By Dorivar Ruiz Diaz, Soil Fertility Specialist, and Dave Mengel, Professor Emeritus, Soil Fertility Specialist
When crop fields appear variable, one question commonly asked is whether this is due to a nutrient problem. An excellent tool that can be used to answer this question is plant analysis or tissue testing.
For corn, soybean, wheat, and other crops, there are two primary ways plant analysis can be used: as a routine monitoring tool to ensure nutrient levels are adequate in the plant in normal or good looking crops, and as a diagnostic tool to help explain some of the variability and problems we see in soybean growth and appearance in fields.
Routine Monitoring Tool
For monitoring nutrient levels purposes, collect 20-30 sets of the upper, fully developed trifoliate leaves, less the petiole, at random from the field anytime between flowering and initial pod set (growth stages R1-4). The top fully developed leaves are generally the dark green leaves visible at the top of the canopy, which are attached at the second or third node down from the top of the stem.
Sampling later, once seed development begins, will give lower nutrient contents since the soybean plant begins to translocate nutrients from the leaves to the developing seed very quickly. Sampling leaf tissue under severe stress conditions for monitoring purposes can also give misleading results and is not recommended.
Figure 1. Example of plant analysis interpretation using the concept of a sufficiency range.
The sampled leaves should be allowed to wilt overnight to remove excess moisture, placed in a paper bag or mailing envelope, and shipped to a lab for analysis. Producers should not place the leaves in a plastic bag or other tightly sealed container, as they will begin to rot and decompose during transport, and the sample won't be usable.
Which Nutrients to Request
In Kansas, nitrogen (N), phosphorus (P), potassium (K), sulfur (S), zinc (Zn) and iron (Fe) are the nutrients most likely to be deficient in soybeans. Normally the best values are the “bundles” or “packages” of tests offered through many of the labs. The packages can be as simple as N, P and K, or can consist of all the mineral elements considered essential to plants. K-State offers a package that includes N, P, K, Ca, Mg, S, Fe, Cu, Zn, and Mn for $32.00.
Table 1. Nutrient content considered “normal” or “sufficient” for soybeans
The data returned from the lab will be reported as the concentration of nutrient elements, or potentially toxic elements in the plants. Units reported will normally be in terms of “percent” for the primary and secondary nutrients (N, P, K, Ca, Mg, and S) and “ppm,” or parts per million, for the micronutrients (Zn, Cu, Fe, Mn, B, Mo, and Al). Most labs/agronomists compare plant nutrient concentrations to published sufficiency ranges. A sufficiency range is simply the range of concentrations normally found in healthy, productive plants during surveys. A diagram explaining this concept is shown in Figure 1.