The ABCs of fall fertilization

By Brad Fresenburg, PhD

When field managers think about fall fertilization, many assume the focus is on cool-season grasses. While this may be true for many regions of the country, the ABC’s of fall fertilization can include some considerations for both warm- and cool-season grasses. The type of fertilizer and application timing has always been and is still important, so using the correct fertilizer at the correct time will maximize growth, recovery and performance; making fall one of the most important time-frames for fertilization. One item that still remains true is the fact that a good fertilization program is dependent on proper soil testing. So as we discuss the ABC’s of fall fertilization, we will remind everyone on the importance of testing our soils, look at the various types of products available, and develop a plan with some of the latest research in mind.

Soil testing is the basis for developing a fertilization program. While the small investment in a soil test can tell the field manager what is needed; a soil test can also tell a field manager what is NOT needed, saving money. Testing should be conducted annually on sand-based fields and every 2 or 3 years on a soil-based field. Another option in conjunction with soil testing is tissue testing. Here, a field manager can determine the effectiveness of their fertility program, by seeing if nutrients are actually being taken up by grasses in sufficient amounts. The following table has been an excellent resource for understanding the role that each nutrient plays in the growth of a plant and the amount of nutrients required for that growth.

A helpful conversion for this table and soil testing reports to convert ppm to pounds/acre is to take ppm times 2 to equal pounds/acre. Refer to “Understanding Soil Tests” at for interpreting a soil test report and managing soil pH. We also need to remember that soil pH plays a role in nutrient availability; therefore pH results are as equally important to knowing nutrient levels. Kentucky bluegrass and bermudagrass grow best when soil pH is between 6.0 and 7.0; tall fescue prefers a soil pH from 5.5 to 6.5. Perennial ryegrass can tolerate a wide range of pH (5.1 to 8.4); however it performs best with a soil pH of 6.5.

Your local Extension Office or soil-testing lab will usually provide recommendations based on your particular location and soil type. It is also important to indicate on the submission form that your soil sample is for turfgrasses so appropriate recommendations can be made.

Choosing a fertilizer is dependent on needs, expectations, time of year, soil temperature and soil moisture. Do you need rapid recovery or not? Does your fertilizer choice require microbial activity and warmer soil temperatures? Or, does your fertilizer choice require soil moisture for hydrolysis?

Complete fertilizers contain some percentage of nitrogen (N), phosphorus (P) and potassium (K), but may also contain an additional component such as iron (Fe) or sulfur (S) or micros. Compound fertilizers contain at least two of the primary nutrients of N, P and K. Homogeneous fertilizers have prills that contain an amount of each fertilizer component within indistinguishable granules. Heterogeneous blended fertilizers are a blend of separate fertilizer components for each of the primary nutrients included. Homogeneous fertilizers are usually thought of as being better for providing an even distribution of nutrients; however with many fertilizers being formulated with lower size guide numbers (SGN), even blended fertilizers distribute well.

Sometimes fertilizers are defined by the fertilizer ratio of N to P to K; therefore a maintenance fertilizer defined by a 3-1-2 ratio could be a 15-5-10 fertilizer. Keep in mind that the P and K component in a fertilizer are truly a percentage of P2O5 and K2O, respectively. To determine the actual amount of phosphorus being applied, the amount of P2O5 needs to be multiplied by 0.44. To determine the actual amount of potassium being applied, the amount of K2O needs to be multiplied by 0.83.

Several nitrogen, phosphorus and potassium sources are available as many of you are familiar with. A vital part to controlled-release nitrogen sources is understanding the method of release: microbial breakdown or hydrolysis or some of both. There are countless fertilizer products available from complete to single nutrients, from quick to controlled-release, and granular versus foliar. Keep quick-release fertilizers to a minimum; establishment/grow-in and rapid recovery. Maintenance fertilizers should include 30 to 50% controlled-release nitrogen forms and usually in a 4-1-2 or 3-1-2 ratio. The guaranteed analysis on every product will indicate what you are purchasing. Simply select fertilizer products based on your needs. Often products are selected based on low cost. However, consider spending a little more if you can find a product that better matches your need and saves on labor by lasting longer. Fertilizers that provide consistent feeding over time are better for your grasses.

Schedules and rates

Fall still remains the most important time of the year to fertilize cool-season fields for recovery from summer and fall seasons as well as building carbohydrates reserves. With fall comes compacted soils and aerification just before an application of fertilizer is an excellent practice for recovery. Fields being used heavily will obviously benefit from both practices.

Research shows that late fall applications of nitrogen will enhance fall color of both cool and warm-season grasses. Early fall applications of nitrogen to cool season grasses should contain 30 to 50% controlled-released nitrogen, especially on sand-based fields. Late fall applications are best when shoots have begun to slow in growth (but remain green), promoting root growth. Late fall applications of nitrogen have also shown to have an impact on root development in the spring. Precautions for late season applications on cool-season grasses include the potential for increased snow mold and winter annual weeds (annual bluegrass, henbit, chickweed, etc.). Understand that phosphorus and potassium applications in the fall is not as critical for timing when correcting soil needs or when just including some amount as part of a complete fertilizer. Fertilizers should never be applied to frozen ground as the potential for runoff is greatly increased.

On the warm-season front, research has indicated that late season (fall) nitrogen applications to bermudagrass will not contribute to winterkill. Maintaining adequate levels of potassium in the medium to high range is important for winter protection. However, studies have shown that additional applications of potassium in the fall, when sufficient levels already exist in the soil, will not increase winter hardiness of bermudagrass. Here a soil test can save on that purchase of a winterizer if potassium levels are found to be adequate. If potassium is needed, applications should be made well ahead of warm-season grasses slowing down in growth. So, late season applications of nitrogen to bermudagrass will allow for more recovery, maintain green color longer, help to build carbohydrate reserves and not contribute to winterkill.

One precaution to note with fall applications of nitrogen to bermudagrass is the increased potential for spring dead spot disease. Avoid late season applications if spring dead spot is beginning to or historically occurs at your facility. Late season nitrogen applications to bermudagrass fields can also contribute to increased winter annual weeds.

Regardless of how well you developed your fertilizer program, if application techniques (swath and overlap) and calibration (half rate applied in two directions) are not properly followed, then you will realize the lack of uniformity in about 7 to 10 days!

Brad Fresenburg, PHD, is assistant extension professor the University of Missouri in Columbia. See for “Understanding Soil Tests,” “Late Fall Fertilization of Athletic Fields,” from Penn State, and “Fertilizers,” University of Tennessee, Extension Publication W161-E. 2008.