Turf & Rec

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Determining the right amount of fertilizer

March 13, 2015  By  Mike Jiggens

How much fertilizer should one apply? “When it comes to fertilizer, there are two questions we need to answer," Dr. Micah Woods, chief scientist at the Asian Turfgrass Center and assistant adjunct professor at the University of Tennessee, said in January at the Ontario Golf Superintendents Association’s conference and trade show in Niagara Falls.

The first question, he said, is to ask whether an element as fertilizer is needed or not. Second, if something is needed, how much should be applied?


Woods said most will agree that if there is enough of an element already in the soil, adding more won’t be necessary. But if there is a deficiency of that element, more will have to be added to ensure there is enough.

The focus of Woods’ presentation was to estimate how much of an element is required. For example, “do we or do we not need to add potassium, and how much of it do we need to apply. The goal is to make sure we never have any nutrient deficiencies.”

To answer those questions, Woods said three quantities need to be estimated. One is the amount of an element the turfgrass will use. Second is the reserve amount in the soil. Third is the amount of an element that is actually present.

“It turns out that from these three numbers, we can determine if we have to apply fertilizer and, if so, how much is required as fertilizer?”

There is a tendency to think of fertilizer in a two-dimensional way, he said, suggesting turf managers think of a turfgrass surface as being so many square metres in size, and fertilizer is applied accordingly.

“For me, that’s a two-dimensional way of thinking.”

Soil, however, is three-dimensional, Woods said, adding when soil is tested, its volume is being measured. It’s not a flat plane, he said, but rather a volume that is as deep as the root zone. The two-dimensional and three-dimensional spaces are connected.

“If we can convert from one way of thinking to another, we find that all those numbers are related and we can easily determine if something is required as fertilizer or not.”

The amount of fertilizer the turfgrass uses can be called “A” while the amount required in the soil—the amount that cannot be allowed to drop below—is “B.” The amount the grass is actually using plus the amount needed as a reserve in the soil is A plus B. The amount already in the soil is “C.” A plus B minus C is the difference between the amount needed and the amount present, Woods said.

If the amount present is more than the amount needed, none of that element is required as fertilizer, he said. For example, the soil potassium is 1,000 parts per million, which is a high level, and over the course of a year the grass plant uses 100 parts per million. The MLSN guideline for potassium is 37 parts per million which means the turf manager never wants to soil to drop below that amount. He will always want that much to be in the soil as a reserve. When the amount used is 100 parts per million and the amount required in the soil is 37 parts per million, the amount needed in this hypothetical situation is 137 parts per million.

Woods said if the soil actually has 1,000 parts per million potassium, the amount present is much more than the amount needed, meaning it doesn’t need to be applied as fertilizer. Yet if the amount present is smaller than the amount needed, the difference between them is the minimum amount of fertilizer required to ensure the soil doesn’t drop too low in that element, running the risk of the plant not being able to get what it needs.

He imagined another situation in which there is a need for 137 parts per million potassium and the soil test potassium was a more normal level of about 50 parts per million. In such a case, the amount needed is more than twice the amount present.

“For sure we need to supply that potassium as fertilizer in that situation.”

A question many will pose, Woods said, is what if there was potassium in the soil, but it wasn’t available to the plant? He said it’s a common question regarding nutrient availability and the presence of an element. That’s why soil testing is done, he said.

Soil tests are calibrated to the specific soil type so that they are measuring a meaningful amount of an element. If a soil test shows there are 40 parts per million of potassium, there is no need to worry about the availability or the non-availability, Woods said. Turf managers are looking at the number from the soil test as a nutrient availability index.

“So the soil test itself is not giving us the total amount of potassium in the soil. The soil test is already incorporating some estimate of availability because we compare the soil test results with so many other soils in how the turfgrasses perform in those soils.”

Woods said to imagine a sand-based putting green from which the clippings are removed and nitrogen is never needed to be applied. It’s grown in for two years, is perfect, nitrogen is discontinued and clippings are collected. Within a year or two, there likely won’t be any grass after a while because in a sand root zone without a lot of organic matter, grass in a soil that is low in nitrogen will eventually stop growing if clippings continue to be removed, he said.

The turf manager can restrict the growth of grass by withholding nitrogen. Grass will always respond to nitrogen by turning a little greener and growing a little faster.

“We think of nitrogen as being the driving force behind how much grass will grow.”

Nitrogen use by the grass plant is not 100 per cent efficient. If one gram of nitrogen per square metre is applied and the amount taken up by the plant is tracked as well as the amount escaping into the atmosphere, the amount remaining in the soil and the amount leached below the root zone, the uptake efficiency of the nitrogen applied can try to be measured.

“Even when they radioactively label it, they can’t even find where it all goes. There’s still some part missing.”

It could be that 50 per cent is found in the plant, 20 per cent in the soil, 10 per cent in the atmosphere and 5 per cent is leached, and the remainder may not be found anywhere, Woods said.

He said that when we think of nitrogen applied to turf, it should be recognized that not 100 per cent of that is actually going to the plant.
“We should do our fertilizer applications in a way—choose products and timings and application methods—that will make those applications as efficient as possible. Even if we do that, we can’t get 100 per cent uptake.”

Woods said he likes to make a “conservative” estimate, defining conservative as trying to ensure there is not a deficiency.

“We’re erring on the side of safety.”

The system he recommends tends to use less fertilizer than is commonly applied. The ways of making these calculations are chosen in an effort to be conservative in not being too aggressive. He said we know 100 per cent of nitrogen isn’t taken up by the plant, but, if we assume it is, it becomes conservative in the way we’re estimating clipping yield.

Woods said there are a couple of assumptions about nitrogen. The more applied, the more growth, and there will always be the assumption healthy turf is desired. Healthy turf will always contain a certain amount of nitrogen. Once the amount of nitrogen applied is considered as well as the amount of nitrogen in healthy grass, a simple calculation can be made to estimate clipping yield. This allows the uptake and the use of having a single nutrient to be estimated.

For bentgrass leaves, he said, there will be about 4 per cent nitrogen as dry weight and about 2 per cent potassium as dry weight, suggesting this defines healthy turf. If nitrogen is 4 per cent, it means that in 100 grams of dry clippings, there are four grams of nitrogen. The first quantity necessary to estimate to determine if a nutrient is required as fertilizer is the amount the grass uses. We never actually measure the amount the grasses uses, however, because no one collects all his clippings and measures what is in them.

But that can be conservatively estimated by realizing the grass doesn’t use all the nitrogen applied, and the maximum growth at 4 per cent nitrogen would be the amount of nitrogen applied divided by 4 per cent. That gives the maximum amount of clipping yield one can have.
“It’s conservative because we actually won’t have that much clipping yield.”

Woods said this slightly overestimates how much potassium, phosphorus, calcium, magnesium and other elements would be used.

“That’s what I mean when I say conservative.”

As an example, four pounds of nitrogen per 1,000 square feet are applied in a year, but the grass won’t use that much nitrogen. It will be slightly less because the uptake is not 100 per cent efficient. The healthy grass that has grown up, he said, may impact about 4 per cent nitrogen in it. He said to take that amount of fertilizer applied, divide it by .04, and it gives the dry clipping yield for that area.

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