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Commercial Horticulture > Comm Hort Blog > Posts > Evaluating the Effectiveness of Your Fertility Program—Using Plant Analysis/Leaf Tissue Testing

    Leaf tissue testing (also called plant analysis) is the best option when deciding if and how much more nitrogen or other nutritional elements needed to meet expected yields.  Leaf tissue testing can help identify any "hidden hunger" that might exist in the crop.  A "hidden hunger" develops when a crop needs more of a given nutrient but has shown no visual deficiency symptoms. 

     Leaf tissue testing is the chemical evaluation of essential element concentrations in plant tissue.  Essential elements include those that are required to complete the plant's lifecycle.  The elements carbon, oxygen, and hydrogen are supplied by the atmosphere and water and generally are not considered limiting.  Growers must place focus on the essential elements supplied by soil or feeding solutions.  Macronutrients — nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S) — are required in the greatest quantities.  Micronutrients — iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), and chlorine (Cl) — are required in very small quantities. 

     Toxicities of micronutrients are equally important and can be as yield limiting as deficiencies.  Plant analysis is effective in diagnosing toxicities of micronutrients.

     The interpretation of plant analysis results is based on the principle that healthy plants contain predictable concentrations of essential elements.  Auburn University's Soil Testing, Forage and Water Testing Lab can provide nitrogen concentrations as well as those of the other macro- and micronutrients of the plant materials to aid in fertilizer application decisions.  A program of periodic leaf tissue sampling and analysis will help you optimize your fertility program and can allow you to correct deficiencies before symptoms become apparent.  The best indicator samples have been identified for most economically important vegetable crops.  In turn this has provided the basis for developing data for which we can compare values from our analysis to those of established, recognized values.  These are called Sufficiency Ranges or Critical Values.  For those crops such as tomatoes which receive the greatest research support, indicator samples have been identified by stage of growth.  In tomato we have sufficiency ranges established for plant tissue samples taken at mid-bloom of the first, second, third, fourth, fifth, and sixth flower clusters.

     Critical values have been defined as the concentration at which there is a 5 to10% yield reduction.  The use of critical values for practical interpretation has limited value.  It is best suited to diagnose severe deficiencies and has little application in identifying hidden hunger.  Symptoms are generally visibly evident when nutrient concentrations decrease below the critical value.  Critical values play an important role in establishing lower limits of sufficiency ranges.  

     Sufficiency range interpretation offers significant advantages over the use of critical values.  First, hidden hunger in plants can be identified since the beginning of the sufficiency range is clearly above the critical value.  Sufficiency ranges also have upper limits, which provide some indication of the concentration at which the element may be in excess.

     Method for Collecting Leaf Tissue Samples for Analysis

  • Each vegetable crop has a specific corresponding plant part that is collected and used to determine foliar nutrient levels.  Often this corresponds to sampling the most recently matured or fully expanded leaves.  Careful sampling ensures the effectiveness of plant analysis as a diagnostic tool.  For major crops, best indicator samples have been identified by stage of growth.  For young seedlings, the entire plant is sampled 1.0 in above the soil level.  For larger plants, the most recent fully expanded or mature leaf is the best indicator of nutritional status.  As some crops, including corn, approach flowering and fruiting, the best indicator of nutritional status is the leaf adjacent to the uppermost fruit (ear leaf).  When unfamiliar with sampling protocol for a specific crop, it is generally acceptable to select the most recent mature leaf as the best indicator of nutritional status.  Detailed information for sampling most vegetable crops can be found at http://www.ncagr.gov/agronomi/saaesd/scsb394.pdf
  • Sample from 20 to 30 plants.
  • Sample across the field, from different rows, and avoid problem areas (low spots, ridges, washed out areas, etc.).
  • Sample when the plants are actively growing (typically between 9 a.m. and 4 p.m.).
  • Do not collect samples from water stressed plants.
  • Forms for submission can be found on the Lab's web site http://www.aces.edu/anr/soillab/
  • Send samples to the AU Soil, Forage and Water Testing Lab in a paper bag.  DO NOT SEND SAMPLES IN A PLASTIC BAG.  Plastic bags will cause your samples to spoil and will impact results.  Contact your local Extension office for information on how to submit leaf tissue samples to you state diagnostic labs (http://www.aces.edu/).

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