Fertilizing the field gives you double the yield you would experience otherwise. Fertilizing can be done at multiple stages; as long as it is done before the plant is matured. You have four types of fertilizer that you may use. They all have the same bonus to yield.
Fertilizer may be in either liquid form or solid for the chemical fertilizers and liquid or solid form from your cattle when you start to raise them. Spreaders will distribute solid chemical fertilizer. Sprayers will distribute liquid chemical fertilizer. Slurry Tankers utilize the liquid manure from your cattle or from the biogas plant. Manure Spreaders will use the solid manure you get once you are feeding your cattle a total mixed ration.
Starting out, a good choice for fertilizer application is the Amazone ZA-M 1501 since it has a substantial spreading width and is relatively cheap. Likewise, many farmers will employ a fast, but lower hp tractor with a 3-point hitch on both the front and rear to pull the sowing machine on the back and the fertilizer spreader/sprayer on the front.
As fertilizer can be expensive for the farmer at the beginning of the game, do NOT operate both machines at the same time. The sowing machines require a much lower speed than the fertilizer. While a sowing machine does not consume seed unless it is over seedable farm land (and then only at a rate to actually seed the farm plot), the fertilizer spreader/sprayer will keep consuming your supply as long as it is on; regardless of the speed you travel or whether you are fertilizing anything. To use the example earlier, the Amazone ZA-M 1501 may be successfully operated up to 17 mph (by the farmer, not a hired hand); nearly twice the speed of most drills.