Ask the Expert: How to Read a Soil Analysis

Soil testing is an important component of turf fertilization. Soil tests help estimate current nutrient availability, help determine what products can help solve deficiencies, and provide overall peace of mind when making decisions about proper fertilization.

While soil tests can be useful, understanding their results can be complicated. That’s why connected with Jeffrey Thrasher, Central’s Liquid Chemicals and Fertilizer Products Manager, to provide some insight on how to read and understand a soil analysis.

From Jeffrey:

Understanding the information in a soil analysis is a good first step to recommending products that will address soil deficiencies. Remember, a soil test is only a snapshot and is subject to sample error, weather conditions, and other factors that can add to the full story. Keep in mind that soil porosity, grass types and condition, application history and other site characteristics are also important. Think of a soil test as a guide map to point you in the right direction. It remains up to the service provider to implement cultural practices that lead to great results.

The top three readings are the most important (not including Buffer pH). Each will impact the other so look at them as a group.
What Each Reading Means

pH

The letters stand for ‘potential’ or the ‘power’ of Hydrogen (H). It measures the concentration of H in the soil solution on an inverted scale. In other words, a lower pH represents higher concentrations of H and greater acidity. Inversely, a higher pH indicates alkaline soil. Neutral is 7 and most plants (and microbes) prefer slightly acidic soil in the 6.0 – 6.9 range.

Balancing pH is the critical first step to a well-managed soil. In the soil there are acid forming cations (elements with a positive electrical charge) principally hydrogen, sulfur and sometimes aluminum and base forming cations, principally calcium, magnesium and potassium. Balancing these nutrients will lead to a healthy pH.

Organic Matter

pH is influenced by a number of factors, including the percent of organic matter (OM). Decomposition of OM by soil microbes creates negatively charged organic acid fractions that attract cations, buffering the pH.

The OM portion of the soil is also a reservoir of nutrients that become available to the plants as the microbes do their work. Ideal soil will have between 2-3% OM. Too much OM can create ‘soggy’ and anaerobic soil, too little creates a host of issues for healthy plant growth. Carbon rich OM is a fertile humus that is the result of steady additions of fresh organic matter. That is one reason why it is recommended that grass cuttings be left to resupply the soil. Not all organic matter is the same. In some cases, the microbes have depleted the carbon leaving behind fats, waxes and lignins that are of little benefit to plants or to a healthy pH. This is often the result of ‘urbanized’ soils that are suffering from compaction and the overuse of synthetic fertilizers.

Cation Exchange Capacity (CEC?)

Cation Exchange Capacity expresses the presence of negatively charged clay or humus soil particles. The cations are not actually bonded to the particles, rather they are held in close proximity floating between the particle and the soil solution. When in the soil solution the cations are available for absorption by plants and a new cation can then be pulled into the electromagnetic sphere of the soil particle.

The existence of these exchange sites is a measure of the soil’s capacity for retaining nutrients essential for plant growth. The balance of cations held on the exchange sites will also determine pH. Sandy soils are low in CEC, clay soils are high in CEC. A typical sandy loam will have a CEC between 5-10, a typical clay loam will be between 15-30. A high CEC does not necessarily mean the soil is fertile.

Buffer pH?
This reading is a lab test done to measure the potential for change is the soil. Sandy soils are more easily altered than clay soils. The recommendations for adjusting pH listed on the bottom line of the analysis are arrived at from this measurement.

Base Saturation
By measuring the percent of the five principal cations base saturation further refines pH and nutrient balance. The cations and recommended percentages are as follows:

  • Calcium: 65 – 75%
  • Magnesium: 10 – 15%
  • Potassium: 3 – 6%
  • Hydrogen: < 10%
  • Sodium: < 1%

The total for all five should be 100% but all of these cations are often not included in an analysis. If the soil being tested has a history of synthetic fertilizer applications, sodium should be included in the analysis. It is important to stay within these ranges because an overabundance of one will displace others on the exchange sites and reduce availability for plant uptake.

Three Solutions for Balancing Nutrients in Soil
  1. Include fresh additions of organic matter with all fertilizer applications. There are few soils that will not benefit from added OM. The enhanced biology has a wide range of benefits. OM has up to 20X the CEC as clay.

Additionally, the nutrient levels in your analysis do not necessarily mean they are plant available. The test is done by applying lab acids that approximate plant biological acids. The true availability can significantly differ. Microbes help to enhance nutrient availability, especially phosphorus.