## Putting green nutrient use and requirements

This information supplements the presentation made on this topic as a GCSAA webcast on 24 October 2013.

For general information about turfgrass management, see Viridescent, the Asian Turfgrass Center blog. Follow Micah Woods on Twitter @asianturfgrass for turfgrass photos, links to interesting articles, and new information about turf management.

### Nutrient Requirement Overview

An excellent introduction to the relative importance of soil physical, chemical, and biological conditions, along with an introduction to this topic, is given in a video by Dr. Doug Soldat from the University of Wisconsin Department of Soil Science. His GCSAA webcast on Managing Soil to Maximize Plant Health provides practical information while putting these various aspects of soil management into context.

When trying to identify turf nutrient requirements, we deal with a persistent problem. The conventional soil nutrient guidelines are too high. Thus, we find high performance turfgrass growing in soils classifed as low in essential nutrients by the conventional guidelines.

From 2001 to 2006 I studied at Cornell University with Dr. Frank Rossi. The focus of my research was nutrient availability to turfgrass in sand rootzones. In my research, and in other recent research, there is a common theme.

Here are a few of the quotes I shared in the presentation, along with links to the full papers:

“The current target ranges of extractable K in sand rootzones promote K fertilizer applications that may be detrimental to turfgrass performance.”

“There were no observed changes in shoot and root growth in response to K fertilization even at low soil test K levels.”

Ebdon et al., 2013. Crop Science. Long-term Effects of Nitrogen and Potassium Fertilization on Perennial Ryegrass Turf.

“The results of this intensive 4-year investigation do not suggest that additional K fertilization is beneficial for bermudagrass quality or clipping yield when elevated soil Na is present.”

Cisar et al., 2013. International Turfgrass Society Research Journal. Potassium and sodium application effects on soil-test values, and turfgrass quality, clipping yield and elemental composition of bermudagrass grown in a sand soil.

### How should we think about nutrient use and plant requirements?

General Principles:

1. Nitrogen controls growth and nutrient uptake.
2. If there is enough of an element in the soil, none is required as fertilizer.

These articles were also quoted in the presentation:

“N supply was the primary determinant of turfgrass growth rate, plant nutrient demand, and nutrient uptake. Nitrogen uptake accounted for over 88% of uptake of all other nutrients. Uptake of P and K were strongly related to tissue N content irrespective of soil test levels.”

Kussow et al., 2012. ISRN Agronomy. Evidence, Regulation, and Consequences of Nitrogen-driven Nutrient Demand by Turfgrass.

“Nitrogen is the primary plant food you are seeking. All the other nutrients, while important, may simply be unnecessary, especially if soil tests do not show a deficiency for that nutrient. Why use and pay for nutrients that the grass does not need or the soil does not require?”

Zontek et al., 2010. USGA Green Section Record. Does the Grass Know the Cost?

### Growth potential and its relation to nitrogen use

The temperature-based growth potential was developed by PACE Turf. The growth potential gives a number from 0 to 1 (or from 0 to 100%), indicating how close the temperature is to the optimum photosynthetic temperature.

Article from 2005 Golf Course Management explaining growth potential

This is the growth potential equation

\begin{aligned} GP = e^{-0.5(\frac{t-t_o}{var})^2} \end{aligned}

where,

GP = growth potential, on a scale of 0 to 1
e = 2.71828, the base of the natural logarithm
t = actual temperature
to = optimum temperature
var = adjusts the change in GP as temperature moves away from to

That looks a bit complicated, but it is easily calculated in a spreadsheet. PACE Turf have prepared just that, in the climate appraisal form of their PACE IPM planning tools.

Note that this information is also available from GCSAA as part of their IPM Planning Guide in the Climate Appraisal section.

I've written about growth potential, the most detailed version being Using temperature to predict turfgrass growth potential (GP) and to estimate turfgrass nitrogen use.

### The Minimum Levels for Sustainable Nutrition (MLSN) guidelines

Because nitrogen controls the growth and uptake of the other elements, it is critical to supply the right amount of nitrogen to the grass. The growth potential, as described in the documents above, provides an estimate of nitrogen requirements.

For the other elements, the MLSN guidelines have been developed to provide guidance on fertilizer requirements. The MLSN guidelines ensure that soil nutrient levels remain high enough to produce excellent turfgrass conditions. Simply make sure that the the selected element is at or above the MLSN guideline, and you can have a high confidence that there will be enough of that element availble to the grass to produce high performance turfgrass.

The MLSN guidelines were developed as a join project between PACE Turf and the Asian Turfgrass Center.

MLSN project page and current guidelines

The MLSN guidelines are updated continually, through the Global Soil Survey citizen science initiative. The Global Soil Survey for Sustainable Turf serves to validate and refine the MLSN guidelines, improving the way we fertilize turfgrass around the world.

Sustainable Soil: Going global, article in GCM

Through some simple mass balance calculations, we can determine how much fertilizer, on an element by element basis, is required. This is done by estimating the harvest of clippings, which contain nutrients, measuring how much of an element is present in the soil, and determining if the element will remain at or above the MLSN guideline with that estimated nutrient harvest through grass clippings.

An example of the calculations was given in the webcast. I've written about the necessary calculations in a number of documents, and for further reading and examples, see:

Understanding Turfgrass Nutrient Requirements

Nutrient Requirements of Tropical Turfgrass

What Fertilizer Should I Use?

A Method for Estimating Turfgrass Nutrient Requirements