By Barry Stewart, Ph.D.
One of the first things I would like to know about my soil is where it came from. What are its origins? Previously, you would have had to obtain a soil survey for this type of information, but the USDA Natural Resource Conservation Service (NRCS) made this quite easy for us with the SoilWeb app. Fire up the app, and, voila, you can find out what soil map unit you are standing on – give or take a couple social distances (12 feet or so). Now, most modern athletic fields are not constructed for on-site materials, but this will give us some idea of what our subsoil might be, as well what the drainage pattern and landscape looked like before our fields were built.
Many fields are constructed using cut and fill operations, so it may be possible to ascertain which places were cut and which places were filled. Also, if you know from where your soil was sourced, you could look up that location and see the soils that exist there and see their properties. The SoilWeb app will give you a visual soil profile. as well as the description of that soil series. You can dig deeper and find out about the suitability of your soil for many different uses. It’s like having a soil survey in the palm of your hand.
While the information from SoilWeb is useful to have, it is far more useful to have data from actual soils samples from your site. But what data do we need? First, I would like to know about the particle size analysis and particle size distribution of my soil. Many athletic field soils are very uniform as they were hauled in with the expectation that the field or the complex was being covered with the same material.
If there are known areas of different soil texture within a field or complex, then more than one sample may need to be analyzed. For athletic field soils that are not sand based, a particle size analysis by the hydrometer (see Figure 1) or pipette method is all that is needed for particle size analysis. For sand-based fields, I would like to know the particle size analysis, as well as the sand size distribution. Many labs will do a particle size analysis for $50 or less, and about the same figure sand distribution.
Once we have the percentages of sand, silt and clay determined, we can place our soil on the soil textural triangle (see Figure 2) and determine its texture. This will go a long way in giving us information about how this soil will perform. Engineers look at soils a bit differently, and tell us that soils behave as the finest 20 percent of the soil particles. This implies that if your soil is 20 percent or more clay, it will behave like a clay. Soil structure may modify this somewhat, but it is something to remember when looking at water movement and infiltration. Knowing soil texture also allows us to contemplate what action we might take to modify our soils. Soils with 50 percent or more sand are candidates for modification by blending in more sand. To improve our porosity, we are trying to get sand-particle-to-sand-particle contact and pore space created by bridging between the sand particles. If we do not have at least 60 percent sand, this will not happen readily. If I were doing a modification project, I would want to get to at least 75 percent sand. For a soil with 40 percent sand or less, I would recommend some type of sand cap that could be applied slowly by yearly topdressing or an all-at-once sand cap.
For soils with more than 80 percent sand, I would also like to know the distribution of the sand particles. The USDA sand size classification scheme is presented in Table 1. Examination of this data will determine if we have a sand that is likely to compact well or one that resists compaction. A soil that has about 20 percent in every sand particle size is termed a well-graded sand, and will have the tendency to compact as the finer particles will fit in the pore space between the larger particles. These are excellent sands for use in concrete and mortar mixes as they pack together well, and a good concrete sand even contains fine gravel. This is why very little very fine sand, silt and clay is allowed in a sand that meets the USGA particle size specification. The best sand for resisting compaction is a poorly graded or uniform sand. This is a sand that has a majority of its particles in one or two adjoined particle sized classes. A USGA spec sand has a minimum of 60 percent in the course and medium sand categories. Because the particles are so uniformly sized, the pores created between them will also be uniform; and since there are few particles from other size classes, there are fewer fine particles to fill these pores. So, knowing particle size analysis and sand distribution allows us to know a great deal about drainage characteristics such as infiltration and hydraulic conductivity, pore space and water retention, potential for water runoff, cation exchange capacity and load-bearing capabilities.
Table 1. USDA soil particle size classes
|Particle size class||Diameter in mm|
|Very Coarse Sand||1.00 – 2.00|
|Coarse Sand||0.50 – 1.00|
|Medium Sand||0.25 – 0.50|
|Fine Sand||0.10 – 0.25|
|Very Fine Sand||0.05 – 0.10 *|
|Silt||0.002 – 0.05|
*USGA classifies fine sand as 0.15 – 0.25 mm and very fine sand as 0.05 – 0.15 mm
The next thing I would like to know about is soil compaction. One of the best tools for finding soil compaction is a trained set of eyes. Look for areas of turf that look thinner than the rest of the field, areas that have standing water after a rain and remain wet longer than other areas, look in high-traffic areas such as near the soccer goal mouth or between the hash marks of a football field. Look for indicator plants that thrive in compacted areas like path rush, goosegrass, knotweed, and Poa annua. Once we have identified areas that may be compacted, we now must take a measurement. Bulk density is the gold standard for measuring soil compaction. Usually a core sampler is driven into the ground with a slide hammer.
A soil core of known volume is brought back to the lab and dried and bulk density is determined. Bulk densities of about 1.6 g/cm2 (see Table 2) begin to impede root growth, and bulk densities of 1.8 stop root growth. Take samples in areas of good turf growth and poor turf growth and compare them. If areas of higher bulk densities are found, those areas will need extra care in terms of aerification.
Table 2. General relationship of bulk density to root growth based on soil texture. USDA NRCS 1996
|Soil Texture||Ideal BD||BD affectingroot growth||BD Restrictingroot growth|
|Sands, Loamy Sands||<1.60||1.69||>1.80|
|Sandy loams, loams||<1.40||1.63||>1.80|
|Sandy clay loams, clay loams||<1.40||1.60||>1.75|
|Silts, silt loams||<1.30||1.60||>1.75|
|Silt loams, silty clay loams||<1.40||1.55||>1.65|
|Sandy clay, silty clay, some clay loams||<1.10||1.49||>1.58|
|Clays <45% Clay||< 1.10||1.39||>1.47|
USDA NRCS, 1996. Soil Quality Information Sheet, Compaction. April, 1996. 2 pages
Measuring soil bulk density is a slow process, but it is unaffected by soil moisture content. Soil penatrometers and Clegg Impact Soil Testers (CIST) give us immediate field readings, but they are affected by soil moisture and, in the case of the CIST, grass conditions. A penetrometer is a good tool for looking for soil compaction – especially with depth – but the readings are dependent on soil moisture, and area sampled per reading is very small, so many readings must be taken. Make penetrations in areas of good turf growth and poor turf growth and compare them. Use the data collected to find areas that need extra attention. The CIST is used to identify areas of surface hardness, which is not the same as compaction, as surface hardness has a contribution from grass. Closely mown grasses (1/2 inch or less) have harder surfaces than taller grass (1 inch) due to more biomass. Surface hardness is also affected by soil moisture. Water lubricates the soil particles, which results in less particle-to-particle friction and softens the surface. The CIST is a useful tool for identifying hard areas of the field, and these areas are likely compacted.
On many athletic fields the compaction is limited to the top one or two inches (25 to 50 mm) of the soil. The best method to alleviate compaction is hollow-tine aerification and knowing the compacted areas so they can be given extra care. The relationship between soil compaction, soil texture and root growth is shown in Table 2. This illustrates that one of the benefits of having a sandy texture is that there is still pore space when the soil is somewhat compacted. Soils with finer textures have little pore space when compacted, and root growth suffers at lower bulk densities.
Barry Stewart, Ph.D., is associate professor at Mississippi State University. He teaches courses in Athletic Field Management, Golf Course Operations, and Plant Science. His current research focuses on athletic field quality and sustainable turfgrass management. He is a part owner of the Green Bay Packers.