Using saltpeter, and the GregTech recipe you can create 20 pieces of fertilizer per craft. Now you might remember that crafting saltpeter with the compressed air cells, nitrogen cells and potassium cells actually consumes the cell. But if you use the GregTech electric crafting table, you can craft the 10 saltpeter, and it will give you your cells back, then you could refill them and repeat the process All you will need is enderpearls for the nitrogen (shouldn't be too hard with an enderman farm in the end or a soulshard/autospawner) and of course some power for the electric crafting table and some sand/dirt for actually making the fertilizer