What Tissue Test Will and Will Not Tell You

Tissue testing has long been utilized as a diagnostic tool but is increasingly being used as part of the overall crop fertility management. This concept is helping agronomists and growers find more effective and efficient ways to provide plant nutrition. It is easy to read too deeply into tissue test result, while missing basic issues. There are a couple of basics to keep in mind when reviewing plant tissue data. In many cases, observation of the crop leading up to sampling is key.

Less than 15% of the plant dry biomass is represented by tissue test data. Much of a plant dry biomass is carbohydrates comprised of carbon, hydrogen, and oxygen that is not reported in the tissue test data. As the carbohydrate content of the plant goes up, the percentage of the plant represented by the nutrients on the tissue test decrease.  A plant that is stunted or stressed due to environmental impacts that do not directly impact the update of nutrients can result in overall normal to high tissue test data values.

Plant growth patterns can impact tissue test data. Just prior to a rapid growth phases, plants accumulate nutrients in preparation. At this point tissue test results tend higher. Once the plant enters the period of rapid growth, the plant begins to accumulate carbohydrates very quickly. These additional carbohydrates effectively dilute the nutrient content of the plant biomass. These growth patterns can also shift the mobile nutrients in, and out, of the plant segment being sampled.

A tissue test is a snapshot in time. It is an evaluation of the nutritional status at the time of sampling. This will reflect the nutrients the plant was able to access in the past but does not give any indication at to predicting nutrient values into the future. This a key reason why management systems with a defined focus on tissue testing a part of an overall fertility plant promote repeated sampling of the same area through the growing season.

Repeated tissue testing of the same area can show how any seasonal patterns and plant development may impact the crops’ ability to access nutrients through the growing season. To make valid assessments of the tissue data, weather data, along with crop observations, are key. For example, periods of dry weather can reduce nutrient availably to the plant, soil water is essential in nutrient movement to and into the plant. Dry weather with normal growth, resulting in normal carbohydrate accumulation, will normally lead to slightly lower nutrient vales in tissue tests, especially nitrogen and potassium. If the dry weather is severe enough to effectively stop plant growth, resulting in reduced carbohydrate accumulation, the tissue test could come back normal to high.

A tissue test can tell you what nutrient is missing in the plant but cannot tell you why. Was the plant unable to access the nutrient, or was the soil void of the nutrient? Often the first instinct is to apply the nutrient that was low in the tissue test. In this situation it is recommended to retest later to see if the nutrient application corrected the issue. A second recommendation is to take a soil test from the same sample locations as the tissue samples to identify if the nutrient is low in the soil or could something like soil pH be impeding the availability of the nutrient.

Do not get too wrapped up in ratios of nutrients in the plant. If a nutrient included in the ratio is deficient, the ratio will be skewed towards the opposite value. This can be seen with or without a calculated ratio if target for normal levels for a given nutrient are included. Ratios do help bring attention to these variances. For example, if one of the nutrients in the ratio is very close to the bottom of a normal or target level, while the other is high the ratio may alert you to abnormality a bit sooner than looking only at the individual nutrient ratings. The reporting of a ratio does not specifically mean there is an interaction between the nutrients.  A good analogy is a brick wall. A brick wall has a defined ratio of mortar and bricks. If you have more bricks, you cannot build any taller of a wall, and the extra bricks don’t impact how much mortar it takes to build the wall.

Trying to predict yield or any future values from tissue testing is difficult. There are more factors than plant nutrition that can impact plant growth and the final yield. If you are looking to start plant tissue test monitoring this growing season, contact your ALGL regional agronomist for details on our plant monitoring program before the sampling season begins. Contact your ALGL regional agronomist with any other questions or tissue testing needs.

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