Soil testing isn’t just for problem areas–it should be performed on a routine basis to determine actual needs for soil and plant health.
It is also an important tool to employ on new properties. Soil sampling and analysis will also give a company a more professional image while ensuring responsible fertilizer applications based on the actual need.
General soil requirements are a challenge to visually detect even to a keen eye, however there are some indicators that can unveil a critical issue. One can scout for areas or patches of yellowing turf and chlorotic leaves, which can be easy to spot indicators, but by examining closer for patterns such as stunted growth and weak stems, you'll realize it as a call to action for a rootzone analysis.
There are other signs to consider, for instance identification of weed species can give you a hint as to what the soil conditions are:
- Redroot pigweed–Can indicate too much Fe or not enough Mn; prefers high K and low Ca & P
- Quackgrass–Improper Fe:Mn ratio
- Bitterweed, trumpet vine, broom sedge–Ca deficiency
- Wild buckwheat–Low P and excess K
- Docks (curly dock)–Low Ca, extremely high Mg, P, K
- Lambsquarters–Low P, high K
- Foxtail barley–Low Ca, high Mg
- Knapweed–Low Ca and extremely low P
- Oxeye daisy–Low P, high K and Mg
Diseases can also give us an idea of what needs to be addressed in the rootzone:
- Low N levels–Rust, red thread, and anthracnose
- High N levels–Leaf spot, gray leaf spot, brown patch, necrotic ring spot, and pythium diseases
- Low K levels–Spring dead spot
- Mn deficiency–Take-all patch
- High pH–Summer patch
While the soil conditions associated with these weeds and diseases are not the only contributing factors, these problems tend to be more severe when the corresponding conditions are in place. Some will also consider soil adjustments when faced with particular problems, an example of which would be Take-all Root Rot. This disease can thrive when the pH is over 6.5, and acidifying the soil can help reduce severity. In any case, promoting healthy turf with a thick canopy without overdoing the fertility can help crowd out weeds and prevent some diseases, which is why soil health is so important.
While the signs above can indicate an issue in the soil, lab analysis will be a much better tool for determining the actual needs or corrective steps to be taken.
In order to conduct soil sampling, a few things are required. Generally speaking, it is recommended to use a soil probe to acquire sub samples. A clean bucket comes in handy to deposit the cores and later crush and mix them up giving you the composite sample. The last would be a sample bag, often provided by the lab.
These samples, or cores, should be taken from multiple areas of the lawn or beds depending on what is to be tested, and samples from different areas should not be mixed as there may be some differentiation in recommendations. Typically, at least five or more locations should be a good example of the surrounding soil. In this case, more is better. Many will use a zig zag pattern, but obtaining samples from all portions of the property is key.
In terms of sports fields or golf courses, samples from each field, putting green, or other area should be tested separately. We also want the soil being tested to be from the depth at which the roots are growing, so focusing on the top three to four inches below the thatch layer should be included.
Most importantly is to remember the old adage, "Garbage in, garbage out." Make sure to only include soil for the best possible results. Thatch, plant material, and other debris should be discarded so they do not skew the testing.
For ornamental beds, a six-inch depth core sample will be a better representative as many landscape plant roots will grow in deeper soil. You can break up the samples and include at least 1 1/2 cups for testing. Make sure to fill out the information form completely and accurately for the best possible recommendations.
Timing is also a critical factor. Wait a minimum of two weeks following any type of fertilizer or soil amendment application to the area to be tested. Also, for many contractors and lawn applicators, business is slowing down during the fall so soil testing can result in an up-tick in revenue.
In addition, this type of testing can be used as a planning tool for what to do, or not do, for the upcoming season.
Lastly is timing of soil amendments. Fall is the best time for lime applications and spring for sulfur. These operations can be performed with the help of your testing results, which can be ready in as few as a couple of days, or in some cases a couple of weeks depending on how they are delivered and the workload of the lab.
Your soil test results should be returned to you with graphs, data, and recommendations depending on the parameters selected. Most basic soil tests will return levels of phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca) along with pH, however this differs from lab to lab. A more complete test may include several other micronutrient levels, organic matter content, buffer pH, and a value called cation exchange capacity or CEC.
Nitrogen levels are typically not included because this value changes so rapidly that by the time you read the report, it is likely that the N values are not the same as when the samples were first taken. In most cases, on residential or commercial lawns, the basic test will suffice. Sampling can also be done in ornamental beds and vegetable gardens in order to optimize plant performance, however the execution is slightly different.
Often when reading soil testing results, my eyes will go directly to pH first. pH is the measure of acidity or alkalinity in the soil and will be the key that unlocks many nutrients making them available to the plants. Most plants including turfgrasses prefer a pH that is slightly acidic or the range of 6.1 – 6.9. Remember that pH of 7.0 is neutral and anything lower is acidic while values over 7 are alkaline.
When soils sway far away from neutral, many nutrients can become "locked-up" or insoluble, rendering them unavailable to the plant no matter how much additional is applied. Below 6.5, magnesium, calcium, potassium, and phosphorus can start becoming unavailable. In more alkaline soils, micros like iron, manganese, boron, copper, and zinc can become locked up. At a pH in the range of 6.5 – 7.0, most nutrients are available. In extreme acidic situations, certain nutrients can become toxic to plants. For instance, below 4.5, aluminum and manganese can be available in toxic quantities. There have also been cases where very acidic soils have affected the breakdown of certain herbicides.
For all of these reasons, pH becomes a critical factor in understanding what needs to be remedied. Soils become acidic over time due to a few natural processes. Rainfall percolates through the soil taking with it calcium and magnesium ions which keep the soil more alkaline, replacing them with hydrogen and aluminum which are acidifying elements. The decay of organic matter by microorganisms will also compound soil acidity. Lastly is what I like to call the human touch, the use of sulfur in fertilizers, i.e. sulfur coated urea, ammonium sulfate, and sulfate of potash will also drive down soil pH.
Irrespective of which manner, the process tends to be slow. Liming is a common practice for raising soil pH which is too low, while adding sulfur is the most well known process for acidifying. The process of pH and soil nutrient adjustment can be a slow one requiring several years of applications and soil tests.
Nutrient levels in the soil will fluctuate throughout the season with the addition of fertilizers and leaching. Typically, these nutrient levels will be shown in a bar graph of sorts indicating whether they are deficient, in range, or in surplus. Along with the quick glance graphs, there should be data revealing the actual levels often expressed in parts per million (ppm). This is what is used to make corrective or maintenance recommendations. These nutrients should ideally fall into the ranges below:
- Phosphorus (P) 50-80
- Potassium (K) 130-220
- Magnesium (Mg) 140-280
- Calcium (Ca) 900-1500
- Sulfur (S) 20-40
- Boron (B) 0.9-1.7
- Copper (Cu) varies
- Iron (Fe) 9-40
- Manganese (Mn) varies
- Zinc (Zn) 3.9-10.9
Cation Exchange Capacity
Also known as CEC, this is a measure of the soil’s ability to hold positively-charged ions which are nutrients for plant use. Soil particles, particularly clay and organic matter, have a net negative charge which will hold positively charged ions in the soil making them available for root uptake. The greater the CEC value, the more ability the soil has to store nutrients.
The CEC is largely determined by the soil particle types–sand, silt, or clay. Sand-based systems tend to be low in CEC, generally in the range of two to four, which indicates that sand does not retain nutrients well. This is one of the reasons why turf grown in soils with heavy sand content will require more frequent fertilization. Loamy soils tend to land in the range of 9 – 26 in terms of CEC value, whereas clays can be 4 – 60 and organic matter 50 – 300. Some of the most common cations held in the soil consist of calcium, magnesium, potassium, ammonium, hydrogen, and sodium.
Organic Matter (OM)
Sometimes called SOM, soil organic matter is a percentage by weight of the soil comprised of decomposed plant or animal material. As mentioned in the previous paragraph, organic matter will help hold vital nutrients in the soil but will also improve tilth, and water holding capacity. In many cases a range of one to eight percent is suitable except in the case of sand-based athletic fields or putting greens. In these circumstances, the range should be between 0.5% and 2.5%. When organic matter content exceeds 3.0%, it can begin to impede the drainage ability.
This is a value that is primarily used by the lab to determine the necessity or quantity of liming material that would be required to raise soil pH to a specified level. It is a measure of residual acidity that is neutralized to raise the pH. In general, the lower the buffer pH, the more lime that will be required to raise soil pH.
The last bit of information that should be included is general recommendations based on the results and the type(s) of plants being grown in the test area. The lab may suggest a specific fertilizer analysis which may require some interpretation based on locally available products. Additionally, if needed, you may see lime or sulfur recommendations based on pH and plants grown.
Soil testing should be conducted on a two to three year rotation in most places, but sandy soils and/or areas of high rainfall and frequent irrigation should be tested annually. Other common times for soil testing include new turf or ornamental establishment or for the function of soil salt reduction. In some cases, specific landscape plants may be selected based on pH or other native soil characteristics. This all falls into the "right plant, right place" theory of landscape design.
Many areas are apt to saltwater intrusion or can only irrigate with salty water sources. These along with areas of salt runoff will face accumulation in soils. Soil salinity can cause a multitude of problems from lost plant vigor to reduced nutrient uptake and inhibition of seed germination, all of which may result in reduced or obliterated sensitive turf or ornamentals. By determining the baseline of salts in the soil solution, you can see if they are gaining ground as remedial operations are being conducted. Throughout the process, several additional tests may be required to record progress.
As you can see there is a lot to be gained from a lab analysis of soils on new and currently maintained properties. Contact the Ewing Technical Services Team with any questions.