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Plant Sap Testing for Nitrogen and Potassium Status of Vegetable Crops

1220 views - 2020-07-09 16:09:30

 

Introduction

 

Soil and plant tissue testing is a valuable tool for determining the fertilizer needs and maximizing the fertilizer efficiency. Soil test is especially useful early in the growing season when plants are too small to collect tissue samples. Plant tissue test for nitrogen (N) and potassium (K) levels during the growing season provides information for diagnosing problems. It is preferred than soil test as the results of which can change quickly by rain or irrigation. Plant sap testing offers advantages over the conventional dry tissue testing being carried out in laboratories. Aside from the lower cost it requires, plant sap testing can be easily done in the field and the results can be obtained quickly which is important in making fertilization decisions.

 

Vegetable growers, consultants, and fertilizer companies can use the LAQUAtwin NO3- 11 or NO3-11C nitrate pocket meters and LAQUAtwin K-11 potassium pocket meter in the field to help manage N and K fertilizer. These meters are waterproof and equipped with built-in thermistor that detects the sample temperature and replaceable sensor with flat membrane that accepts as little as 0.3 ml sample (0.05 ml with sampling sheet B). They can measure plant sap in just a few seconds and display ion concentration reading expressed in either mg/L or ppm unit. The LAQUAtwin NO3-11 and NO3-11C nitrate pocket meters can measure nitrate ion (NO3 - ) and nitrate-nitrogen (NO3-N).

 


Method

 

Meter Calibration

Prior to sample measurement, calibrate the meters using the standard solutions included in their kits or prepare two standard solutions having concentrations that are ten-fold apart. Samples should be read within the calibrated range of the meters. Readings outside the calibrated range should be considered inaccurate. Dilute the sap with deionized or distilled water if the reading is above the calibrated range and take the dilution factor into account when calculating the original sap concentration.

1. Select the desired unit in the settings of each meter.

2. Calibrate the meters using their respective standard solutions According to the instruction manuals.

    • If LAQUAtwin NO3-11 is set in nitrate-nitrogen (NO3-N) mode, the readings of 150ppm and 2000ppm nitrate standard solutions will be calibrated as 34ppm and 450ppm NO3-N, respectively.
    • If LAQUAtwin NO3-11C is set in nitrate-nitrogen (NO3-N) mode, the readings of 300ppm and 5000ppm nitrate standard solutions will be calibrated as 68ppm and 1100ppm NO3-N, respectively.

When the LAQUAtwin pocket meters are used in the field, it is recommended that they are operated under a shade and recalibrated frequently throughout the day as they are sensitive to temperature changes. It is best to collect petioles from the field and analyze them with the meters indoor. Refer to Technical Tip: LAQUAtwin Ion Sensor Maintenance Procedures for conditioning, cleaning, and storing the meters.

 

Sample Collection and Preparation

The nitrate level in plant can vary throughout the day. To obtain consistent test results, reasonable standardization of time, temperature, and weather conditions under which sampling is carried out will help.

1. Collect the petioles (leaf stems) of about 20 most-recently-matured leaves—those leaves that have reached maximum size and have changed from a juvenile light green color to a dark green color. Refer to Figure 1 to help you identify petioles of some vegetable crops. There are exceptions however—for example, the roots of onions are used to etermine the nitrate levels.

Figure 1: Leaves and petioles of some vegetable crops

 

      • Take the leaves from different plants to ensure that the sap sample is representative of the field or the area being tested.
      • Petioles may be stored at room temperature (70°F, 21.1°C) in a plastic bag for up to 2 hours There are longer storage options that do not produce appreciable changes in sap N or K concentrations according to studies conducted in Florida— fresh, whole (unchopped) petioles can be placed in a plastic bag and stored on ice in a cooler for up to 8 hours or frozen overnight. Cold petioles must be warmed to room temperature before crushing so that the temperature differences between sap and meter do not affect the results.

2. Chop and mix the petioles. Take a subsample of chopped petioles for crushing.

3. Squeeze the sap from petioles using a garlic press, lemon press, or hydraulic sap press.

4.  Measure the fresh sap using the LAQUAtwin pocket meters within 1-2 minutes of pressing.

5. Record the readings. 

 


Results and Benefits


Over the growing season, vegetable crops differ widely in their nutrient needs and patterns of uptake. With regular plant sap testing and good record keeping, the trend of N and K concentrations can be followed over the season to gain insight on the nitrogen and potassium dynamics of  the crop. It is important to note the plant growth stage at sampling to know what sufficiency standard to apply. There are guidelines for various crops at various growth stages developed and published by universities and research institutes based on their research and field experience. Tables 1 and 2 contain ranges that are suggested critical values.

 

Crop

Crop Developmental Stage

Fresh Petiole SapConcentration (ppm)

NO3-N

K

Broccoli and Collard

Six-leaf stage

800-1000

NRz

One week prior to first harvest

500-800

 

First harvest

300-500

 

Cucumber

First blossom

800-1000

NR

Fruits three-inches long

600-800

 

First harvest

400-600

 

Eggplant

First fruit (two-inches long)

1200-1600

4500-5000

First harvest

1000-1200

4000-5000

Mid harvest

800-1000

3500-4000

Muskmelon

First blossom

1100-1200

NR

Fruit two-inches long

800-1000

 

First harvest

700-800

 

Pepper

First flower buds

1400-1600

3200-3500

First open flowers

1400-1600

3000-3200

Fruits half-grown

1200-1400

3000-3200

First harvest

800-1000

2400-3000

Second Harvest

500-800

2000-2400

Potato

Plants eight-inches tall

1200-1400

4500-5000

First open flowers

1000-1400

4500-5000

50% flowers open

1000-1200

4000-4500

100% flowers open

900-1200

3500-4000

Tops falling over

600-900

2500-3000

Squash

First blossom

900-1000

NR

First harvest

800-900

 

Strawberry

November

800-900

3000-3500

December

600-800

3000-3500

January

600-800

2500-3000

February

300-500

2000-2500

March

200-500

1800-2500

April

200-500

1500-2000

Tomato (Field)

First buds

1000-1200

3500-4000

First open flowers

600-800

3500-4000

Fruits one-inch diameter

400-600

3000-3500

Fruits two-inch diameter

400-600

3000-3500

First harvest

300-400

2500-3000

Second harvest

200-400

2000-2500

Tomato (Greenhouse)

Transplant to second fruit cluster

1000-1200

4500-5000

Second cluster to fifth fruit cluster

800-1000

4000-5000

Harvest season (Dec.-June)

700-900

3500-4000

Watermelon

Vines 6-inches in length

1200-1500

4000-5000

Fruits 2-inches in length

1000-1200

4000-5000

Fruits one-half mature

800-1000

3500-4000

At first harvest

600-800

3000-3500

zNR-No recommended ranges have been developed

Table 1. Guidelines for plant leaf petiole fresh sap nitrate-nitrogen- and potassium-testing

 

 

Crop Crop Developmental Stage NO3-N
Broccoli Mid growth  1000 - 1600
  Button formation 800 - 1200
  Preharvest  600 - 1000
Cabbage1 Cupping  1200 - 1500
  Early heading  1000 - 1200
  Mid heading  700 - 900
Cantaloupe  Early flower  1000 - 1200
  Fruit bulking  800 - 1000
  First harvest  700 - 800
Cauliflower  Mid growth  1000 - 1600
  Curd development  700 - 1000
  Preharvest  500 - 800
Celery  Mid growth  600 - 800
  Preharvest  400 - 600
 Lettuce  Early head formation  400 - 600
  Preharvest  350 - 500
Onion2 Bulbs 0.5 - 1.5 inches  350 - 500
Pepper  Vegetative growth  900 - 1200
  Early flower/fruit  700 - 1000
  Fruit bulking  700 - 1000
  Preharvest  700 - 900
Sweet Corn  Entire season  600 - 700
Tomato  Vegetative growth  700 - 900
  Early flower/fruit  600 - 800
  Fruit bulking  500 - 700
  Preharvest  400 - 600
Watermelon  Early flower  900 - 1100
  Fruit bulking  700 - 900
  First harvest  500 - 700

  Table 2: Guidelines for fresh petiole sap nitrate-nitrogen

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