June means warm weather and long days and spring planted vegetable crops are growing rapidly. Monitoring the mineral nutrient status of vegetable plants is important to evaluate fertility programs and to make adjustments. Recommended fertility programs for vegetable crops are given in the Commercial Vegetable Production Recommendations publication for Delaware and surrounding states. See the website of the University of Delaware 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. Tools that are available include tissue testing, petiole sap testing, or the use of instruments such as a chlorophyll meters or NDVI sensors to monitor plant nutrient status.
Tissue testing involves taking samples from the plant (most commonly leaves) at various times during the growth period 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. Chlorophyll meters are used to measure “greenness” of individual leaves and NDVI sensors are used over top of crop canopies to measure the amount of green foliage.
When taking tissue samples, specific procedures should be followed to obtain reliable results. For whole leaves, the sample should not have any stem material. For sweet corn or onions, the leaf is removed just above the attachment point to the stalk or bulb. For compound leaves (beans, tomatoes, etc.), the whole leaf includes the main petiole and all the leaflets. With heading vegetables like cabbage take the outermost whole wrapper leaf. For young plants, the whole above-ground portion of the plant is sampled.
Most tissue tests are done using the most recently matured leaves (MRML) for analyses. Most-recently-matured leaves are leaves that are full size and have changed from the young leaf light-green color to a darker green color.
For each sample take 25 to 50 individual leaves. More accuracy in determining the actual nutrient status is derived from a larger sample size. Leaves of the same age (physiological age and position) should be removed from each sampled plant (the MRML). Plants damaged by pests, diseases, or chemicals should be avoided as well as plants with dust accumulation. Samples should be air-dried before shipment and paper (not plastic) bags should be used to ship or samples to the testing lab.
Tissue test results are interpreted using critical value tables. Results are commonly placed in the following categories:
Deficient – nutrient levels are below a critical value and plants are being affected. Corrective measures will be needed with additional fertilization.
Low – nutrient levels are below a critical value and plants may be affected. Corrective measures may be needed with additional fertilization.
Adequate – nutrient levels are in a range for normal growth.
High – nutrient levels are above the range for normal growth and may indicate over-fertilization.
Very High – nutrient levels are above the range for normal growth may be damaging to the plant or may indicate luxury consumption
In some lab results low and deficient categories are combined and very high may not be used unless a toxicity is detected.
Petiole sap testing is useful for monitoring nitrogen and potassium and can give very quick results with the use of portable meters. For sap testing, petioles collected from most recently matured leaves (MRML) are used for analyses (see above). A random sample of a minimum of 25 petioles should be collected from each field or zone of 20 acres or less. Leaves with obvious defects or with diseases should be avoided. Sampling should be done the same time of day (best between 10 AM and 2 PM).
To take petiole samples, collect whole leaves from the plant and then remove the leaf blades and leaflets. A petiole of several inches in length remains. Petioles are chopped into about one-half inch segments, crushed in a hand press, and sap is collected in a cup. Follow the instructions for the specific meter you are using to analyze the sap.
Petiole sap results are normally given in the expected range for good growth at a given crop stage.
We have added critical tissue test values and petiole sap test values to the Commercial Vegetable Production Recommendations for many vegetable crops. These can be found here online.
The chlorophyll meter is a tool that is useful to monitor nitrogen status. Test plants are fertilized with extra nitrogen so they become fully green. These test plants are then compared with the crop with normal fertilization. Again 25-50 MRML leaves are tested by clamping the sensor head to the leaf and recording the reading. The sensor should be placed in the portion of the leaf blade without large veins, midrib, or folds. Major differences between test plants and normally fertilized plants indicates lower nitrogen status and that additional nitrogen may be necessary.
NDVI sensors have been used for on-the-go sensing of crops for nitrogen status. High nitrogen test strips are used to compare with sensor readings in the field. There is the potential for on-the-go nitrogen sidedressing of crops such as sweet corn using this technology.
For more information:
Gordon Johnson, Extension Vegetable & Fruit Specialist
University of Delaware