Selecting the best crop fertilizer can depend on the two primary factors of what you are trying to grow and the characteristics of your soil. Plants need three basic nutrients to grow, which are phosphorous, nitrogen, and potassium, though they can also require a variety of micronutrients in varying degrees. If your soil is deficient in any one of the three primary nutrients, then you need to choose a crop fertilizer that contains those substances. It is also important to consider the specific crops, since some plants need a certain level of acidity or alkalinity in the soil, in addition to specific nutrient levels, in order to thrive. Compost and manure can both provide much of the needed nutrients, though you may also want to select a liquid, or pellet-based, crop fertilizer based on your situation.
There are many different types of fertilizers that can be used to aid in plant growth or amend poor soil. Some natural fertilizers, such as compost and manure, contain a wide variety of nutrients. Others are used to impart just one nutrient, such as nitrogen from fish meal, or phosphorous in bone meal. Chemical fertilizers can also be designed to impart a specific mixture of the main nutrients required by crops, or to target a specific element that your soil might be lacking. The choice between natural and chemical fertilizers can depend on whether you are trying to grow organic crops, or if there are any specific regulations in your area about the types of fertilizers you can use.
One of the most important factors to consider when looking for a crop fertilizer is your soil. It is important for soil to contain phosphorous, nitrogen, and potassium, so you may want to have it tested. If your soil is lacking in any of these key nutrients, then you should select an appropriate fertilizer. You may want to choose a fertilizer that contains all of the nutrients even if the tests show your soil is in good shape, since that can help prevent your crops from depleting the soil.
It is also important to consider the specific plants you are trying to grow when you choose a crop fertilizer. Some plants require a specific nutrient or pH balance, while others are capable of producing certain nutrients and discharging them into the soil, which is the case with beans and nitrogen. If you want to plant a crop that is a heavy feeder, and requires a lot of one particular nutrient, then you should choose a fertilizer that is capable of providing for those needs.