Now that summer is here and lawns and gardens are all growing and flowering and producing, it is not too late for one more dose of fertilizer before the real heat begins. But which one do you choose? Stores have many different brands and mixtures, each one with different uses. Some fertilizers have a pre-emergent weed preventer, some are labeled “weed and feed” and contain weed killer, and others only display numbers—21-0-0, 15-5-10, 13-13-13 or other combinations. All fertilizers will show numbers (N-P-K) that indicate what proportions of nutrients are in the bag. It is these numbers that are most important when choosing a fertilizer. Let’s look at these numbers and what they mean to you and to your plants.
The first number is the amount of nitrogen (N). Nitrogen is a part of the chlorophyll molecule, which is responsible for the rich, deep green leaves in your lawn and plants. Nitrogen is an essential element for healthy plants, and without it your plants will be slow growing, stunted, and tend to have a general yellowing of leaves. As nitrogen is water soluble and can leach out of the soil after heavy rains, it needs to be added in the lawn or garden from time to time. It is generally a good idea to choose a fertilizer with a slow release form of nitrogen whenever possible, so that the impact of the nitrogen will be spread over several weeks instead of all at once.
The second number in the fertilizer is phosphorus (P). Phosphorus is useful in plants for flower and seed formation, root development, resistance to disease, and increased stalk and stem strength. Phosphorus is not water soluble, and does not leach out of the soil like nitrogen.
The third number is potassium (K). Potassium is second only to nitrogen as a plant nutrient as it is responsible for water regulation in plants, the flow of nutrients up and down the entire plant, as well as providing energy for chemical processes in the plant. Potassium is known to be helpful in the drought resistance of plants as it helps plant roots take up water. A potassium deficiency will cause plants to be more susceptible to pests and disease organisms.
Now, which formula to buy? The first step should always be a soil test to determine the needs of your soil. The Texas A&M AgriLife Extension office has free soil test bags available, and the routine soil test costs only $12. When you receive the results of your soil test, it will show the amounts of nutrients currently in your soil, and will include advice on which nutrients to add, and how much to add, depending on your gardening goals. It should also be mentioned that when you go to the store to purchase your fertilizer, if the numbers are the same, the $10 bag is just as good as the big name $35 bag. Nitrogen is nitrogen, and if you buy the less expensive product, the plants cannot tell the difference! Happy gardening!