Manufactured and modified sand-based soil profiles on golf courses have one main objective in common: good soil drainage that yields healthy turf and firm playing conditions. Uniform soils without layering are key to good drainage as they allow the free flow of water via gravity and water cohesion and adhesion to soil. However, from the moment turf is planted and the first sand topdressing application is made, soil layering begins. In severe cases of soil layering, mitigating the negative impacts can be a daunting task that requires aggressive and disruptive cultivation.
So, what exactly are we talking about when we refer to soil layering? Soil layers are simply differences in soil texture – intentionally or unintentionally created – that slow water flow through the soil profile. Layering can occur at varying depths in the soil, but for the purpose of this article we will focus on soil layering in the upper rootzone portion of a putting green.
Even the smallest layer can create turf health and playability issues. While there are several negative impacts of soil layering, the common cause of these issues is slower infiltration due to the varying adhesion forces of differing soil textures. More often than not, layering results in excess soil moisture at the surface that causes shallow rooting, increased disease incidence and severity, and soft playing conditions.
Water Flow Dynamics
In order to understand how soil layering negatively impacts turf health and how to address layering issues, we must understand how water moves through soils. The primary forces influencing water movement in the soil are gravity and water cohesion and adhesion.
The role played by gravity is fairly straightforward – it’s the force that drives water flow downward. However, adhesion and cohesion are more complex. Soil moisture overcomes the force of gravity and can move laterally and vertically due to cohesion and adhesion. Water cohesion is the property that attracts water molecules to one another, and adhesion is its attraction to other surfaces, such as soil particles and organic matter (Gardner, 1988).
Soil particles with greater surface area have greater adhesion forces than soils with less surface area. The three soil texture classes are sand, silt and clay – which are the largest to smallest particles, respectively. Clay particles, being the smallest, have the greatest surface area in a given volume of soil and the greatest adhesion force. Sand particles are the largest and have the least surface area in a given volume of soil, which gives them the lowest adhesion strength. This is why sands drain well and are the most desirable soil for golf courses.
Organic matter also plays a significant role in water flow as it directly and indirectly holds soil moisture. In some cases, such as when a soil is at field capacity, organic matter has a higher water-holding capacity than mineral soils of a similar volume (USDA NRCS, 2008). Organic matter’s ability to hold water depends on the exact source but it is high relative to sand (Bigelow, et al., 2000).
More uniform soils allow water to flow more freely. Obviously, the rate of flow is highest in sand and slows in silt and clay, but the absence of soil layers maximizes flow for a given soil type. When soil layers are present, water flow is slowed due to the differing adhesion forces of varying soil textures. It isn’t until enough pressure builds above a change in texture – or water cohesion forces exceed adhesion forces – that soil water can flow through the layer. This principle is utilized to create the perched water table in USGA putting greens. However, when soil layers occur in the upper rootzone, agronomic and playability problems occur.