If you look at the mineral composition of a grass plant, about 90 percent by weight is just three elements — carbon, hydrogen and oxygen. These three elements make up both the structural carbohydrates (cell walls, etc.) and nonstructural carbohydrates (sugars, starches). But even though they’re such a large percentage of the plant, we don’t apply these elements as a fertilizer. Instead, the plant takes them up mainly from carbon dioxide and water. As seen in Figure 1, applied fertilizer sources that supply nitrogen, sulfur and other elements can contribute, but it’s a small contribution. The rest of the essential elements are minerals found in the soil that are absorbed mainly by plant roots, but also through the leaves if certain foliar fertilizers are used.
The first step in developing a fertility program is to take a soil test, which will tell you what levels of each nutrients are present and provide recommendations for additions, if necessary. The soil test gives information about the pH of the soil. Soil pH should ideally be close to 7.0 (neutral). When the soil pH is too low or too high, a nutrient can be present in adequate amounts, yet be chemically unavailable for uptake by the plant roots. The most common example of this is an iron or manganese deficiency due to chemical unavailability when the soil pH is too high.
Another important number on the soil test is cation exchange capacity (CEC).
Note from Figure 1 that, with a couple of exceptions, the mineral nutrients are taken up by plants in the univalent (1+) or divalent cation (2+) form, which is why the measure of a soils cation exchange capacity is important to understand. CEC is also an important measure, as it guides how you apply fertilizer
If you have a low CEC soil (tends to be high in sand), then the soil holds less nutrients. To compensate, apply less fertilizer but more frequently.
On high CEC soils (tend to also be high in clay or organic matter) the soil holds more nutrients, meaning that you can fertilize less often but with higher amounts.
The six macronutrients are nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. They’re called macronutrients because they’re present in the plant in relatively larger amounts compared to the micronutrients. The macronutrients are further divided into primary (nitrogen, phosphorus and potassium) and secondary (calcium, magnesium and sulfur). The distinction is that the primary macronutrients tend to be added as fertilizer with regularity, whereas the secondary macronutrients tend to be supplied only when a soil test indicates a need.
Phosphorus is very important to plant survival. When a plant harvests energy from sunlight, the energy transfer system in the plant uses a molecule called ATP (the P stands for phosphate). Phosphorus is also present in nucleic acids and phospholipids.
Despite these important roles, a typical fertilizer bag contains much less phosphorus than nitrogen and potassium. There are three reasons for this. One is that, examining Figure 1, we see that phosphorus plays a critical role in the plant, but is present in much smaller quantities compared to nitrogen and potassium. A second reason is that phosphorus is relatively immobile in a soil system, so supplemental applications aren’t needed as often. The third reason phosphorus is applied in smaller quantities is because it’s implicated in causing eutrophication of surface waters such as lakes and streams. Eutrophication is the phenomenon where a species of blue-green algae (bacteria) multiply rapidly in phosphorus-enriched water and result in oxygen depletion and water quality degradation.
You may be thinking, “How can a nutrient that isn’t mobile in the soil move into the water and cause eutrophication?” Two likely avenues are movement of soil sediment into waterways and runoff losses of applied fertilizer phosphorus.
Except at construction time, movement of soil sediment is minimal. You can do a lot to help prevent runoff losses of phosphorus by doing two things: only apply it if a soil test indicates that it’s needed and prior to application, perform core aerification, vertical mow, or conduct another practice that will increase infiltration into the soil rather than runoff. Also, it’s not just turfgrass seedlings that use extra applied phosphorus. Weed seedlings do as well – avoid applying phosphorus when weeds are germinating. Thus, an application for supplementing mature turfgrass should be made sometime during the summer.
Potassium is the other primary macronutrient. Unlike the other elements mentioned thus far, it’s not part of any structure or component of the plant. Rather, potassium plays a role in regulating water balance in plant cells as well as the correct functioning of certain enzymes. Because of the way the potassium works in the plant, it’s implicated in improving a plant’s ability to deal with environmental stresses, such as heat and drought.
Synergistic relationships between nitrogen, phosphorus and potassium in turfgrass were found in the late 1970s. But research to conclusively quantify the ability of potassium to improve, for example, drought tolerance, has been difficult to conduct.
Over the years the trend in fertilizer analysis has changed. In 1980, a typical fertilizer analysis for use in turf might have been 32-2-4. When the use of high amounts of potassium became trendy (around the early 1990s), the typical analysis might have been 22-2-22. Today, there tends to be a mix of philosophies about the merits of higher potassium fertility. I’ve seen cases where high potassium levels make a difference and others where it didn’t. If trying higher potassium levels to improve stress tolerance, leave a check plot so you can document the benefit.
The two most commonly used fertilizer sources of potassium are potassium chloride and potassium sulfate. Potassium chloride is a less expensive source, but it has a much higher potential to cause physiological burn (chlorosis to the leaf tissue caused by contact of the fertilizer with the leaf; see Figure 2). A standard practice when fertilizing is to wash the fertilizer into the soil by lightly irrigating following application. If this practice isn’t followed, then the use of fertilizer materials that have high physiological burn, especially when applied on hot or dry days, can result in some unwanted yellowing of the leaves.
The secondary macronutrients are used by the plant in relatively larger quantities, but in most cases are present in adequate amounts to satisfy growth needs. These nutrients tend to be added based on the results of a soil test. Several sources of these elements are available for use as a fertilizer (Figure 3). These elements are also commonly added to fertilizers designed to supply micronutrients to turf.
Calcium is a constituent of cell walls and is required for the growth of meristems and cell division. Calcium deficiencies are most likely to occur in lower pH soils or on constructed athletic fields in which the sand used is silica based. When a grass plant is deficient in calcium, one of the signs is reddish-brown, younger leaves.
Magnesium is the atom in the center of the chlorophyll molecule. It’s also required for protein synthesis, phosphorylation and enzyme activation. Deficiency symptoms include yellow leaves with a reddish tint at the edges. Magnesium deficiency isn’t common.
Sulfur is a component of some of the amino acids. In parts of the U.S., sulfur deficiencies are rare because the burning of high sulfur coal results in acid rain that supplies an adequate amount of sulfur for plant growth. Also, sulfur is found in many fertilizers (think sulfur-coated urea). Sulfur deficiencies manifest as a yellowing of the younger leaf tissue.
Each of the micronutrients are just as important as the macronutrients, and without any of them the plant can’t survive. Most of them function either in the production of chlorophyll or the formation of carbohydrates.
Micronutrients are present in the plant in much smaller quantities. The focus of some of the research in micronutrient management for turfgrass has been on potential toxicity if over-applied or in the soil at too high of a concentration. There were concerns about zinc and chlorine toxicity, but research shows this to not be an issue. Boron, however, can occasionally reach toxic levels if irrigating with effluent water. Copper toxicity has been reported, but is rare. Molybdenum and nickel, as you can see in Figure 1, are the two elements found in the lowest concentration in the plant. (I haven’t heard of a documented case of a deficiency of either of these elements.) Manganese deficiency, on the other hand, is somewhat more common and manifests as a yellowing of the leaf, but the leaf veins and tips remain green.
Labs test for micronutrients and can identify potential nutrient deficiencies. Some of the elements are naturally more or less likely to be deficient. Many of the micronutrients tend to be available in sufficient quantities in the soil, or are delivered as “contaminates” of N-P-K fertilizers. If deficient, many of these elements can be supplied by way of a micronutrient package sold by fertilizer vendors. Micronutrient deficiencies are more frequently observed on sandy soils with lower CEC.
The most frequently seen micronutrient deficiency is with iron. Not because there isn’t enough of it. Rather, the chemistry of iron in the soil is what causes the issue. Depending on the pH of the soil and the parent material of the soil, iron tends to be present in sparingly soluble salts. Plant roots excrete substances called chelating agents that solubilize the iron so it can be taken up. To fertilize with iron, you can purchase inorganic sources (Figure 3). This should only be done to correct an iron deficiency in the soil (somewhat rare). Also, at higher pH levels – above 7.5 – the issue isn’t the amount of iron present, but rather chemical availability.
A more common use of fertilizer iron for turfgrass is to cause a rapid greening of the turf for aesthetic reasons. Iron isn’t a component of chlorophyll but it is a co-factor in its production, and fertilizing with iron results in rapid increase in chlorophyll leading to darker green leaves.
In fact, over-application causes a blackening of the turf (it’s ultra-dark green). This is temporary because iron isn’t mobile in the plant. Once the blackened leaves are mowed off, the symptom disappears. To get the benefit of the rapid darkening of the leaf tissue (for a game or event), chelated forms are typically applied. These are more expensive but usually necessary to deliver the intended response.
An understanding of the role that each element plays in growth and development is useful when designing a fertility management program. Applying the most appropriate fertilizer at the right time – based on the results of a soil test – is one of the primary practices to achieve successful field management.