How do I Choose the Correct Fertilizer?

Plants need a continuous supply of nutrients; however, sometimes the nutrients found naturally in the soil are not enough. In that case, fertilizer is needed. Choosing the correct type can be an overwhelming task. Most garden centers have shelves packed high with bags and containers of fertilizer — each type serving a specific purpose.

Nitrogen (N), phosphorus (P), and potassium (K) are the main nutrients that plants need to survive. Complete fertilizer contains those three ingredients and they are listed clearly as the N-P-K ratio. So, if a package states that its N-P-K ration is 10-6-6, then it contains 10% nitrogen, 6% phosphorus, and 6% potassium.

Nitrogen is generally the nutrient that plants and lawns need most. So, the first number or nitrogen should usually be the highest of the ratio. In addition, nitrogen is usually water-soluble or fast-release forms and/or in water-insoluble or slow-release forms. Water-soluble nitrogen satisfies the plants needs quickly, while insoluble nitrogen must be broken down by organisms in the soil before the plants can be fully satisfied.

Fertilizer can be either natural or chemical/factory-made. Natural varieties use compounds that would otherwise become waste, such as animal waste, dead organisms, and meals made from soybeans, blood, cottonseed, bone, alfalfa, and kelp. In most cases, natural fertilizer has lower nutrient levels than that which is factory-made, and is often more expensive than that created in a factory. To see the greatest improvement in plants and lawns, natural fertilizer should be applied in early spring or late fall.

Natural blood meal has an N-P-K ratio of 13-0-0. It is a good source of nitrogen and can be scratched into the soil around plants. Natural cottonseed meal has an N-P-K ratio of 6-2-1. It acidifies the soil when it fertilizes it and is usually used when soils are alkaline and the plants require more acidity, such as azaleas. Natural fish pellet fertilizer has an N-P-K ratio of 8-5-1 and is great for vegetable beds.

Factory-made or chemical fertilizer is usually mass-produced. It has higher levels of nutrients and has more soluble nitrogen than natural varieties. Lawns and plants can burn and become destroyed if too much of this type is used. Consequently, it is recommended that it is applied to moist soil and that the soil is irrigated again after it is applied, as well.

Both natural and chemical fertilizers can be purchased in liquid or solid forms. Liquid types supply nutrients to the roots immediately. They most commonly have an N-P-K ratio of 20-20-20. However, they need to be applied more frequently.

Solid fertilizer can be spread across a lawn or near plants and release nutrients slowly, over time. It does not wash away from rain or water systems as easily as liquid. There are several different N-P-K ratios available for solid fertilizers: 10-10-10 for all purpose use; 10-20-20 for vegetables; 6-10-4 for flowers; and 29-3-4 for lawns.

In general, fertilizers formulated for plants grown for their leaves and for lawns have higher nitrogen levels. Those formulated for flowering plants and fruits have higher phosphorus levels. There are also specialty varieties for vegetables, roses, fruits, and lawns. Understanding the N-P-K ratio and the kinds that are available should make choosing the correct type easier.