By Dave Mengel, soil fertility specialist, Kansas State University

Many farmers are asking questions about secondary and micronutrient nutritional status of their wheat crop. Unfortunately, soil testing is not as accurate for many nutrients as we would like. Nutrients such as iron, or manganese for example are always present in large quantities in the soil, but they are not always available. Factors such as soil pH, organic matter content, and soil oxygen content have huge effects on their availability. Other mobile nutrients such as nitrogen, sulfur, and chloride tend to accumulate in the subsoil, and require the use of a 24-inch profile sample to best estimate availability. 


Plant analysis is an excellent alternative. Plant analysis is also an excellent "quality control" tool for wheat growers interested in high yield wheat management to use to look for "hidden hunger" issues. This spring we have already used plant analysis to help diagnose problems with sulfur deficiency on sandy soils in south central Kansas.


There are two primary ways plant analysis can be used: as a routine monitoring tool to ensure nutrient levels are adequate, and as a diagnostic tool to help explain some of the variability in wheat growth we see in fields this time of year. Keep in mind however that any plant stress (drought, heat, frost, etc.) can have a serious impact on nutrient uptake and plant tissue nutrient concentrations. Sampling under stress conditions for monitoring purposes can give misleading results, and is not advisable.


We have good calibrations to use for monitoring purposes: at the late tillering to joint stage, and at boot and early heading stages. At late tillering to joint, collect whole plant samples by cutting off 20-30 plants at the ground level from various locations around the field. At boot to early heading collect 40-50 flag leaves at random from the field. In both cases the plant material should be allowed to wilt over night to remove excess moisture, placed in a paper bag or mailing envelope, and shipped to a lab for analysis. Do 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.


Once the samples arrive in the lab they will be oven dried for 12-36 hours, ground, digested so all the nutrients are brought into solution, and then analyzed. Generally the results will be sent out within 4-7 working days after the sample arrives in our lab, depending on the complexity of the analysis requested.


The data returned from the lab will be reported as the concentration of nutrient elements, or potentially toxic elements, in the plants. 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. It can be thought of as the range of values optimum for plant growth. The medical profession uses a similar range of normal values to evaluate blood work.


The sufficiency ranges change with plant age (generally being higher in young plants), vary between plant parts, and can differ between varieties or hybrids. So a value slightly below the sufficiency range does not always mean the plant is deficient in that nutrient, but it is just an indication that the nutrient is relatively low. However, if that nutrient is significantly below the sufficiency range, then one should ask some serious questions about the availability and supply of that nutrient.


Levels above sufficiency can also indicate problems. High values might indicate over fertilization and luxury consumption. Plants will also sometimes try to compensate for a shortage of one nutrient by loading up on another. This occurs at times with nutrients such as iron, zinc, and manganese. In some situations very high levels of a required nutrient can lead to toxicity. Manganese is an example of an essential nutrient which can be toxic when present in excess.


Plant analysis is also an excellent diagnostic tool to help understand some of the variation seen in the field. When using plant analysis to diagnose field problems, producers should take comparison samples from both good/normal areas of the field, and problem spots. Collect soil samples from the same good and bad areas. Don't wait for boot to take diagnostic samples. Early in the season (prior to stem elongation) collect whole plants from 20-30 different places in your sampling area. Later in the season take the uppermost, fully developed leaves (those with leaf collars visible). Handle the samples the same as those for monitoring.


The following table gives broad sufficiency ranges for wheat early in the season, prior to jointing and later in the season at boot to early heading. Keep in mind that these are the ranges normally found in healthy, productive wheat.












































 


 


Growth stage


Nutrient


Unit


Whole plant at tillering-jointing


Flag leaf at boot to heading


Nitrogen


%


3.5-4.5


3.5-4.5


Phosphorus


%


0.3-0.5


0.3-0.5


Potassium


%


2.5-4.0


2.0-3.0


Calcium


%


0.2-0.5


0.3-0.5


Magnesium


%


0.15-0.5


0.2-0.6


Sulfur


%


0.19-0.55


0.15-0.55













































 


 


Growth stage


Nutrient


Unit


Tillering-jointing


Boot


Iron


ppm


30-200


30-200


Manganese


ppm


20-150


20-150


Zinc


ppm


15-70


15-70


Copper


ppm


5-25


5-25


Boron


ppm


1.5-4.0


1.5-4.0


Aluminum


ppm


<200


<200


Plant analysis is an excellent tool to monitor the effectiveness of your fertilizer and lime program, and a very effective diagnostic tool. Producers should consider adding this to their toolbox.