Potassium and Phosphorus Deficiency

I get pictures of people’s lawns all the time. Now that we are in June, I knew I would start getting pictures of those lawns struggling to green up. Dormancy was clearly no longer the explanation. How did I know? The grass on the right was existing grass and the grass on the left was new turf from a local sod farm that operates with a strict fertility program.

Of course I requested a soil sample. The results? The visual stressors appear to be a dormant style grass but the scientific results diagnosed a potassium and phosphorus deficiency. The pH was 7.2 and is on the cusp of being high but will hold that for monitoring. I did suggest we address this problem with fertilizer and a P & K booster. The takeaway point, a soil sample not only diagnoses the deficiency but recommends the right amount for remediation. No guessing and performing a lab-based soil test provides a prescriptive approach that works. 

Let’s break apart P & K to better understand their roles in lawn health. Let’s start with Phosphorus (P). P is the second number in a fertilizer mix N-P-K and it is the most heavily regulated nutrient in our fertilizers. Most states have implemented a no phosphorus fertilizer regulation or encourage soil testing before applying phosphorus. There are few exceptions such as starter fertilizer for new turf installations that allows the new grass to get an extra boost during root establishment. When phosphorus is applied and there is already enough for optimal plant uptake, the over abundance will run off in our watershed and cause pollution and algae bloom, which is very harmful. Notwithstanding phosphorous is a critical component, and if your soil sample comes back with low levels, it is imperative to apply just the right amount for plant health. Now that you know the risk of the over application of phosphorus, let’s discuss the effects of phosphorus deficiency. 

Phosphorus is usually recognized as an essential element for root health, and ultimately it is responsible for overall plant health, and when not addressed, plants will enter stress conditions, becoming susceptible to pests and diseases and ultimately death. In between soil testing recommendations (at least annually according to the USDA), one way to naturally recycle phosphorus is to leave grass clippings after mowing. Purdue Extension recommends, “When turf-grass clippings are regularly recycled during mowing, phosphorus is recycled on-site and the need for applying phosphorus fertilizer may decrease slightly over time. By contrast, when clippings are regularly removed from these areas, then the turf can become phosphorus deficient and require supplemental phosphorus fertilizer.” Keep those clippings and closely monitor your phosphorus use – not only does your lawn need it but our water quality depends on it as well. 

Potassium is the third number in the fertilizer combination N-P-K and most people recognize potassium as a winterizer preparation. While that is a best practice, it is also the nutrient that the grasses use the most. When potassium is at the right level, it is protecting the grass from drought stress, winter injury, and diseases. So although it does have a very important role for winter hardiness, it also keeps your lawn strong against drought conditions and diseases for the other times of the year. Potassium also helps with your lawn’s turgor pressure in the cell walls. Turgor pressure is water pressure within the cell walls. As the potassium deficiency increases, the failing turgor pressure will cause the plant to wilt and fall over. 

Why is potassium less regulated in fertilizers among states? According to Penn State Extension, “Although it [potassium] is readily leached into groundwater, potassium is not a major pollutant in surface water and groundwater in the United States. It rarely is present in concentrations toxic to people or aquatic life, and it does not deplete water of oxygen.“ There is less concern with harmful environment affects and since not all potassium levels in the soils are readily available, a professional, lab-based soil test is the answer. 

Next time your lawn just seems to be “off,” it might be showing you the beginning of a larger problem underneath where all the health begins…in the soil.

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