This spring I took part in the Global Soil Survey for the 3rd time. I did my soil tests in early April 2015 before the grass started growing so that I would know what fertilizer I would have to add in order to keep my soil levels above the MLSN guidelines.
As you can see in the top 2 rows of the above table, the soil test results were all above the MLSN guidelines except for Potassium.
I was then able to estimate the amount of each nutrient that the plant would use based on the amount of nitrogen applied. Where K was half, P 1/8, Ca 1/8, Mg 1/20, and S 1/40 the amount of nitrogen applied. Now this assumes that all the clippings were removed, which they more or less were. We always collect clippings on our greens.
I was then able to compare the amount of nutrients used by the plant to the amount added as fertilizer. I then converted this number from g/m2 to ppm by multiplying the mass by 6.7 to give me a net gain or loss. As you can see, in 2015 there was a net loss of P,Ca and MG simply because I applied very little or no amount of most nutrients. This is partially why I was able to make such a decrease in the amount of fertilizer applied this year.
You can also see that I applied exactly the amount of K as the plant used. This left me with a 2 ppm deficit as I started the year with a 2 ppm deficit. Clearly I should probably add more K. Even though I am theoretically below the MLSN guidelines, my greens are still in great shape. I will be making a light K application the next time I take my sprayer out just to be sure that K levels stay at a safe level over the winter.
I thought it would be fun to see how long the supply of each nutrient in the soil would last me while still staying above the MLSN guidelines. This assumes that the amount of fertilizer I applied was all held in the soil. We know this isn't the case and I will demonstrate that next. Either way, I theoretically have a 21 year supply of P, 14 year supply of Ca, 11 year supply of Mg and a 243 year supply of S. I applied a lot of S, and I have a hunch that if I tested my soil right now it wouldn't be much over 15ppm as most of it likely leaches or is wasted as far a fertilizer is concerned.
Now all these numbers except for the soil tests are theoretical. Is there a way that I could compare them to actual numbers?
In 2014 I did the GSS soil test in early June. Here were the results of that.
I then used the amount of fertilizer added between the dates of the two soil tests and was able to make a prediction of the amount of nutrient used.
I was then able to compare the theoretical deficit during the winter of 2014/2015 to the actual numbers tested in April 2015. I compared how much it should have gone down according to this math vs how much it went down according to the test.
As you can see the estimated difference for K was 7.8 ppm less than the actual difference. That's a difference of about 1.2g/m2. Why would its loss be lower than anticipated? Probably because some of my nitrogen applications were poorly timed, essentially being wasted therefore not contributing to turf growth in a way that increased the use of potassium. Or it could just be the margin of error by using math to explain an incredibly complex natural process with many variables.
This math estimated the Mg change almost perfectly. It was way off with Ca and S though. Now why would this math be way off? Because it assumes that that soil can hold onto whatever you apply. It also assumes that whatever I apply actually enters the soil! This is a great way to illustrate waste.
The math estimated that the amount of sulfur in the soil would go up by 87.94 ppm. This is because I applied a lot of sulfur. In reality, it actually went down! Where did all this sulfur go? Probably leached.
I also estimated that the amount of Ca would go up by 28 ppm when it actually went down by 17 ppm. Again, there is a huge loss here which means one thing. Waste. I am basically throwing my money away if the calcium applied was intended to go into the soil for the plant to use. Maybe the calcium applied was taken up by the plant leaves? I doubt that because it was applied as 2 heavy granular gypsum applications. I bet most of it was either leached or washed off the surface of the turf or picked up by the mowers. Gone. Either way, it was not held by the soil.
So with all the waste in 2014 I set out to reduce that waste in 2015. This is why I applied so little fertilizer. If it was in the soil, it would make no difference to the plant, and the soil tests if I applied that type of fertilizer. This is the basic concept with the MLSN guidelines. Only apply fertilizer if it is needed. With this math, you can determine how much you will need to stay above the guidelines and ensure that your grass has all the nutrients it needs to be healthy and awesome.
In 2015 I still applied lots of S as it is a component of some of the fertilizer I apply such as ammonium sulfate and potassium sulfate. Therefore I get it whether I need it or not. The fact is that I like the S but not from a fertilizer standpoint. I like it because of its acidifying and anti-fungal properties.
So there you have it. A useful way to look at where the fertilizer you apply goes and if it actually makes a difference. A single soil test is a useful snapshot to see what is currently there, but comparing soil tests over the years with theoretical plant uptake numbers can help you understand how efficient your fertilizer practices really are. Eliminate waste, save money and reduce the potential impacts your fertilizer use has on the surrounding environment.