Two of the most important macronutrients for plant growth and development are potassium and phosphorus. A fertilizer that has both of these important nutrients can really help plants grow by making sure they get enough potassium and phosphorus. This guide will tell you everything you need to know about fertilizer that has phosphorus and potassium in it.
The Vital Roles of Potassium and Phosphorus in Plants
Potassium and phosphorus serve a range of critical functions in plants
Potassium
- Regulates water balance and transpiration
- Activates enzymes and aids in photosynthesis
- Promotes root growth and nutrient uptake
- Strengthens stems and increases disease resistance
- Improves quality and shelf life of fruits and vegetables
Phosphorus
- Essential for cell division and development of new tissue
- Promotes root, flower, fruit, and seed development
- Improves flowering and fruiting
- Boosts seed production
- Enhances maturity and hardiness
Deficiencies in either potassium or phosphorus can lead to a range of issues such as weak stems, stunted growth, poor flowering and fruiting, increased disease susceptibility, and reduced yields.
Selecting the Right Potassium and Phosphorus Fertilizer Ratio
Fertilizers will have a three number ratio on the package that represents the percentage of nitrogen (N), phosphorus (P2O5) and potassium (K2O).
For example, a 10-20-10 fertilizer contains:
- 10% nitrogen
- 20% phosphorus pentoxide
- 10% potassium oxide
To add more potassium and phosphorus than nitrogen, look for ingredients with higher P and K percentages, such as
- 5-15-30
- 8-25-15
- 10-30-20
Slow release fertilizers can provide longer-lasting nutrient availability.
Pick a ratio that fits the needs of your plants and the amount of nutrients in the soil. Soil tests help determine any potassium or phosphorus deficiencies.
Top Potassium and Phosphorus Fertilizer Options
Some high-quality fertilizer options rich in phosphorus and potassium include:
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Bone meal – Provides a slow release source of phosphorus and calcium.
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Rock phosphate – A mined mineral phosphate fertilizer with long-lasting phosphorus.
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Kelp meal – Supplies potassium, along with other nutrients and growth promoters.
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Greensand – Provides steady potassium release plus trace minerals.
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Blood meal – A fast-acting source of nitrogen and potassium.
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Bat guano – An excellent organic potassium fertilizer.
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Wood ash – Supplies potassium, phosphorus, and other nutrients.
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Cottonseed meal – A renewable source of phosphorus and potassium.
Always check labels for the NPK percentages and nutrient sources.
Proper Application Rates and Timing
Follow package instructions for applying potassium and phosphorus fertilizers:
- Apply at recommended rates for given area or volume
- Spread evenly over soil surface or mix into potting media
- Water after application to incorporate nutrients
- Reapply every 4-8 weeks during active growth
- Use lower doses more frequently instead of heavy individual doses
- Avoid overapplying which risks fertilizer burn
Target fertilization to periods of rapid growth and development. Use in gardens before planting and when plants are growing quickly. For lawns, fertilize during active growing seasons. Conducting soil tests helps determine optimal application rates and schedules.
The Benefits of Proper Potassium and Phosphorus Fertilization
With the right fertilizer ratio and proper application practices, potassium and phosphorus fertilization provides:
- Healthier root systems
- More robust vegetative growth
- Improved flowering and fruit production
- Higher quality fruits and vegetables
- Increased resistance to disease and stress
- Better plant hardiness and winter survival
- Higher crop yields
- More efficient water use
Supplying adequate potassium and phosphorus ensures plants can complete their entire lifecycle and reach their full genetic potential.
Key Takeaways on Fertilizer with Potassium and Phosphorus
- Potassium and phosphorus play vital roles in plant growth, development, and reproduction.
- Look for fertilizers with higher phosphorus and potassium percentages to provide extra levels of these nutrients.
- Quality organic and mineral sources include bone meal, rock phosphate, kelp meal, greensand, and others.
- Follow label instructions carefully for proper application rates and timing.
- Proper potassium and phosphorus fertilization promotes vigorous, healthy, high-yielding plants.
By understanding their roles and using the right fertilizer products, potassium and phosphorus can give your plants a major boost. Your fruits, vegetables, flowers and landscapes will thrive with a fertility program targeted to their potassium and phosphorus needs.
The phosphorus recommendation in this example is based on the phosphorus maintenance equation in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.
Phosphorus Maintenance Equation:
lb P2O5 /acre to apply = YP x CR
- Yield Potential (YP) in bushels per acre for corn grain.
- Crop Removal (CR) in pounds of P2O5 per bushel. Corn grain removal for phosphorus is 0. 37 pounds of P2O5 per bushel.
The equations for P buildup and maintenance recommendations are provided in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.
To develop the potassium (K) recommendation, use Figure 3 (Table 23) of the Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa. Locate the yield potential (top of the chart) and the soil test value (left side of the chart). For this example, the corn yield potential is 150 bushels/acre, and the K on the soil test is reported as 100 ppm, which is below the “optimal” range. The CEC of this soil is 10 meq/100g (Table 1). The K recommendation is 85 pounds of K2O per acre, which is based on crop removal plus additional K to build soil test levels to the critical level.
The potassium recommendation in this example is based on the potassium buildup equation in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.
Potassium Buildup Equation:
lb K2O/acre to apply = [(CL – STL) x (1 + (0.05 x CEC))] + (YP x CR) + 20
- At a CEC of 10, the Critical Level (CL) is 100 ppm.
- Soil Test Level (STL) in ppm
- Cation Exchange Capacity (CEC) in meq/100g
- Yield Potential (YP) in bushels per acre for corn grain
- Crop Removal (CR) in pounds of K2O per bushel. Crop removal is 0. 27 pounds of K2O per bushel of grain corn .
The equations for buildup and maintenance recommendations are provided in Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa.
Calculating the amount of fertilizer needed to meet the nutrient recommendation requires two additional steps.
The first step is to read the fertilizer label and locate the percentage of the target nutrient in the fertilizer source. This percentage is used to calculate the amount of fertilizer needed to meet the nutrient recommendation. Commercial fertilizer sources are required by law to give a guaranteed analysis of the nutrients claimed on the label. The first three values given on the fertilizer label are nitrogen, phosphorus, and potassium (N-P-K). A fourth number may be included for nutrients such as sulfur or other micronutrients that are guaranteed on the label.
Nitrogen (N) is shown as pounds of N per 100 pounds of product. Phosphorus (P) is shown in pounds of P2O5 per 100 pounds of product. Potassium (K) is shown in pounds of K2O per 100 pounds of product. For example, in Table 3 below, 11-52-0 is the fertilizer source needed to meet the phosphorus requirement, and 0-0-60 is the fertilizer source needed to meet the potassium requirement.
In the second step, the amount of fertilizer product to be applied needs to be calculated from the guaranteed analysis. Calculating the amount of nutrients applied in addition to the target nutrient is useful to understand their amount and potential impact on nutrient management. For example, the fertilizer 11-52-0 being used to meet phosphorus requirements also contains nitrogen. Pounds of fertilizer to apply per acre and the calculation of additional nutrients applied are shown in Table 3.
Table 3. (PDF Download) Nutrient and Fertilizer Recommendation Worksheet with Desirable pH and Nutrient Ranges. (Completed Example Report)
Tables for other crops are in Tri-State Fertilizer Recommendations for Corn, Soybean Wheat, and Alfalfa. Appendix A contains a blank template that can be used for your fields.
Appendix. (PDF Download) Nutrient and Fertilizer Recommendation Worksheet.
Culman, Steve, Anthony Fulford, James Camberato, Kurt Steinke, Laura Lindsey, Greg LaBarge, Harold Watters, et.al. 2020. Tri-State Fertilizer Recommendations for Corn, Soybeans, Wheat, and Alfalfa, Bulletin 974. PDF. extensionpubs.osu.edu/tri-state-fertilizer-recommendations-for-corn-soybean-wheat-and-alfalfa-pdf
LaBarge, Greg. 2022. “Interpreting a Soil Test Report” (AGF-0514). Ohioline, The Ohio State University. ohioline.osu.edu/factsheet/agf-0514
Mullen, Robert, Edwin Lentz, and Maurice Watson. 2016. “Soil Acidity and Liming for Agronomic Production” (AGF-505). Ohioline. The Ohio State University. ohioline.osu.edu/factsheet/AGF-505-07 Topics:
Don’t fall for high-phosphate fertilizers
FAQ
Which fertilizers are high in potassium and phosphorus?
You can use NPK(S) 8:20:30(2) since it is high in potassium and phosphorus and low in nitrogen. It is best for fall use. It is particularly valuable for crops that require large amounts of available phosphorus and potassium in the soil.
What fertiliser has phosphorus and potassium?
Phosplus is a highly concentrated Phosphorus and Potassium liquid fertiliser. It contains mono-and di-potassium salts of phosphorus acid (potassium phosphite).
What is a good source of both potassium and phosphorus?
Double jeopardy foods that are high in both potassium and phosphorus include dairy products, nuts, seeds, chocolate and some whole-grain foods. More phosphorus and potassium can be found in whole grain hot cereals like oatmeal and wheat flakes than in refined cereals.
How to add phosphorus and potassium to soil naturally?