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Department of Agronomy

Plant, Forage & Grain Analysis


Plant, forage, and grain samples are sent to the KSU Soil Testing Lab for nutritional status and nitrate testing. Nutrient analysis includes nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), zinc (Zn), iron (Fe), copper (Cu), manganese (Mn), sulfate-sulfur (SO 4 -S), chloride (Cl).

The length of time required for analysis of plant, forage, and grain samples depend largely on the moisture content of the sample. If the samples (plant samples specifically) are split after sampling to allow drying to begin, the total laboratory time will be reduced. Immediately after submission to the lab, the sample will be dried in an oven, followed by analysis. Results will then be reported to the producer by phone or email.

Available Testing

Sulfuric Acid Hydrogen Peroxide Digestion

Prepared plant material (0.2500g) is weighed into a 50 ml Kimax digestion tube. In a fume hood, 2ml of concentrated sulfuric acid is added to the tube, then 1ml 30% peroxide. After oxidation reaction has subsided, the tube is placed on a digestion block at 3000C for 20 minutes. Tube is then cooled to room temperature and another 1 ml of peroxide is added before returning to the digestion block.

This is repeated until the acid in the tube is clear and colorless. After cooling, the sample is diluted to 50 ml with ddH20 and mixed by inverting the tube twice. Analysis for Potassium (K + ) is by Flame Atomic Absorption using sulfuric acid/hydrogen peroxide digested plant material.

Salicylic-Sulfuric Acid Digestion

Prepared plant material (0.2500g) is weighed into a 75ml Techator digestion tube. Boiling chips and 10 ml of the Salicylic-sulfuric acid mixture is added. After 2 hours, 0.6 grams of sodium thiosulfate is added and each tube mixed on a Vortex mixer. Samples stand overnight. Tubes are placed in a cold Techator digestion block in a fume hood with a fume capture system attached and the temperature is set at 2000C.

After 1 hour, the temperature is increased to 3850C for 1 hour them samples are removed from the block and allowed to cool. One catalyst tablet (SST-Auto from Alfie Packers) is added to each and the tubes are returned to the hot block for 3.5 hours. After cooling, samples are diluted to 75ml using ddH2O and mixed by inverting the tube 3 times.

Nitric-Perchloric Acid Digestion

Prepared plant material (0.2500g) is weighed into a 50 ml Kimax digestion tube. Boiling chips and 5 ml of Nitric acid is added in a fume hood. Tubes are covered with saran wrap overnight. 5ml of Perchloric acid is added to predigested plant material and tubes are placed on a cold Techator digestion block. Heat is set at 2000C and digests are heated for 2.5 to 3 hours or until white fumes appear and the acid is clear (and colorless when cooled to room temperature).

Tubes are diluted to 25 or 50 ml with ddH2O and mixed by inverting twice. Analysis for Ca 2 + , Mg 2+ , K + , Cu + , Fe 3+ , Mn 2+ , Zn + , SO 4 2- is by Flame Atomic Absorption or ICP Spectrometry using Nitric-Perchloric digested plant material.

Nitrogen & Phosphorus

Digested plant material from either a sulfuric acid/hydrogen peroxide digest or a Salicylic-Sulfuric digest is analyzed simultaneously using the Technicon AAII auto analyzer with the colorimetric Industrial Method 334-74W/B using separate channels.


Sample Collection

Forage Collection

Plant sample collection is very specific for each crop and for the appropriate plant part. If samples are not taken from the correct plant part or are collected at the wrong time, there may not be information to interpret the results.

For specific plant sampling instructions, please contact the KSU Soil Testing Lab. If there are not specific instructions, the rule of thumb is to sample the upper mature leaves.

Below is a list of common crops, sampling times, and brief instructions for each.

  • For corn, collect 15-20 ear leaves or the leaf above and opposite the ear shoot at green silk.
  • For sorghum, collect 15-20 flag leaves at flowering.
  • For soybeans, collect 25 sets of the top, fully developed leaflets, without the petioles, when the first pods are ½ to 1 inch in length.
  • For wheat, collect 25-30 flag leaves at flowering.

Forage sampling can include sampling in a grazing system and sampling in a hay production system.

  • For cut hay, sample 10-15 areas and take 1 subsample for each 40 acres. If the hay is baled, probe 1 sample for every 5 round bales or 100 square bales. Combine the samples and submit one subsample.
  • For grazing systems, cut a small area at a normal grazing height from 10-15 areas in a field and submit one random subsample.

Contact Us

Soil Testing Laboratory

K-State Research and Extension
2308 Throckmorton PSC
1712 Claflin Road
Manhattan, KS 66506-5503

Ph: +1-785-532-7897

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