0 Organic Matter - Moisture content

In part two of our organic matter series, we will discuss the impact of organic matter on moisture contents, and moisture contents on organic matter accumulation in our greens. We will also look at how this affects some of the management decisions we make when trying to manage both moisture and organic matter content in our greens.

Measuring moisture content and its significance

Through combining organic matter results with moisture content readings, either in combination with Loss on Ignition (LOI) testing or onsite testing, significant data and experience between the two measurements has been developed.

Through regular moisture testing (using a Fieldscout TDR 100), we have started to develop clear target moisture levels for course managers to work within, depending on each green's individual circumstance. For some, target moisture ranges are primarily driven by the target species content, such as fescues or bentgrass. For others of us, the moisture content of our greens is driven by desired greens firmness levels.

However, for many clubs, having any control of moisture contents in greens during wet periods, least of all, what we have experienced in 2012, can often be an element that falls outside our control.

The vast majority of clubs in wet periods spend large portions of the year with little or no control over moisture content in greens. This is often commonly attributed to rainfall alone but, with increasing amount of information being processed, we are finding that greens moisture content can be brought under better levels of control, despite weather experienced. The answer, for many greens, lies in its organic matter levels.

Default moisture setting

Determining the moisture content contained within the upper profile (0-6.5cm) during periods of perceived soil saturation can tell you a lot about the nature of your greens and, indeed, organic matter contained within them.

For many, it is often perceived that saturated greens during wet periods are largely a result of weather and, therefore, outside of our control or, similarly, put down to an underlying drainage problem. Perhaps, in some unique cases, this may be correct but, with ever increasing amounts of organic matter and moisture content results, we are finding moisture contents, although not completely controllable, can be managed better through creation of healthier soil profiles and lower organic matter content, giving lower year round average 'default' moisture contents. This has several agronomic advantages to course managers and their clubs.

The impact of organic matter on moisture content throughout the year

Depending on various organic matter levels, we are finding moisture contents to behave in various ways through the year. Using a typical UK climate - noting variability from north to south - the model (right) provides general classification of moisture behaviour in relationship with organic matter (LOI %) contained within the upper 0-40mm.

In contrast, where moisture contents are suitably low, the retentive nature of the upper profile is notably reduced and target moisture contents can be achieved for larger portions of the year.

Again, where organic matter is well above desirable ranges, moisture contents will spend large portions of the year outside of desirable moisture ranges and, indeed, out of our manageable control, leading to several negative agronomic characteristics.

Achieving low moisture contents with high organic matter

This model isn't to say we can't achieve low moisture contents with high organic matter. At points in the year, regardless of organic matter levels, sustaining low moisture contents can be achieved.

Managing moisture content at low levels (15-25%) with high organic matter (12%>) content is more than possible, but does come with very high risks and''knife-edge' management. Dry patch almost becomes a certainty when trying to drive moisture contents low with high organic matter, and intensive preventative management is required.

The depth of moisture retention

It is easy to generalise and the term soil moisture content in itself could be considered just that. Looking further into soil moisture, it can be further divided into soil moisture at bespoke depths, for example 0-30mm, 30-60mm, 60-100mm and so on.

On average, moisture content in the upper 0-30mm of the soil profile has been 20-40 times higher than that of moisture content at 40-60mm deep in the profile. Further decreases in moisture content at depth are commonly experienced until heavier silt/clay soils occur or, in USGA greens, no increase is experienced. It is not surprising to state this trend replicates itself in the presence of organic matter content, which generally decreases at 20mm depths in the soil profile.

This information alone, when applied to a course manager's own greens, can assist greatly in deciding the requirements of sand dressing material or products on greens, and at which depth in the soil profile they are required.

The impact of higher moisture content on organic matter accumulation

Throughout this article moisture contents have been referred to as a percentage figures. If we estimate 40-50% of the soil profile is solid mass (silt, sand, clay and organic matter), the rest is made up of water (moisture) or air to varying extents.

For decades we have been taught about air and its importance within the soil profile. The benefits of air in the soil include healthier plant rooting, freer exchange of nutrients and their availability and increased soil microbial activity.

Historically, getting air into the soil has been talked about in terms of machines and products that encourage air in the rootzone, with benefits often short-lived. Seldom have we heard aeration being talked about in terms of managing lower average moisture contents annually.

Saturated upper soil profiles

Where we have organic matter content that encourages excessively high average moisture levels, air levels within the soil profiles will be inherently lower for larger portions of the year. With lower average air content within the soil, we can expect less of the benefits of aeration.
Perhaps one of the key benefits of higher air content in the soil is natural breakdown of organic matter. The exact quantification of the amount of organic matter broken down by microbial content is difficult to estimate but, to help imagine its potential, simply watch grass clippings and leaf piles decrease in size.

Without air and the presence of excessively saturated rootzones, this breakdown in organic matter either doesn't occur or operates at a significantly reduced rate, further accelerating the rate of organic matter accumulation in the upper soil profile, or increasing the amount of dilution or physical removal required, at notable cost in both resource, time and lost golfing revenue.

As a result of this, it needs to be recognised that organic matter accumulation will vary significantly depending on rainfall, type of green construction, green shaping and capacity to shed water and the amount of organic matter already present. Some clubs will have to work harder at reducing organic matter than others in order to achieve its successful management, which will subsequently result in better and easier moisture management.

Getting to grips with our moisture contents

The first step to getting control of moisture content is knowing exactly where it is. This means having a monitoring programme in place as, without this, we can only make guesstimates as to its exact level, but this is insufficient in our changing economic and environmental climate.
Upon identification of higher annual moisture content than desirable, the next step is to determine whether drainage, rootzone and/or organic matter is the cause of this moisture retention.

Upon determination of excessive organic matter causing higher average moisture contents - noting this is the most common of causes - there is very little in the way of alternatives to reducing your greens' average annual moisture content than the reduction of organic matter in the upper soil profile.

Reducing organic matter is a difficult operation that can rarely, if ever, be implemented without impact and some level of disruption of greens surfaces.

In the next article in the organic matter series, we cover the management and reduction of organic matter.

Charles Henderson HND, BA, BASIS, RIPTA,
Agronomy Director
Sports Agronomy Services Ltd. www.sportsagronomyservices.com

Maxwell PWS soil analysis can provide Organic Matter Benchmarking.

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