Grass seed is dependent upon a good, nutritious soil to grow most effectively and fully. The challenge is choosing the right fertilizer and knowing when it should be switched to a different fertilizer, if at all. In some cases, choosing the wrong fertilizer could cause the entire yard to suffer and possibly prevent the grass seed from germinating at all. Therefore, understanding fertilizers is very important for grass seed growth.
Not all fertilizers are meant to promote unfettered growth of all sorts of plant material. Generally, those manufacturing fertilizers assume that the vegetation you want to grow is already started. Some may put pre-emergent herbicides in fertilizer to stunt seed germination, thinking that anything that could be growing up after application would be undesirable. This could be fatal to a newly-seeded lawn.
Starter fertilizers do not have pre-emergent herbicides as they are formulated specifically for seeds and new plant growth. Most packages will clearly mark if a fertilizer can be used as a starter fertilizer. In most cases, this fertilizer will be high in phosphorous, an element critical for good seed growth. A fertilizer mixture of 1-2-1 is often recommended for grass seed.
Choosing when or if to use fertilizer with grass seed is something that often depends on the circumstances. For example, if the pH balance of your lawn is out of sync, then you may need to add lime or other materials to get the acidity correct. The time of year also makes a difference, with fertilizers playing a greater role during cooler times of the year. A soil test may help best determine what your specific needs are.
Remember that even after the grass seed emerges, it may not be time to switch away from a starter fertilizer. Grass varieties are often labeled as cool season or warm season grasses. If you live in an area where your lawn is a mixture of both cool season and warm season grasses, then it may be that one type has emerged but the other one has not yet.
Even a starter fertilizer is capable of damaging a new plant, including grass. Therefore, if you are interested in fertilizing around grass seed, keep in mind that fertilizers should contain very little or no urea or diammonium phosphate. These two elements may have some benefit for more established plants, but can be detrimental to new plants.