Plant Food 101

Understanding Fertilizer

Ever wonder what those three numbers on the front of a bag of fertilizer mean? Numbers like “5-10-5″ or “10-10-10″. It’s very simple: those numbers represent the three most important nutrients for plant growth; N, P, and K. Each number reflects the percentage of nitrogen, phosphorus, and potassium contained in the fertilizer. Understanding the purpose of each of these nutrients can help increase your gardening success.

Key Nutrients

Nitrogen is listed first because it is one of the most important nutrients of all—it is responsible for the lush green growth of leaves and shoots. Plants cannot survive without nitrogen, and it is available in limited quantities in the soil. Gardeners use fertilizers high in nitrogen when they want to boost the rate of growth in spring and early summer.

Over fertilizing with nitrogen can have negative effects though. It can cause a plant to grow too rapidly resulting in soft, weak plant tissue. Soft tissue is easily damaged and can then attract an array of sap-sucking insects. It’s also easy to”burn” your plants with an overdose of nitrogen. This will appear as yellow to brown areas on the tips of leaves and new shoots. Always follow the recommended application rate on the fertilizer package. Avoid using high nitrogen fertilizers from mid-summer onward, since this is when most plans have ended their most active growth phase and require other nutrients to help develop flowers, fruits, and seeds.

Phosphorus, the second nutrient on fertilizer labels, is required by plants in smaller amounts than nitrogen, but it’s still an essential nutrient that is available in limited supply in the soil. Phosphorus supports physical processes of plant growth, including water uptake, photosynthesis and nutrient absorption. It is especially important for flowering, fruiting, and root growth.

Potassium, the third nutrient on fertilizer labels, is the key nutrient that supports overall plant health, reproduction, and disease resistance. Potassium is almost as important as nitrogen because it affects every part of a plant from its roots to its new shoots.

Research has shown that a 3-1-2 ratio of nitrogen, phosphorus, and potassium is ideal for plant growth. So any fertilizer with a ratio of 6-2-4, 9-3-6, 12-4-8, etc. is considered a balanced fertilizer. It’s best to do a soil test to determine if your soil needs more or less of certain nutrients. Soil test kits can be purchased or you can contact your nearest state university cooperative extension office to see if they offer soil testing (expect to pay a fee).

Natural Fertilizer Sources

For those who prefer to feed their plants organically there are many natural sources of fertilizer. Compost is an excellent source of organic nutrients—it contains a balanced ratio of plant foods but at a lower quantity than synthetic fertilizers. Organic sources of nitrogen include blood meal, fish emulsion, manure, and cottonseed meal. Organic phosphorus is provided by crushed limestone or bone meal. Sources of organic potassium are wood ashes, kelp and seaweed, and greensand (a sedimentary rock formed by ancient sea beds that is mined and crushed to a granular form). Organic fertilizers are just as effective as synthetic ones and are generally considered better for the environment.