The physical properties of natural turf sports field soils profoundly impact the performance of the turf growing on them. Soils that are compacted, have poor structure, or are fine textured will likely be poorly drained, have high water retention, and poor aeration. Because these soils are poorly aerated and wet, the turf develops shallow roots and has low density.

The physical properties of a soil such as infiltration rate, aeration porosity, and water retention are influenced by three factors: soil texture, soil structure, and soil density. Modifying any one or more of these will result in a better soil environment for your turf.

The soil texture refers to the percentage of sand, silt, and clay present in a soil. Sandy loam soils, which will contain from 50 to 85% sand, are generally preferred for topsoil sports fields. Finer textured soils tend to be very prone to compaction and poorly drained.

It is commonly thought, then, that by simply adding sand to a soil that you will improve the drainage and performance characteristics. Unfortunately, some of the worst sports fields we have seen were where people made attempts to modify the texture of the soil through sand additions.

Small additions of sand to a soil will actually do little good in terms of improving soil physical properties. In fact, we have seen sports field soils constructed of mixes containing 85% sand that were better suited for road bases than sports fields. What went wrong, and why don’t sand additions make the expected improvements in a soil?

In order to see any improvement in a soil by adding sand, you must add a sufficient amount so that the sand particles bridge or come in contact with one another. When this occurs, you get the creation of large pores between the sand grains. It is only then that you see improvement in the physical performance and compaction resistance of the soil. In most cases, you are looking at having to add at least 80% sand to soil; in other words a sand based system. Failure to do so will almost always results in very hard, compacted fields that don’t drain.

The particle size distribution of the sand you use to modify the soil is important as well. Coarser, uniform sands will do a better job of bringing about improvement in your soil. We would recommend a sand with the following particle size range:

Sand Classification	Particle Diameter (mm)	% retained
Gravel	> 2.0	0 - 5%
Very coarse sand	1.0 - 2.0	5 - 20%
Coarse sand	0.5 - 1.0	30 - 60%
Medium sand	0.25 - 0.5	30 - 60%
Fine sand	0.10 - 0.25	0 - 15%
Very fine sand	0.05 - 0.1	0 - 3%
Silt and clay	<0.05	0 - 3%

Aside from selecting the best sand for modifying your soil, you also need to know how much sand to add to the soil to bring about improvement. Soil physical testing labs such as Hummel & Co. specialize in this type of testing. Performance testing involves making mixes with different proportions of your sand and soil (and perhaps organic matter), and running them through standard ASTM (American Society of Testing and Materials) tests for infiltration rate, porosity, and the distribution of air and water filled pore space.

The bottom line is that modifying the texture of your soil is an all or nothing proposition. Failure to follow the steps outlined will likely result in a disappointing if not disastrous outcome.

Do you ever wonder why some native soil fields perform well while a poorly conceived and constructed sand based fields don’t? It is probably due to the native soil field having good soil structure, where sand based root zones are single-grained structureless soils. Soil structure is the arrangement of the finer mineral components of a soil into larger clusters or aggregates.

A granular type structure, which is very desirable, has the soil particles arranged into pea sized or smaller aggregates. Collectively, these particles act much like a coarse sand would, the pore space between the aggregates providing aeration and infiltration. Unlike sand, however, these soil aggregates also have small pores within them that hold water. How nice it would be if we could all have granular structured soils in sports fields.

Unfortunately, as these finer textured soils become compacted because of heavy use, the soils lose their structure or actually develop into undesirable blocky or platy type structures. Routine core aerification will help break these larger aggregates up. At some point in time, however, it may become necessary to plow, disk, and harrow the soil to restore good structure. This obviously will put a field out of commission for a while, but if you can work out some type of rotation, even if its on a ten year cycle, it may be one of the best long term practices for reinvigorating your soils, and ultimately your fields.

The addition of organic matter to a soil, especially a soil that is low in organic matter, will help in the long term to maintain aggregate stability. Well decomposed organic matter, or humus, is actually a good cementing agent that holds the aggregates together. Working coarse textured organic matter sources such as sphagnum peat or good quality composts will actually help open the soil matrix up some.

[Published in 1996 Western States Turfgrass Conference Proceedings]