a plant into the garden. Over time, you will cover most of the bed.
For information about these products go to the Tool Shed file about Water Absorbing Polymers