Types and Forms of Nutrient Sources - Phosphorus (P)
Phosphorus is important in stimulating early root growth and promoting early plant vigor. Phosphorus moves very little in the soil with most of it bound tightly to soil particles. Soils naturally high in phosphorus generally provide sufficient phosphorus to sustain plant health and growth for many years without supplying additional phosphorus from fertilizers.
There are three reasons for correcting phosphorus-deficient soil prior to establishment. First, since phosphorus moves very little in the soil, it is desirable to mix the phosphorus throughout the root zone. This is relatively easy prior to seeding but much more difficult after establishment. Second, seedling plants with newly developing root systems are most likely to be affected by lack of phosphorus. Third, mixing phosphorus into the soil (rather than applying it to the surface) reduces the chance for phosphorus to move into lakes and streams. Fully developed turfgrass root systems can absorb phosphorus from a much larger volume of soil, reducing plant deficiencies.
A soil testing laboratory can determine the current level of available phosphorus in a particular soil. This inexpensive procedure is the best way to accurately determine the phosphorus requirements of a lawn. Remember, it is currently illegal to apply any phosphorus-containing fertilizer to a lawn in Minnesota without verifying a need by a reliable soil test, such as from the University of Minnesota Soil Testing Laboratory, or at the time of seeding or sodding. The only other exception is use by golf course personnel who have gone through a certification training program on the use of phosphorus fertilizer in golf course applications.
Research recently completed at the University of Minnesota indicated that when soil test levels of P are considered high there is potentially greater loss of phosphorus and increase runoff volume when lawns are thin and poorly fertilized (Fig. 7.2) compared to a healthy, dense lawn. In addition, this same research indicated that there is no significant difference between lawns fertilized with nitrogen (3 pounds of N per 1000 square feet applied annually over three applications), and potassium only (no phosphorus applied) compared to those fertilized with a low rate of phosphorus (1 pound of P per 1000 square feet applied annually over three applications) (Fig. 7.3). Hence, the results of the study support the no-phosphorus-lawn-fertilizer-restriction as acceptable turfgrass density and quality could be maintained on an established lawn without additional phosphorus application when soil test levels of P were high. For a complete report of this research see Minnesota Department of Agriculture report in the Appendix.
Figure 7.2. Thin, unfertilized lawns have a greater potential for water volume and phosphorus loss.
Figure 7.3 Very little difference in turfgrass quality occurred when N and K were applied vs. N, P and K were applied.
Links to more information about the Minnesota Phosphorus Law can be found in Table 7.6.
Table 7.6. Additional Phosphorus Resources
More detail pertaining to the Minnesota Phosphorus Lawn Fertilizer Law can be found in Chapter 18C.60 of Minnesota Statues.
Options for disposing of leftover phosphorus-containing lawn fertilizer - online factsheet.
Proceed to Types and Forms of Nutrient Source - Potassium.