How Much Phosphorus Do You Actually Need?

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.

The MLSN Guidelines. Figures are in ppm.

The MLSN Guidelines. Figures are in ppm.

The Mehlich 3 figures are sourced from the latest MLSN Guidelines from 2014 while the Bray and Olsen figures are sourced from the 2012 MLSN update.As for the Ammonium Acetate figures, I will leave it to the always helpful Micah Woods to explain from a recent email conversation below

For K, Ca, and Mg, I expect ammonium acetate is used by the lab for every
pH. So you can make an estimate by using conversion equations such as

I think there may be a little bit of trickiness involved with that,
because those units are lbs per acre, and the MLSN guidelines are in ppm
and we make those recommendations for a 4” (10 cm) soil depth. This is all
understandable once one works through it, I assure you.

From the Cornell equation, for K, it is written as:

(M3-K in ppm * 0.84) x 2 = Ammoniumacetate K in lbs/acre

If you have ammonium acetate K in ppm already, that means the lab already
divided the lbs/acre by 2. So to convert to estimated values of Mehlich 3
K, you take your potassium (K ppm) and divide in by 0.84. That gives you
the estimated K ppm if it was done by Mehlich 3.

And that same process could be done for Ca and Mg.

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. 

Now while these results may not be staggering, they are realistic.

If you are new to the MLSN Guidelines and want to know more or perhaps how to start at your facility, you can see how we started the journey in this previous post and for more information on the calculations check out this post.