Using Fertilizer on Sandy Loam Soil

Some nurseries sell triple phosphate (just P). If you are not 100% organic gardener, any balanced all purpose fertilizer (10-10-10 or 13-13-13) should be fine.
As far as the plants are concerned, fertilizer is fertilizer. Organic in this case is just an environmental issue.
Wood ash is also a good source of Potash(K). K is vey stable and sticks to soil. N is water soluble and will leech out quickly unless the soil is rich in organic matter. P is semi stable.
Good luck.