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Phosphorus & Potassium, Two Essential Turf Macronutrients

By Achille Correggia, B.Sc. (Agri.) Agronomy, A.PAg/CCA-ONT

Nutrients that are required for both the growth and development of turf are considered essential nutrients. A nutrient is considered essential if (a) in its absence turf is unable to complete a normal lifecycle, or (b) that element is part of essential plant function(s). Essential nutrients can be separated into two categories: (a) macronutrients and (b) micronutrients (also called trace elements). The distinction between the two categories simply indicates the difference in nutrient concentration within the turf tissue, with macronutrients being required in larger amounts than micronutrients. This difference does not represent the importance of one nutrient category over another as both categories are equally vital for healthy turf.

Two nutrients classified as macronutrients are phosphorus and potassium, the function and availability of both these macronutrients will be discussed in more detail.

Phosphorus

In order to manage soil phosphorus levels for economical turf management and environmental land stewardship turf managers must understand the different forms of phosphorus with in the soil. Soil phosphorus can exists in three completely separate and different forms; soluble (1%), organic (40-50%) and inorganic (40-60%). These three different forms are in constant equilibrium with each other, meaning phosphorus is constantly moving from either a soluble or insoluble form within the soil. It is important to know that phosphorus only in its soluble form is available to turf.

With sufficient phosphorus levels the overall quality of turf is enhanced as phosphorus is involved in many fundamental processes including, photosynthesis, nitrogen fixation, protein and carbohydrate metabolism and most importantly root development. The table below provides ideal phosphorus levels for a proper fertilization program, this information can help determine if soils contain sufficient amounts of available phosphorus for proper turf development.

Optimum Phosphorous Range

Figure 1. Median Bray P-1 equivalent soil test levels in ppm.

Soils that are lacking adequate amounts of phosphorus could result in turf showing signs of deficiencies, these signs can include; reductions in turf density and root growth, reddening or yellowing of leaf margins, death of mature leaves and poor seed development. Phosphorus deficiencies in turf can also be described as a dark green colour that progresses to a purplish to reddish purple colour in the older leaves; visually turf stands will appear wilted which may be confused with the onset of drought stress.

The availability of phosphorus to turf can be affected by numerous factors including soil moisture, soil temperature, fertilizer application, and soil clay content, but the primary factor in phosphorus availability is soil pH. Soil pH levels do not have a direct affect on phosphorus, but are an indicator on how certain minerals will interact with phosphorus in the soil; this interaction will affect the availability of soil phosphorus. Soils with a pH level less than 5.0 or higher than 7.0 will have reduced phosphorus availability. At pH levels lower than 5.0, phosphorus will react with high levels of iron and aluminum, creating iron or aluminum phosphate minerals. Soil with a pH level higher than 7.0 has a high concentration of calcium which will react with phosphorus, producing unavailable calcium phosphate.

A possible solution to correcting low or high pH soils is the addition of lime to acidic soils with a pH lower than 5.0 and elemental sulphur to basic soils that are well above a pH of 7.0. The amount of lime that needs to be applied to an acidic soil can be easily calculated from the soil's buffer pH and the limestone's agricultural index (A.I). The addition of sulphur on the other hand to basic soil is somewhat more difficult to predict and correct. A typical soil can contain anywhere from one to five percent lime. At the five percent level, the top three inches of soil can contain up to 25 tonnes of lime. These higher calcium levels may require an application of up to eight tonnes of elemental sulphur to amend this pH problem, not a practical solution for established turf. These high sulphur rates would need to be applied prior to turf establishment, which make soil testing prior to turf establishment so vital in correcting any soil problems. On established turf no more than 5 pounds of elemental sulphur per 1000 ft2 should be applied in one application.

Potassium

After nitrogen and phosphorus, potassium is the third most likely essential nutrient to limit plant productivity. Many soils are rich in potassium although only one to two percent of soil potassium is readily available for turf uptake. The remaining potassium is stored in two different forms known as slowly available and relatively unavailable. Slowly available potassium typically constitutes one to ten percent of the potassium in a soil profile while the remaining ninety to ninety-eight percent is relatively unavailable.

Correct potassium levels are vital for chlorophyll, protein and starch production, turf movement (stomata opening & closing), as well as turf respiration. Potassium is especially important in helping turf adapt to environmental stresses. Good potassium nutrition is linked to improved drought tolerance, enhanced winter hardiness, better resistance to certain fungal diseases, and greater tolerance to insect pests.

Figure 2. Median ammonium acetate equivalent soil test K levels in ppm.

A deficiency in potassium will typically begin in older leaves as chlorosis, followed by necrotic lesions (spots of dead tissue) at the leaf margins. The most common causes of potassium deficiencies are under fertilization, restricted root growth from soil compaction (potassium uptake is through root interception) and low cation exchange capacity on constructed sand soils. Sandy soils have minute amounts of clay and organic matter, resulting in very few exchange sites. Potassium, as all other cations are subjected to rapid leaching from sand based low organic matter soils. In these types of soils it is better to maintain lower K values and apply K fertilizers with multiple applications annually. The table below provides ideal potassium concentrations for environmentally and economically sound potassium fertilization programs. Based on the information below, turf managers can determine if their soils contain adequate amounts of potassium for proper turf development.

Optimum Potassium Range

The uptake of potassium by turfgrass is often quite large. Turfgrass will typically absorb far more potassium than is required for proper turf development if sufficiently large quantities of potassium are present within the soil. This tendency for excessive uptake is called luxury consumption because the excess potassium absorbed does not increase turf quality. This excessive uptake of potassium by turfgrass is stored in the blades of the plant; the removal of grass blades from the area through mowing will also remove this collected potassium. On average 3 to 4 lbs of K/1000 ft² are removed annually when clippings are exported from a mown site. The combination of turf's tendency to over consume large quantities of potassium combined with the removal of grass clippings during mowing may potentially lead to potassium deficiencies within the soil, eventually affecting the health of turfgrass.

What does this mean to a turf manager?

Turf managers need to consider several factors when reviewing test results for phosphorus levels. If results indicate low levels, they need to ask themselves what are the causes for this deficiency. One common cause for inadequate phosphorus levels is a high (above 7.0) or low (below 5.0) soil pH, if this is the situation turf managers need to correct this problem by either applying sulphur or limestone. If a soil pH problem is not addressed, any phosphorus addition through fertilizer applications can become bound up in forms (ex. iron phosphate or calcium phosphate) that are not available for turf grass uptake. With regards to potassium, turf managers need to be aware of turf's tendency of luxury consumption and soil's minute levels of available potassium. Disregarding these two vital factors along with other limiting factors that were mentioned above may quickly lead to deficient potassium concentrations in the soil and eventually within the turf.

Without proper levels of available soil phosphorus and soil potassium the overall health of turf could be greatly compromised. Weakened unhealthy turf would become more prone to insect and disease invasion which would certainly increase operating costs. Finally and most importantly is the public's enjoyment that comes from a high quality recreational turf area, any decrease in turf's quality would be greatly diminish an individual's pleasure.

Reference

Ministry of Agriculture, Food and Rural Affairs Publication 611 (1998). Soil Fertility Handbook. Toronto, Ontario, Canada: Queen’s Printer for Ontario.

Brady, N.C., Weil, R.R. (1996). The Nature and Properties of Soils. (11th Edition). Upper Saddle River, New Jersey, USA: Prentice Hall.
Hopkins, W.G, (1995). Introduction to Plant Physiology. Toronto, Ontario, Canada: John Wiley & Sons Inc.

MDS Harris Laboratory Servicses. (no date). Potassium in Turf Grass. Retrieved July 20 2006, from
http://turf.mdsharris.com/education_train/pdfs/phosphorus_CG.pdf

MDS Harris Laboratory Servicses. (no date). Phosphorus in Turf Grass. Retrieved July 20 2006, from http://turf.mdsharris.com/education_train/pdfs/Potassium_in_turf_Grass.pdf

A&L Canada Laboratories Inc.. (no date). Retrieved July 20 2006, from
http://www.al-labs-can.com/techs/tech_SoilOptLevels.html

Fixen, P.E. “Soil Test Levels in North America.” “Better Crops with Plant Food”. Volume 90, No. 1 (2006): 4-7.

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