Fertilizers
Fertilizers can be applied to the plots only while they are sowing to increase or decrease the growing time of the crops. Only one type of fertilizer can be applied to each plot at a time.
Fertilizers can be cancelled, but it will cost you 2 eggs per plot and the original cost won't be refunded.
Fertilizers only changes the growing time, and it will give you the same ratio of yield, as the total time has changed. To keep things fair, it will also adjust the sow cost accordingly with the same ratio when growing starts. Thus you will get a refund if you are using a faster fertilizer, or you will be charged if you are using a slower fertilizer.
Fertilzers will not give you an advantage, it is fair and even across all types and crops and tractor levels.
You can use the calculator in the game to see exactly the tons you will get for each type and crop combination.
The following types of fertilizer are available:
Ultra Slow:
Crops will grow 300% slower, but you can get ±340% more tons per plot. Cost: 2 eggs per plot, maximum 12 eggs within 3 minutes. 

Extra Slow:
Crops will grow 200% slower, but you can get ±260% more tons per plot. Cost: 1 eggs per plot, maximum 6 eggs within 3 minutes. 

Slow:
Crops will grow 100% slower, but you can harvest ±180% more tons per plot. Cost: $100 per plot 

Fast:
Crops will grow 25% faster, but you can harvest ±80% less tons per plot. Cost: $100 per plot 

Extra Fast:
Crops will grow 50% faster, but you can get ±60% less tons per plot. Cost: 1 eggs per plot, maximum 6 eggs within 3 minutes. 

Ultra Fast:
Crops will grow 75% faster, but you can get ±40% less tons per plot. Cost: 2 eggs per plot, maximum 12 eggs within 3 minutes. 
Example: Pumpkins on 1 plot with 100 yield (tractor level 0) with sow cost of $14400
Speed  Grow Time  Total Time  Yield  Sow Cost Change  
Normal  1  6300  8640  100  0 
Ultra Fast  0.25%  1575  3915  45  $7875 
Ultra Slow  400%  25200  27540  319  +$31500 