Fertilizer is advanced bone-meal created by crafting bone-meal with scrap in a shapeless recipe. It can be used with Crop Blocks to speed up the growth process.
Every Crop keeps track its own nutrients. Fertilizer adds 100 to this value, if it is currently <= 100. Maintaining a higher nutrient value may be required for some more advanced plants to grow and it also likely affects growth of plants. The nutrient value decreases over time if there is a crop planted in the crop block. Some biomes may affect nutrition.
Bonemeal can also be used to fertilize plants, however is unknown how much it increases the nutrient value.
There is currently no method (as of 1.71) to determine the nutrient value a crop has at any given moment.
Fertilizer can either be produced using these shapeless recipes or as a byproduct of running a fermenter.
See: Crafting Guide
However, we strongly advertise our new product, the "FAP - Fermented Agricultures Phosphor" fertilizer. It's creation recipe is considered secret and opposed to many rumors, it does NOT, really not, totally not, persist of macerated bones and recycled scrap. Any rumors claiming FAP is created from human corpses are entirely incorrect. As well, we would like to inform you about Mr Kennys death being an tragic, but unavoidable and for sure unintended accident. Application of FAP to crops appears to have an currently semi-unknown effect on crop growth.