Interpreting Your Soil Test Reports

Soil tests allow you to understand soil fertility in your fields. The accuracy of a soil test starts during the sampling process. Samples must be taken at a consistent depth and in a random nature. The objective is to collect multiple small samples representative of the field majority. Avoid areas where animals congregate, areas of over or under-performing vegetation, fence rows, tree lines, etc. For soil sampling instructions, visit Soil Sampling Instructions from Penn State Agricultural Analytical Services Lab.

Soil Fertility Submission Forms site. Now we wait. Delivery and processing take time, so remember to send your samples before you need them. Receiving the results as an email versus waiting for them to be returned through the mail can speed up the process.

No "uniform" soil analysis report exists, meaning results and analysis can differ depending on the provider. This article will break down the soil test reports from the Penn State Agricultural Analytical Services Laboratory (Images 1 and 2). In images 1 and 2, divide the soil tests into four sections.

The first section comprises the information you filled out when you sent the soil sample and forms -- name, contact, and field information.

In the second section, we start to get into the measured soil fertility levels- pH, phosphorus (P), potassium (K), and magnesium (Mg). Phosphorus, potassium, and magnesium are essential plant nutrients measured in parts per million (ppm). As mentioned, all labs and analysis reports are different, so if looking at other labs' results, ensure they are not reporting in pounds per acre (lbs/acre). Soil pH measures the acidity or alkalinity, and pH levels will range from 0-14, with <7 indicating an acidic pH, seven a neutral pH, and >7 a basic pH. The pH scale is based on a logarithmic scale, meaning the difference from a pH of 6.0 is 10x more acidic than a pH of 7.

In section 2 of figures 1 and 2, you'll find pH, P, K, and Mg levels represented by a numerical value and a bar chart describing them as below optimum, optimum, and above optimum. The measured soil fertility values are nothing without research on how crops respond under varying soil fertility conditions. Researchers test crop response to nutrient additions and develop curves, Image 3. When soil test levels are below the optimum, there is a greater chance the crop will respond to additional nutrient supply, positively impacting growth and production. As soil nutrient levels increase, the likelihood of crop response to additional nutrients decreases. The area where crop response begins to reduce becomes the beginning of optimum. In the optimum zone, adding nutrients is less likely to affect crop production. Above optimum could be a concern regarding some nutrients like phosphorus. Where soil phosphorus levels are higher than crops can utilize, there is potential for pollution and water quality concerns. 

You may notice that nitrogen (N) is not measured. Although nitrogen is a macro plant essential nutrient, required in larger quantities, and is often the limiting factor in plant growth, it is challenging to measure in the field. Nitrogen is a dynamic nutrient, moving quickly in the environment from one form to another. Because N is so difficult to measure and moves through the environment in so many ways, N is recommended based on crop needs—more on nitrogen in section 3.

Nutrient recommendations are reported in section 3 of the soil test report. The upper portion of section 3 is used for the recommendations of limestone and magnesium. Limestone is reported as pounds per acre required to reach a target pH of 6.5 and reduce soil acidity. Limestone recommendations are based on material with a calcium carbonate equivalency (CCE) of 100. Many materials can have values above or below this 100 CCE benchmark. Understanding CCE is becoming more critical with the variety of liming products becoming available. This section also has magnesium recommendations since magnesium deficiencies are usually remedied with a high magnesium content lime application. Using a high Mg lime will treat soil pH and Mg simultaneously. More information on calcium carbonate equivalency and liming can be found in the Penn State Extension article Soil Acidity and Aglime.

Following section three, you will next find recommendations for plant nutrient needs. In this section, you will see the cropping year, the crop, expected crop yield, crop nutrient needs, and laboratory notes.

Recommendations from the Lab are based on the crop you are growing and the year of rotation. If you produce a grass crop, the nitrogen recommendations will be higher than those on a legume crop. Likewise, the nutrient recommendations will be higher in a crop that is being mechanically harvested, like corn silage, when compared to pasture. At their essence, recommendations are designed to keep nutrients from being the limiting resource to the crop, while maintaining economic and environmental balance. When all nutrients are in the optimum range, recommendations are based on crop removal, meaning nutrient additions are to replace the nutrients removed by the harvested crop. When nutrients are below optimum, recommendations are based on a building approach. The nutrient additions are designed to get the soil nutrients to the optimum range over the three years of the soil test.

As mentioned, nitrogen is based on average crop uptake and expected yield. This differs from phosphate and potash, which are recommended from soil measurements as P₂O₅ and K₂O, respectively. You must recognize how your recommendations and nutrient levels are reported.

Below the recommendation are the laboratory notes. Often in this section, you will find information about split applications of nitrogen, recommendations on application timing, animal health issues, etc. The laboratory note section can provide valuable insights and tips into proper applications and nutrient timing.

At the bottom of the page, section 4 contains many analytical results or other tests that can be purchased at an additional fee. A few keys to notice in section 4 are: Acidity is not the same as soil pH; these are very different things. As we see in Image 2. The soil pH is 5.2, and the soil acidity is 8.7. When purchasing additional tests such as organic material, nitrate-N, or salts, they will be found in section 4. Also, trace mineral concentrations for zinc, copper, and sulfur are found in this section. Although recommendations are not provided for trace minerals, a typical range table can be found on the back of your soil test result.

Reading soil test results can be overwhelming and seem challenging to process. This will help take some of the pressure off. If you have any concerns, comments, or questions about soil testing, please don't hesitate to contact your local Penn State Extension Office.