With corn, plant analysis can be especially valuable for managing secondary and micronutrients which don’t have high quality soil tests available, and providing insight into how efficiently applied nutrients are being used.

What and when should producers sample? For general monitoring or quality control purposes, 15-20 ear leaves or the leaf below and opposite the ear, should be collected at random from the field at silk emergence, before pollination, and the silks turning brown. Sampling under stress conditions for monitoring purposes can give misleading results, and is not recommended. 

How should samples be handled, and where should producers send the samples? The collected leaves 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. Most of the soil testing labs working in the region provide plant analysis services, including the K-State lab. 

What nutrients should producers analyze for? In Kansas nitrogen (N), phosphorus (P), potassium (K), sulfur (S), zinc (Zn), chloride (Cl) and iron (Fe) are the nutrients most likely to be deficient. Recently questions have been raised by consultants and others concerning copper (Cu), manganese (Mn) and molybdenum (Mo). Most labs can analyze for most of these. Normally the best values are the “bundles” or “packages” of tests offered through many of the labs. They can be as simple as N, P and K, or can be all of the 13 mineral elements considered essential to plants.

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 percent for the primary and secondary nutrients (N, P, K, Ca, Mg, S, and Cl) 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. 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 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. Values on the low end of the range are common in extremely high yielding crops. 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.

The following table gives the range of nutrient content considered to be "normal" or “sufficient” for corn early in the season (less than 12" tall) and later in the season, at silking. Keep in mind that these are the ranges normally found in healthy, productive corn.

Nutrient Content Considered “Normal” or “Sufficient” for Corn at Two Growth Stages

 

 

Growth Stage

Nutrient

Units

Whole plant, less than 12” tall

Ear leaf at green silk stage

Nitrogen

%

3.5-5.0

2.75-3.50

Phosphorus

%

0.3-0.5

0.25-0.45

Potassium

%

2.5-4.0

1.75-2.25

Calcium

%

0.3-0.7

0.25-0.50

Magnesium

%

0.15-0.45

0.16-0.60

Sulfur

%

0.20-0.50

0.15-0.50

Chloride

%

not established

0.18-0.60

Copper

ppm

5-20

5-25

Iron

ppm

50-250

30-200

Manganese

ppm

20-150

20-150

Zinc

ppm

20-60

15-70

Boron

ppm

5-25

4.0-25

Molybdenum

ppm

0.1-10

0.1-3.0

Aluminum

ppm

<400

<200

Keep in mind also that any plant stress (drought, heat, soil compaction etc) can have a serious impact on nutrient uptake and plant tissue nutrient concentrations. So a low value in the plant doesn’t always mean the nutrient is low in the soil and the plant will respond to fertilizer, rather that the nutrient may not be available to the plant. 

Levels above sufficiency can also indicate problems. High values might indicate over fertilization and luxury consumption of nutrients. 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. Plants will load up on iron at times, in an attempt to compensate for low zinc. 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. This can occur at very low soil pH, generally well below 5.

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, try to take comparison samples from both good/normal areas of the field, and problem spots. Also collect soil samples from the same good and bad areas since physical problems such as soil compaction often limits the uptake of nutrients present in adequate amounts. Don’t wait for tasseling or silking to sample. Early in the season (prior to the 6-7th leaf, or roughly a foot high) collect whole plants from 15 to 20 different places in your sampling area. Later in the season, but prior to tasseling, collect 15-20 top, fully developed leaves (those with leaf collars visible). Handle the samples the same as those for monitoring, allowing them to wilt to remove excess moisture and avoiding mailing in plastic bags.