The surface of "slow release fertilizer" beads gradually erode, say at 1 centimeter per year. With a spherical bead, the resulting curve of "fertilization rate" would decline with time e.g. it would be proportional to (1-t)^2 for 0<t<1 for a 1cm-radius bead. It might be better to have beads with constant fertilization rate 1. QUESTION: is there a shape (or set of shapes) whose surface area stays constant as they erode? The answer in 1 and in 2 dimensions is "yes." In 3D the answer is less obvious, but I have a (presently rather messy) argument the answer also is yes. More advanced question: you might want the rate to increase with time (as your plants grow bigger they need more)... or be some other curve... WHICH curves F(t) are achievable fertilization-rate curves? -- Warren D. Smith