CSU Extension Master Gardeners Notes 711 explains when Phosphorus and Potassium application are needed. Continue reading for details.
A soil test is the best method to determine the need for phosphate and potash. With a fertilizer containing nitrogen and phosphate and/or potash, the application rate is always based on the nitrogen percentage because nitrogen is most critical to plant growth.
Phosphate and potash fertilizers are best applied in the spring or fall, when they can be tilled into the soil
Phosphate levels are adequate in the majority of established Colorado gardens. Deficiencies are most likely to occur in new gardens where the organic matter content is low and in soils with a high pH (7.8 to 8.3). Excessive phosphorus fertilizer can aggravate iron and zinc deficiencies and increase soil salt content. Routine application of compost or manure will supply the phosphorus needs in most garden soils in Colorado.
Where phosphorus levels are believed to be low, the standard application rate without a soil test is ¼ to 1-pound triple super phosphate (0-46-0) or ammonium phosphate (18-46-0) per 100 square feet.
Potassium levels are naturally adequate to high in most Colorado soils. Deficiencies occasionally occur in new gardens low in organic matter and in sandy soils low in organic matter. Excessive potash fertilizer can increase soil salt content.
Routine applications of compost or manure will supply the potassium needs for most garden soils in Colorado.
Where potash levels are believed to be low, the standard application rate without a soil test is ¼ to ½ pound potassium chloride (0-0-60) or potassium sulfate (0-0-50) per 100 square feet.