Tissue Testing and Petiole Sap Testing for Vegetables

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Recommended fertility programs for vegetable crops are given in the Commercial Vegetable Production Recommendations publication for Delaware and surrounding states. See http://ag.udel.edu/extension/vegprogram/publications.htm for an electronic version.

While these recommendations should be the base of a fertility program, additional monitoring of plant nutritional status is recommended, especially for highly managed crops such as those grown in plasticulture where fertilizers can be injected through the drip irrigation system.

Tissue testing involves taking samples from the plant at various times during the growth period, most commonly leaves, and sending them to a laboratory for mineral nutrient analysis. Petiole sap testing involves taking leaf petioles and expressing the sap which is then tested for nitrate and/or potassium using portable meters.

When taking tissue samples specific procedures should be followed to obtain reliable results. The following are recommendations from the University of Florida.

“The sample is a whole leaf sample and it should not contain any root or stem material. For sweet corn or onions, the leaf is removed just above the attachment point to the stalk or bulb. For compound leaves (carrots, peas, tomatoes, etc.), the whole leaf includes the main petiole, all the leaflets and their petioliules. For heading vegetables, it is most practical to take the outermost whole wrapper leaf. When sampling particularly young plants, the whole above-ground portion of the plant may be sampled.”

Most commonly the most recently matured leaves (MRML) are used for analyses. Most-recently-matured leaves (MRML) are leaves that have essentially ceased to expand and have turned from a juvenile light-green color to a darker-green color.

“A proper leaf sample should consist of about 25 to 100 individual leaves. The same leaf (i.e., physiological age and position) should be removed from each sampled plant. Plants damaged by pests, diseases, or chemicals should be avoided when trying to monitor the nutrient status of the crop. Individual plants, even side-by-side, may have a considerably different nutrient status. Therefore, by sampling a sufficiently large number of plants, the error due to this variability can be minimized. More accuracy in determining the actual nutrient status is derived from a larger sample size.”

“Samples are often contaminated by fungicides, nutrient sprays, soil, or dust. Data obtained from contaminated leaf samples will be misleading. Decontamination of some dust or soil is best accomplished by quickly rinsing in a dilute non-phosphate detergent solution (2%) followed by two distilled water rinses. Tap water should not be used because it can be high in certain nutrients such as Ca, Fe, Mg, or S. Leaf samples should be washed quickly to minimize the leaching of certain nutrients (especially K) from the leaves.”

“Following rinsing, the sample should be blotted dry with absorbent paper. The samples should be air-dried for several hours before shipment. If a plant analysis mailing kit is not available, the samples should be wrapped in fresh absorbent paper and placed in a large envelope (plastic bags must not be used). The sample should be shipped or delivered immediately to the soil and plant analysis laboratory. An air-dried sample, if loosely packed to avoid rotting, will last two to three days before decomposition begins.”

“If the samples must be held for any length of time before shipping, they should be dried at 150°F in a ventilated oven (leave the door ajar) until dry weight is constant. Once dried, the sample can be placed in a plant analysis mailing kit or a large envelope. This ensures the integrity of the sample until shipping is possible.”

Petiole sap testing is useful for monitoring nitrogen and potassium and can give very quick results with the use of portable meters. The following are guidelines for petiole sap testing from the University of Florida:

“For sap testing, petioles collected from most recently matured leaves (MRML) are used for analyses. Most-recently-matured leaves (MRML) are leaves that have essentially ceased to expand and have turned from a juvenile light-green color to a darker-green color. A random sample of a minimum of 25 petioles should be collected from each “management unit” or “irrigation zone.” Management units larger than 20 acres should be subdivided into 20-acre blocks. Leaves with obvious defects or with diseases should be avoided. Sampling should be done on a uniform basis for time of day (best between 10 AM and 2 PM), and for interval after rainfall or fertilization.”

“Whole leaves are collected from the plant and the leaf blade tissue and leaflets are then stripped from the petiole. A petiole of several inches in length remains. Petioles are chopped into about one-half inch segments. If analysis is not to be conducted immediately in the field, then whole petioles should be packed with ice and analyzed within a few hours of collecting. Given more extreme environmental field conditions (high temperature and bright sun), more dependable results are obtained by making measurement in the lab or office than outdoors.”

“Chopped petiole pieces are mixed and a random subsample (about ¼ cup) is crushed in a garlic press, lemon press, or hydraulic press (obtainable from HACH Co., Table 4). Expressed sap is collected in a small beaker or juice glass and stirred.”

Follow the instructions for the specific meter you are using to analyze the sap. If sap has too high of concentration of nitrate or potassium for your meter, then you will need to dilute the sap to conduct the test.

Information on tissue testing and petiole sap testing for vegetables including tables with recommended levels at different growth stages can be found at this site http://edis.ifas.ufl.edu/ep081.

The following are recommended values for watermelons: 

Plant Tissue Macronutrient Ranges for Watermelons at Different Growth Stages

Crop Plant Part Time of Sampling Status - – - – - – - – - – % – - — – - – - -
N P K Ca Mg S
Watermelon MRM* leaf Vining before flowering Deficient <3.0 0.3 3.0 1.0 0.25 0.2
Adequate range 3.0 0.3 3.0 1.0 0.25 0.2
4.0 0.5 4.0 2.0 0.5 0.4
High >4.0 0.5 4.0 2.0 0.5 0.4
Toxic (>) - - - - - -
MRM leaf First flower Deficient <2.5 0.3 2.7 1.0 0.25 0.2
Adequate range 2.5 0.3 2.7 1.0 0.25 0.2
3.5 0.5 3.5 2.0 0.5 0.4
High >3.5 0.5 3.5 2.0 0.5 0.4
MRM leaf First fruit Deficient <2.0 0.3 2.3 1.0 0.25 0.2
Adequate range 2.0 0.3 2.3 1.0 0.25 0.2
3.0 0.5 3.5 2.0 0.5 0.4
High >3.0 0.5 3.5 2.0 0.5 0.4
MRM leaf Harvest period Deficient <2.0 0.3 2.0 1.0 0.25 0.2
Adequate range 2.0 0.3 2.0 1.0 0.25 0.2
3.0 0.5 3.0 2.0 0.5 0.4
High >3.0 0.5 3.0 2.0 0.5 0.4

*MRM – most recently matured leaf with petiole.

Petiole Sap Nitrate and Potassium Concentration Ranges for Watermelon

Crop Stage of Growth Fresh Petiole Sap Concentration (ppm)
Watermelon Vines 6-inches in length

Fruits 2-inches in length

Fruits one-half mature

At first harvest

4000 to 5000

4000 to 5000

3500 to 4000

3000 to 3500

1200 to 1500

1000 to 1200

800 to 1000

600 to 800


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