Understanding Soil P & K

Nitrogen (N), phosphorus (P), and potassium (K) are among nutrients essential for plant growth and are the three most added via fertilizer. While nitrogen typically is added each season, phosphorus and potassium may or may not need to be added and are often misunderstood.

Plants visibly respond to nitrogen with green color and vegetative growth. Phosphorus is key in root development, flowering, and fruiting of plants. Potassium helps improve stress tolerances of plants, such as temperature extremes, drought, and disease resistance, and works with other nutrients in essential growth processes. Nitrogen is constantly changing form and very unstable in soils, whereas phosphorus is slow to change and extremely stable. Potassium falls in-between.

Compared to nitrogen, phosphorus and potassium deficiencies are much more difficult to determine by looking for visible plant symptoms. Accurate levels of available phosphorus and potassium in soils can be determined by soil testing. Soil test results will indicate these levels, and, if needed, proper amounts of fertilizer to add.

Use soil test results to apply proper rates for plants being grown and to avoid surpluses of P and K in yard and garden soils. Elevated levels of one nutrient impact the availability of other nutrients to plants. In addition, excessive phosphorus is a major contributing factor to algae problems in lakes and ponds. Besides misapplied fertilizer, potential sources of phosphorus in waterways include both soil and organic matter washing into lakes and failing septic systems.

Wisconsin law restricts use of phosphorus on lawns to only certain situations based on soil tests. Research has shown, however, that phosphorus fertilizer properly applied directly to lawns does not move off target as often suspected. Pay close attention when fertilizing lawns and gardens to avoid off-target fertilizer application, such as fertilizer landing on paved driveways, sidewalks, or streets, as it could readily move with rainfall and end up in waterways.

Rock phosphate is a primary phosphorus fertilizer source but is extremely slow in releasing a form available for plant uptake. For this reason, sulfuric acid combined with rock phosphate creates faster-acting superphosphate or triple superphosphate found in fertilizer products. Likewise, ammonia and phosphoric acid are combined to create ammonium phosphate also used in fertilizers.

Potassium sulfate, which is mined and ground rock, is a popular potassium source for fertilizers. Potassium chloride, sometimes listed as muriate of potash, is also widely available. Many garden fertilizer products contain potassium.

Spend time determining fertilizer needs of your yard and garden plants for this coming season. Soil test results provide accurate levels of phosphorus and potassium, along with soil pH. Sound adjustments can then be made accordingly.