Edited By: Ruth Burke
Question 1: What do the numbers mean on the fertilizer package?
The ideal fertilizer is designed to meet the nutritional requirements of the plant it serves. There are at least 17 essential mineral nutrients that are required by plants to grow and reproduce. These essential nutrients are classified as macronutrients and micronutrients. The three major mineral elements (macronutrients) of a typical fertilizer are nitrogen, phosphorus, and potassium—often referred to by their chemical symbols "N", "P" and "K.” So, a fertilizer with a guaranteed analysis of 25-3-5 would consist of 25% nitrogen (N), 3% phosphorus (P205), and 5% potassium (K2O). In greenhouses or controlled growing environments, some fertilizers will also contain secondary macronutrients (like calcium, magnesium, and sulfur) and micronutrients (like iron, boron, or manganese). In outdoor settings, plants are frequently able to acquire most of the macronutrients and micronutrients that they need from the soil surrounding them. However, N, P, and K are usually needed in the largest quantities and soils can become depleted over time if they are planted with demanding crops and not amended with additional fertilizer.
Question 2: Why are the N-P-K numbers more important than the other nutrients?
No one essential nutrient is of greater importance than any other. All the essential elements are necessary for proper development of turf grass, but most lawn fertilizers developed for outdoor use contain the three nutrients that are in highest demand: N, P, and K.
Nitrogen is an essential part of all living matter. It is the basis for amino acids that combine to form proteins. Nitrogen is associated with above-ground vegetative growth and density of turf, as well as its deep green color. Deficiency is noticed in turf that has turned light green or yellow. The blades start dying at the tip and progress along the midrib until the entire leaf is dead.
Phosphorus is the key nutrient in seedling development since it contributes so much to initial root development and seed formation. It is directly related to the vital growth process. Deficiency is most likely to be observed in seedling growth when new seedlings are slow to develop. On established grasses the leaves tend to turn purple.
Potassium (also known as potash) is found in large quantities in the plant. Potassium is associated with winter hardiness and disease resistance in turf. Deficiency will appear with the blades becoming streaked with yellow, turning brown at the tips, and eventually dying. Susceptibility to disease and winter injury is also increased in potassium deficiencies.
Question 3: There’s a lot of talk about adjusting soil pH before doing anything else. How do I measure and adjust accurately?
A soil may be alkaline, acidic or essentially neutral. The alkalinity or acidity of a soil is measured by its pH. All pH values occur somewhere in a scale running from 0 to 14. A pH of 7 is neutral with numbers less than 7 indicating acidity and numbers over 7 being alkaline. A pH range of 6.5 to 7 is good for raising turf grass. This ideal pH range creates a beneficial environment in the soil surrounding the roots of the plants. The overall “availability” of the 17 essential nutrients is at its highest in a pH range of 6.5 to 7. When the pH strays too far from this ideal, nutrients become less available in the soil and more difficult for the plant to use.
In acidic soils, it is common to add high-calcium lime which raises the surrounding soil pH. In the event of a basic soil, sulfur (in various forms) can be used to acidify excessively alkaline soils (another word for basic soils). We recommend that you conduct a soil test to determine if the pH needs to be raised, lowered or left as is—as well as to reveal the fertilizer deficiencies (or over applications!) in your soil. Your local agricultural extension office can provide you with recommendations for local labs that can analyze your soil samples.
What about amending the soil’s secondary macronutrients or micronutrients?
Occasionally, you will see deficiency symptoms in your turf that are caused by secondary or micronutrient deficiencies. These are commonly seen in new construction zones where turf or seed was applied over a thin layer of top soil on gravel fill or in regions where winter salt application created a temporary acidic soil zone that caused micronutrients to become unavailable for a short time. Two of the most common secondary macronutrient or micronutrient deficiencies are sulfur and iron.
Sulfur is an essential part of certain amino acids and proteins. Together with nitrogen, this element makes new protoplasm for plant cell growth. Deficiency symptoms are like that of a nitrogen deficiency in that the leaves will turn light green or yellow, then turn brown, and eventually die.
Iron plays an integral part in chlorophyll production and is also used in many enzymes. It is responsible for giving turf its deep green color. Deficiency symptoms include chlorotic or even white young leaves due to a reduction or loss of chlorophyll.
It is very easy to overfertilize with secondary macronutrients or micronutrients and you can cause more harm than good if you’re not careful. A complete soil test will determine if any of these secondary or minor elements are needed.