A superphosphate is a fertilizer that contains a blend of ground phosphate rock and sulfuric acid. When sulfuric acid is added to the ground phosphate, it makes the normally water-insolulable rock dissolve in water so that plants can absorb it. When amended into soil, superphosphates help promote root growth in pasture plants such as clover. Plants grown in a phosphate-poor soil such as clay soil benefit from amendments with superphosphates.
Dig a soil sample from several locations in your garden before amending to determine the needs of your soil. To do this, scoop about a teaspoonful of soil from the surface of your garden and place it in a plastic bag. The dig down three inches and scoop up another teaspoonful. Add this to the same plastic bag and mix. Do this in several locations, placing soil in different plastic bags.
Create a label with a magic marker and a roll of masking tape. Write on each label the location on your property where you dug your soil sample. Stick this label onto each plastic bag.
Take your soil to your local college-run extension service to have the soil analyzed. This will tell you the nutrient make-up of the soil as well as the pH. The analysis will also suggest what amendments you should add to the soil, and how much of each amendment you should add per 100 square feet.
Select a superphosphate from your local garden center.
Till the soil to break it up to a depth of 12 inches.
Spread superphosphates over the topsoil in the amount recommended by your soil analysis.
Turn the superphosphates into the soil to mix it into the soil.