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Phosphorus and Potassium

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Supply phosphorus and potassium based on soil test results.

  • Soil testing is the most accurate method for determining P and K fertilizer requirements.
  • Phosphorus and potassium are expressed in their respective oxide forms, P2O5 and K2O, for the purposes of fertilizer grades and recommendations.

Understand that special considerations are necessary for application of phosphorus- containing materials.

  • For mature turf, phosphorus application is rarely needed on most soils unless a deficiency is indicated by a soil test.
  • When soil tests indicate P is needed, use rapidly-available sources of P for new seedings to ensure adequate levels of soluble phosphorus for young grass shoots. Note that some organically approved mineral sources of P may not release available phosphorus quickly enough for rapid turfgrass development (e.g., rock phosphate, colloidal soft rock phosphate).
  • When soil test phosphorus levels for established turf are below Optimum (Very Low or Low; see Table 8, Interpretation of soil test categories), the recommended application rate for P is intended to meet immediate turf phosphorus needs in addition to gradually raising soil test levels into the Optimum range (see Table 17, UMass soil test phosphorus application guidelines).
  • Applying P in conjunction with cultivation (aeration, dethatching, etc.) will facilitate incorporation into the root zone and reduce the potential for phosphorus loss.
  • When soil test phosphorus levels are in the Optimum range very little, if any, P is needed for established turf.
  • When soil test phosphorus levels are Above Optimum, no P is needed for establishment or maintenance.
Fertilizer Analysis Salt Indexa CaCO3 Equivalentb Lbs. needed to supply 1 lb. P2O5
Table 16. Characteristics of common phosphorus containing fertilizer sources
Mono-ammonium phosphate 11-52-0 2.7 58 1.9 (also supplies 0.2 lbs. N)
Di-ammonium phosphate 18-46-0 1.7 75 2.2 (also supplies 0.4 lbs. N)
Super-phosphate 0-20-0 0.4 0 5.0
  1. Relative burn potential compared to sodium nitrate. (>2.5 = high, 2.5 -1.0 = moderate, <1.0 = low)
  2. Lbs. of CaCO3 (limestone) needed to neutralize the acidity of 100 lbs. of applied fertilizer.
Table 17. UMass soil test phosphorus application guidelines.
  Soil test phosphorus level
Very Low Low Optimum Above Optimum
lbs. P2O5 / 1000 sf / year
Turf Establishment 2.0 – 2.5 1.0 – 2.0 0.5 – 1.0 0
Turf Maintenance 1.5 – 2.0 0.5 – 1.5 0 – 0.5 0

Limit P input to the lowest possible level needed to achieve adequate turf quality and prevent deficiency.

  • Soil test phosphorus levels should not exceed the environmental critical concentration (40 ppm Modified Morgan extractable P) in order to protect surface water quality. When extractable phosphorus exceeds the environmental critical concentration, the risk of dissolved phosphorus loss in subsurface water flow or runoff in amounts that pollute surface water is significantly increased. As with N, the potential for leaching of P is greater on sandy root zones.
  • To avoid phosphorus overload in the turf system, nutrients from all sources in the management plan should be factored into total P applied. Organic amendments, retained clippings and many compost materials can contribute P into the turf system.
  • Where soil test phosphorus levels are Excessive (greater than 40 ppm P), no P containing materials should be applied and active steps should be taken to minimize surface runoff from the site.
  • Natural organic sources of P, whether approved in organic programs or not, do not pose a lower risk to water resources than synthetic fertilizer P. Where organic fertility programs are in place and soil test results indicate Above Optimum levels of P, the addition of P containing fertilizer, soil amendments or topdressing materials should be avoided.

See Table 12 for a listing of natural organic nutrient sources containing P.

Apply potassium in accordance with soil test results and management goals

  • When soil test K levels are below Optimum (Very Low or Low), application of K fertilizer will generally improve turf health. Even when soil test K levels are in the Optimum range, turf health may benefit from a modest application of K. See Table 8, Interpretation of soil test categories.
  • While every fertilizer application may not include K, those applications preceding stress periods are good times to supplement K and to correct for soil K deficiencies.
  • Where no P is needed, apply N and K over the growing season following a ratio of approximately 3-0-2 or 4-0-2.
  • Early fall applications in particular are often made with a fertilizer containing N and K to improve winter survival without over-stimulating growth.

Potassium chloride (KCl) is the most common K source used in turf fertilizers because of its lower cost and moderate burn potential (Table 15). Potassium sulfate (K2SO4) is used in high-grade turf fertilizers because of its low burn potential (low salt index). 

Fertilizer Analysis Salt Indexa CaCO3 Equivalentb Lbs. needed to supply 1 lb. P2O5
Table 18. Characteristics of common potassium containing fertilizer sources.
Muriate of potash (KCl) 0-0-60 1.9 0 1.7
Sulfate of potash (K2SO4) 0-0-50 0.9 0 2.0
Potassium nitrate (KNO3) 13-0-44 5.3 -23 2.3
  1. Relative burn potential compared to sodium nitrate. (>2.5 = high, 2.5 -1.0 = moderate, <1.0 = low)
  2. Lbs. of CaCO3 (limestone) needed to neutralize the acidity of 100 lbs. of applied fertilizer.
Table 19. UMass soil test potassium application guidelines
Management level Soil test potassium level
Very Low Low Optimum Above Optimum
lbs. K2O / 1000 sf / year
Normal 3–4 2–3 1–2 0
Intensivea 4–5 2–4 1–2 0
  1. Use intensive recommendations for heavily used or intensively managed turf such as sports turf, or golf greens and tees.

See Table 12 for a listing of natural organic nutrient sources containing K.