One doesn't need to apply supplementary potassium (K) after a rain, as I wrote in part 1 of this series, because such applications will invariably lead to application of way more K than the grass can use. In part 2, I showed a calculator that makes an estimate of how much K is reasonable to apply as fertilizer, based on how much K the grass will use.
Looking at this with soil test data, these four charts show what happens to K in the soil over time.
In 2002, I applied nitrogen (N) and K every two weeks to L-93 creeping bentgrass maintained as a putting green in Ithaca, New York. I collected soil samples every eight weeks. This summarizes what happened during the summer of 2002.
At the start of the experiment, before applying any N or K, the Mehlich 3 K was 86 ppm, and the water extractable K was 8.3 ppm. I've added a horizontal line at each of those levels, to indicate what the starting level of soil K was in this experiment.
Then the treatments started, N and K every 14 days. When no K was applied, what happened? The soil K went down. That is to be expected, because the grass uses K, so when the grass is growing one expects the soil K to go down if no K fertilizer is added.
What happened when a moderate amount of K was added? Over these 16 weeks in the summer of 2002, I applied 12 g N m-2, and the K rate supplying 13 g K m-2 in that time is close to a 1:1 ratio. From June to July, the soil K went up at that rate, because that is slightly more K added as fertilizer than the grass can use. Then from July to September, the soil K in plots supplied with the 1:1 ratio went right back to where they started the summer. The reason for the decrease is discussed below.
What happens when the K applied is way more than the grass can use? The highest rate in the experiment supplied 50 g K m-2 over this time period, roughly a 1:4 ratio of N to K. And the soil test levels went up, because when one supplies a lot more K than the grass can use, that's what happens.
Why was the soil K higher in late July than in September? That is because the irrigation of this area was increased in August, and the rain + irrigation from the end of July to the time the samples were collected in September was double the evapotranspiration (ET). From the start of the experiment until the samples were collected in late July, the rain + irrigation was just slightly higher than the ET.
The grass performance was good in all the plots, and equally good no matter if no K was applied, if a moderate rate of K was applied, or if the highest rate of K was applied.
There were six rain events with > 25 mm (> 1 inch) of rain during these 16 weeks. Adding K after rain would have accomplished nothing, other than supplying even more K than the grass would use, and supplying K that would mostly be leached out sometime in the future. By supplying the amount of K the grass uses, one can maintain a pretty stable level of soil K. Of course if the soil K is well above the MLSN guideline, then no K is needed at all, because the grass can get all the K it needs from the soil.
For more details about the experiment, see this paper from Soil Science.