Nitrogen, along with phosphorus, potassium, calcium, magnesium, and sulfer, is essential to plant survival. Nitrogen is not only a major component of plant cells, but it is also an intregal part of the plant's ability to perform photosyntehis (the process by which plants produce the energy they need to live.) In short, no nitrogen = no farming.
One of the major components of fertilizer is nitrogen. The theory is that if you increase the amount of nitrogen that's in dirt the plant will be better able to produce strong roots and fuel its growth. If there isn't enough nitrogen, crops tend to grow smaller and have weaker root systems. Of course, there are diminishing returns. Too much nitrogen in the soil is just as bad (and in some ways worse) than having too little. It's harder to get rid of extra nitrogen than to put more in.
There really isn't a way to grow thing without fertilization. Even the woods, where people don't spray chemicals, fertilization still happens through decay and animal droppings. Otherwise you couldn't get much to grow. It isn't what we think of as fertilizing in a farming sense, but it's nature's version so it still counts.
Farmers need to put nitrogen into the ground to replace what they are taking out, otherwise they'd need to constantly move to new places where they havne't yet worn out the soil. Normally this is done via chemicals, which are potent and require less planning. There are alternatives to using created chemicals, and here are a few in no particular order:
- Animals provide a good deal of fertilizer through their waste products,
- Crop rotation makes sure that crops don't exhaust the soil,
- Leaving a portion of a field "fallow" to allow nature to replenish nutrients,
- Growing a non-commercial crop in a portion of field, such as clover, that is then plowed under to replace nitrogen,
- Keeping appropriate crop density,
- Genetically modifying crops to require less nitrogen.
In short, if you don't have nitrogen you don't have a crop. Nitrogen is in fertilizer, and so that's one of the reasons farmers use it. Whether natural or unnatural, fertilization has to occur in order to grow things, but farmers have alternatives to spraying chemicals if they don't want to.