## Using Waterfall Charts to Look at Soil Nutrient Levels: potassium as an example

##### 19 March 2013

I have recently described a method that can be used to estimate nutrient requirements for any turfgrass, grown at any location. This method makes use of the temperature-based growth potential and the minimum levels for sustainable nutrition (MLSN) guidelines.

This method makes a few basic assumptions about turfgrass management.

- The soil can hold a fixed amount of nutrients; it cannot hold an unlimited amount.
- The plant takes up a limited amount of nutrients; the plant does not take up an unlimited amount.
- Application of more nutrients than the soil can hold, or than the plant can take up, provides no benefit to the grass, nor to the soil, and is in fact inherently wasteful.

This is essentially a mass balance approach, in which we consider the rootzone and the turfgrass as a system and we account for the nutrients that enter and leave that system. We can use a waterfall chart to visualize the individual and cumulative effects of the nutrient addition and the nutrient loss from our turfgrass and rootzone system.

This type of chart is described on Wikipedia as:

The waterfall chart is normally used for understanding how an initial value is affected by a series of intermediate positive or negative values. Usually the initial and the final values are represented by whole columns, while the intermediate values are denoted by floating columns. The columns are color-coded for distinguishing between positive and negative values.

Let's look at potassium (K) as an example, developing the chart step-by-step. I first make a frame for the chart. I've labeled the y-axis with K expressed in units of ppm (mg/kg).

Next, I draw a horizontal line, in blue.

This line is at 35 ppm, which is the MLSN guideline level for K. What the MLSN guideline means, in simple terms, is that we have a high level of confidence that turf will perform well and will have access to ample amounts of an element, as long as the soil level remains at or above the MLSN guideline. For K, we are confident that the turf will perform well and will have access to ample K when the soil K, as measured by the Mehlich 3 soil test extractant, is at or above 35 ppm.

Then I add on a column to represent the amount of K actually in the soil, as determined by a soil nutrient analysis. In this example case, the soil test value is 70 ppm. This is a typical value for Mehlich 3 K in a sand rootzone. Note that this is more than the 35 ppm MLSN guideline, and we consider this soil test level as our starting point. Any positive amount of an element, or an addition of that element to the system, I will represent with a black color. Any loss of an element from the system will be represented in red.

Next I add a column for annual plant uptake. This is based on the assumption that leaf clippings will be collected and removed from the system. This removes whatever elements are in the leaf clippings, including K. We can note a few things here:

- The color of the "Annual Plant Uptake" bar is red, meaning this is a loss of K from the system.
- ‒54 is shown at the bottom of the bar, which indicates 54 ppm of K is estimated to be lost from this system by annual plant uptake.
- 54 ppm is equivalent to 8 g/m
^{2}(or 1.6 lb./1000 ft^{2}) when expressed on a mass per area basis, and this assumes average rootzone depth of 10 cm and a soil bulk density of 1.5 g/cm^{3}. - This amount of annual uptake and removal from the system is what we would expect when average leaf K content on a dry matter basis is 2% and when the annual dry matter clipping harvest is 400 g/m
^{2}. This would be typical of creeping bentgrass greens in a climate such as Tokyo or New York. - Notice that if we simply combine the soil test level at the start of the season, which is 70 ppm, and then account for the annual plant uptake, wich is estimated to be 54 ppm, then we drop the amount of K in the system to less than the MLSN guideline, all the way down to 16 ppm.

But we don't want the K to drop below the MLSN guideline. To prevent this, some K will be applied as fertilizer. Let's look at the chart now that the "Fertilizer Applied" column has been added.

- The "Fertilizer Applied" column is in black, indicating that this is an addition of K to the system.
- The amount added, 67 ppm, is equivalent to 10 g/m
^{2}(or 2 lbs/1000 ft^{2}). This assumes the K is added to the top 10 cm of the rootzone and that the soil has a bulk density of 1.5 g/cm^{3}. - Notice that the
**Soil Test**column has a length of 70, the**Annual Plant Uptake**column has a length of 54, and the**Fertilizer Applied**column has a length of 67. This is useful in comparing the magnitude of the amounts in the system. The amount of**Fertilizer Applied**is greater than the amount of**Annual Plant Uptake**, and the**Fertilizer Applied**is similar in amount to the level of K in the**Soil Test**at the beginning of the season. - The amount of K in the system, because of the K addition through fertilizer, has now increased to above the MLSN guideline of 35 ppm.

To complete the chart, I add a final column, to show the amount of K "Remaining in Soil". This column has a length of 83, showing that there is 83 ppm of K in the system. This is 70 minus 54 plus 67. And it indicates that when we have a starting level of 70 ppm of K in the soil, with a harvest through plant uptake of 54 ppm, and a fertilizer addition of 67 ppm, then we expect to have 83 ppm of K remaining in the system (in the soil) and that this level is well above the MLSN guideline.

I find this to be an interesting graphical approach to depicting soil nutrient levels and their relationship to the MLSN guidelines. If we would look at nitrogen, or calcium, or magnesium, or phosphorus, using this approach, we can understand more clearly how the amount in the soil is related to the plant uptake, and how that helps to determine the annual nutrient requirement as fertilizer.

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