Zero! Except after a scheduled aeration. I exaggerate but that is indeed how much I thought I needed to apply in the early days of my career. Phosphorous is bad, it makes the plant puffy and the leaf soft. (Sigh) Don't really know what I was thinking back then but I am so happy I have now educated myself and know some great people that are so willing to give their time to answer my questions.
So it has been around 8 months since we started following the MLSN guidelines and we have been very happy with the results, not just from a turf performance point of view but from the financial side as well! It is easy to think you are now an expert at this point, growing great turf and saving the club a lot of money which can be spent elsewhere but it is easy to make mistakes as we all know. I will now share with you some of mine, so that you may learn from them as I did.
One particular point I had overlooked with our new approach was the relationship between the MLSN guidelines, soil pH and the testing methods of the laboratory. The particular lab we use for our soil fertility analysis uses the Ammonium Acetate method for the Ca, K, Mg, Na and for Phosphorus they use the Bray method for our Fairways as the pH is below 7.5 (most likely due to the amount of ammonium sulphate we apply) and the Olsen method for our greens and tees as the pH is above 7.5. This is important as in some cases the figures vary greatly which will impact on your turf performance.
Another oversight was while I was having no issues with the calculations to determine the amount of an element I needed to apply and the forecasted use of the element by the plant, I could not understand why my forecasted ppm value for P was so far off the actual P quantity in the soil test reports. I then realised my mistake, which some may call amateur but what I realised was that I was using the Phosphorus quantities on the fertiliser bag (P2O5) instead of actual P to make the calculations and once I adjusted the equation, my forecasted figure was less than 1ppm from the actual result of the soil analysis! Read this blog post from Micah for more info regarding the P2O5 conversion.
Now back to the question at hand. How much phosphorus do you actually need? Well it largely depends on the lab testing method and your pH. For me, I only need to stay above 6ppm in my greens and tees and 30ppm in our fairways. Also, you should always remember that the MLSN Guidelines are in fact only guidelines and the figures quoted below are the minimum amount required for the plant, you should always aim to be above that minimum.
Some of you may be asking "What about root growth?". Well I can only tell you from my experience following the MLSN over the past 8 months is that we have seen a steady increase in root depth over the last 12 months which I attribute to suppling sufficient amounts of Phosphorus (no visual P deficiencies observed), our cultural program and our heavy focus on maintaining a consistent daily volumetric water content of between 15 to 22% in our greens.