Tissue samples are often submitted to the lab with the sample ID ‘s of “good” and “bad”, and sometime the tissue test data results are very similar. The dry weather this year has increased the appearance of these samples. Sometimes the “bad” sample will have higher nutrient concentrations than the “good” sample.
It is advisable in tough growing conditions to take both a “good” and “bad” sample. In some cases, the samples should be labeled “bad” and “really bad”. Even the better appearing plants may be struggling and result in low tissue test values, just not as low as the poor appearing plants.
Tissue testing lab methods are a complete acid digestion of the plant materials. The concentration is the relative amount of a given nutrient within a defined volume of plant biomass. Changing either the total amount of nutrient in the plant or changing the overall volume of plant biomass will impact the results.
The impact of nutrient uptake and plant size on tissue test results when comparing two samples.
If nutrient availability in the soil is not limiting, there is no reason to expect the tissue test data between a “good” and “bad” sample to be significantly different. If a plant is limited by physical or environmental factors leading to reduced plant growth, the biomass volume of the impacted plant will be less. Equally decreased nutrient uptake by the impacted plant will lead to a less total nutrient in the plant tissue tested. Often the decrease in plant biomass is correlated to the relative decrease in nutrient uptake. This leads to a very similar sample nutrient concentration. If the plant biomass is severely impacted while nutrient uptake continues, the impacted plant could result in elevated nutrient levels. Notes and pictures taken at the time of sampling can be very valuable in interpreting plant tissue data.
When a nutrient deficiency is occurring, it normally only impacts one or possibly two nutrients. When all or several of the nutrients are shifted, then external forces like lack of water limiting mass flow uptake of nutrient or soil compaction reducing root mass may be the cause. This is why taking a soil test close to the sampling location of the tissue test is very helpful. If the tissue test is low and the soil test is low, there is a lack of supply. If the tissue test is low and the soil test is good, then there is a lack of access.
Getting a tissue test report back from the lab showing that both the “good” sample and “bad” sample have adequate nutrient concentrations to support plant growth does not mean the tissue test did not tell you anything. It means the issue affecting the growth of the “bad” sample is most likely not directly related to specific nutrient deficiency. Contact your ALG agronomy representative for support using plant tissue data in diagnosis situations.