How to Best Use Diagnostic and Soil Lab Services

By Dave Han, Ph.D. and Jim Jacobi, Ph.D.

Getting the most out of your soil testing

Soil testing is vital to successfully growing any plant. Turfgrasses are no exceptions. But turfgrasses grow very differently to most other crops, so sampling and implementing recommendations are different in turfgrasses from most crops. Here is how to get the best information and benefit out of your soil testing for turfgrass.

How turfgrasses make soil testing a bit more complicated

The main difference between turfgrass and row crops or vegetable gardens is that turf is perennial. Because it is not replanted every year, there is no time when lime or fertilizers can be incorporated into the soil throughout the entire rootzone. Turfgrass also covers the ground completely, ideally leaving no bare soil showing, so any lime applied to the top of the turf must work its way through the thatch to even reach soil, then move down through the soil profile. This means that changes in soil pH due to lime applications, for example, happen very slowly in soils growing continuous turfgrass. A good rule of thumb is that most turfgrass roots are in the top three inches or so of the soil, though of course there are exceptions both ways. It can take months or even years for lime to work its way down that far into the soil profile. If lime is applied to soil under turfgrass on a regular basis, a large difference in pH over very small changes in depth can build up over time. This can have a big influence on the way the grass grows. We will return to this issue later.

Collecting a good sample

The whole point of soil testing is to give growers (in this case, turf managers) the information they need to make sure that the soil will support the healthiest plants possible. For this reason, it is essential to understand how turfgrasses use the soil, especially where their roots are and how deep to sample. As previously mentioned, most turfgrass roots are in the top few inches of the soil. For this reason, it’s not necessary to collect a 6- or 8-inch deep sample for most turfgrass applications. In fact, this can backfire. When soil the turfgrass is not using is mixed in with soil where roots are growing, the soil test results might not accurately reflect the nutrients that the grass can actually get and use from the soil where it is growing. Conversely, a sample that is too shallow might also give skewed results. The basic idea is always to sample soil that the roots are actually growing in, or where you want roots to be growing in the case of a new establishment.

There are many guides to taking a soil sample, including those from Alabama Extension (see Home Soil Testing: Taking Sample http://www.aces.edu/pubs/docs/A/ANR-0006-A/ index2.tmpl or The Basis of Soil Testing in Alabama https:// www.aces.edu/anr/soillab/forms/documents/ay-324A.pdf). The key is to make sure you are testing representative samples. If the area (lawn, putting green, field, or whatever it may be) to be tested is known to have uniform soil, then simply collecting 12–20 individual samples of the proper depth, mixing the soil together then sending in a pint of that mixture will do. If there are obvious differences (including, but not limited to differences in soil type, texture, drainage, etc), then samples (again, composites of 12-20 subsamples of the proper depth) should be pulled from each distinct area. For large areas, grid sampling can be employed. If a problem is suspected in a given area due to poor grass growth, then be sure to send in samples from both the good and the bad areas.

Finally, if the grass is having problems at the soil surface — for example, stolons are not pegging down properly, or root growth near the top of the soil is not good — then it is a good idea to send in samples subdivided by depth. Take soil’s cores and divide them into the top 1/2 inch, then the next 1/2 inch, etc. This is more difficult and time consuming to sample, but it can provide some valuable insight that a bulk sample of the top 3–4 inches can’t. Remember that lime applied to the surface takes a long time to work its way down and this can lead to a pH gradient by depth. It is entirely possible for a turf that has been limed in the past to have a bulk pH in the top 3 inches of 6.5 (great for grass growth) but for the top 1/2 inch to be 7.7 (this will inhibit rooting of stolons)! Subdividing by small units of depth or taking very shallow samples can pinpoint problems like this.

Proper soil testing can help you get the information you need to identify and correct soil problems. Taking the time to think about how to get the best sample will pay off in the long run.

Submitting a sample for problem diagnosis

The first step to controlling disease and insect problems of turfgrass is an accurate diagnosis. When you can’t diagnose the problem or need a second opinion, consider submitting a sample to a diagnostic lab. Just remember, the quality of the turf sample and supporting background information is key to getting an accurate result from a diagnostic lab.

To diagnose turfgrass problems, we need at least a 6-inch square by 3-inch deep piece of turf. Golf course cup cutter plugs also are good, especially when sampling greens and tees. If the symptoms are variable across an area, include more than one sample when possible. Take the sample from the outer border between the healthy and diseased turf, where 2/3 of the sample is diseased and 1/3 is healthy. Don’t take a sample from dead or bare areas. And always make sure to take the sample before any fungicides or other chemical treatments have been applied. Fungicide applications, especially systemic fungicides, made before taking the sample can make the isolation and identification of fungal pathogens tough, masking the true cause of the problem.

Submit the sample as soon as possible, if you can’t mail or deliver the sample that day, keep it in a cool location out of direct sunlight. Wrap the soil and roots in aluminum foil to keep the sample intact. Do not place the sample in a plastic bag or add water or wet paper towels which encourage secondary non-pathogenic fungi and bacteria. Pack the sample in newspaper or other packing materials so the sample won’t shift or tumble in the box. Make sure to mail samples so they will arrive by Friday. A sample that sits for 2–3 days before getting to the lab on Monday, may arrive at the lab in poor condition, making it unusable for diagnosis.

Nearly all diagnostic labs have a submission form that should be filled out providing all the necessary information on the timing of the symptoms, recent weather, cultural practices (aerification, topdressing, etc.), and any other unusual problems or events that have occurred, such as an irrigation leak. Also list all fertilizers and pesticides, especially fungicides, applied in the last 30 days. Provide as much information as possible when filling out the submission form. The more information provided with the sample, the more accurate the diagnosis and control recommendations will be.

Don’t forget to send 2–3 digital pictures along with the sample as they are always helpful when diagnosing disease and insect problems. Include digital images that clearly show the pattern of damage and the appearance of the surrounding healthy turfgrass. Include pictures from varying distances. In most cases, pictures from 5 feet or more are best. If the damage seems related to site topography or shade patterns include a wide angle or ‘landscape’ image. Seeing a picture of the whole area can often help better understand the problem. Close up shots of less than a few feet typically are not needed, unless it’s something that might not be seen after shipping. And always check the images to make sure they are in focus.

fHopefully, these tips will help you make better use of diagnostic services and get the most out of those times when you need some help diagnosing a problem or need a second opinion.