Using Tissue Testing, Sap Testing and the Pre-Sidress Soil Nitrate Test (PSNT) to Assess Nitrogen Needs in Vegetable Crops

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

Nitrogen management in vegetable crops has often not been given the priority it deserves. Growers have fertilized according to crop needs using recommendations from published sources and from experience. However, as nitrogen (N) prices increase and as there is continued concern on reducing nitrogen losses to the environment (ground and surface waters), growers should consider using other tools to determine nitrogen needs for vegetable crops.

Nitrogen is a difficult nutrient to manage because it is in a constant state of change and is mobile and subject to losses. Nitrogen exists in both organic and inorganic forms. It is added to the soil with fertilizers, manures, crop residues, and cover crops (particularly legumes). Plants take up N as nitrate (NO3) or ammonium (NH4) but this is only a portion of what is removed from soils. Nitrate is very subject to loss by leaching with heavy rains and N can also be lost as a gas by volatilization of ammonia from the surface and denitrification (loss as N2 gas or oxide forms), most commonly with soils that are saturated with water.

To complicate matters, nitrogen undergoes many transformations in soils. Nitrogen is released as ammonium through mineralization of organic matter as it is decomposed by soil microbes. Ammonium is then transformed to nitrate by nitrifying bacteria. Soil microbes can also take up nitrogen making it immobile and temporarily unavailable. These cycles in the soil are influenced by temperature, moisture, soil chemical properties such as pH, and the composition of organic materials from crop residues.

The amount of nitrogen available at any particular time from fertilizer and organic matter will affect vegetable growth. Several tools and techniques are available to assess the nitrogen status of vegetable crops and then adjust nitrogen fertilization accordingly.

Quick tests for nitrogen status of vegetables have been developed using sap expressed from vegetable plants. Petioles, midribs, or stems will be used depending on the crop. Sap is analyzed with a portable nitrate tester (Cardy nitrate meter). This technique is especially useful in drip irrigated vegetables where nutrients can be added through the irrigation water. Guidelines have been developed for different crops and are given in Table 1.

Table 1. Guidelines for Plant Fresh Sap Nitrate-Nitrogen-and-Potassium-Testing.
(Petioles from recently matured leaves are used unless otherwise indicated)

Crop Crop Developmental Stage

Fresh Petiole Sap Concentration (ppm)

NO3-N K
Cabbage (midrib)  Cupping
Early heading
Mid heading
1200-1500
1000-1200
700-900
 
Sweet Corn (lower stem) All stages 600-700  
Broccoli and Collard Six-leaf stage
One week prior to first harvest
First harvest
800-1000
500-800
300-500
NR*
Cucumber First blossom
Fruits three-inches long
First harvest
800-1000
600-800
400-600
NR
Eggplant First fruit (two-inches long)
First harvest
Mid harvest
1200-1600
1000-1200
800-1000
4500-5000
4000-5000
3500-4000
Muskmelon First blossom
Fruit two-inches long
First harvest
1100-1200
800-1000
700-800
NR
Pepper First flower buds
First open flowers
Fruits half-grown
First harvest
Second harvest
1400-1600
1400-1600
1200-1400
800-1000
500-800
3200-3500
3000-3200
3000-3200
2400-3000
2000-2400
Potato Plants eight-inches tall
First open flowers
50% flowers open
100% flowers open
Tops falling over
1200-1400
1000-1400
1000-1200
900-1200
600-900
4500-5000
4500-5000
4000-4500
3500-4000
2500-3000
Squash First blossom
First harvest
900-1000
800-900
NR
Strawberry November
December
January
February
March
April
800-900
600-800
600-800
300-500
200-500
200-500
3000-3500
3000-3500
2500-3000
2000-2500
1800-2500
1500-2000
Tomato (Field) First buds
First open flowers
Fruits one-inch diameter
Fruits two-inch diameter
First harvest
Second harvest
1000-1200
600-800
400-600
400-600
300-400
200-400
3500-4000
3500-4000
3000-3500
3000-3500
2500-3000
2000-2500
Tomato (Greenhouse) Transplant to second fruit cluster
Second cluster to fifth fruit cluster
Harvest season
1000-1200
800-1000
700-900
4500-5000
4000-5000
3500-4000
Watermelon Vines 6-inches in length
Fruits 2-inches in length
Fruits one-half mature
At first harvest
1200-1500
1000-1200
800-1000
600-800
4000-5000
4000-5000
3500-4000
3000-3500

*NR-No recommended ranges have been developed
Information from University of Florida and UC-Davis

Plant tissue testing is another alternative to assess the nitrogen status of soils. Recently matured leaves are sampled and sent to a laboratory for analysis. The University of Florida lists the critical values at this site http://edis.ifas.ufl.edu/EP081. Examples using sweet corn and watermelon are given in Table 2 and Table 3.

Table 2. Critical (deficiency) values, adequate ranges, and high values for macronutrients for sweet corn

Plant Part* Time of Sampling

Status

- – - – - – - – - – - – % – - – - – - – - – - -

N

P

K

Ca

Mg

S

Whole seedlings 3 leaf stage

Deficient

<3.0

0.4

2.5

0.6

0.25

0.4

Adequate Range

3.0

0.4

2.5

0.6

0.25

0.4

4.0

0.5

4.0

0.8

0.5

0.6

High

>4.0

0.5

4.0

0.8

0.5

0.6

Whole seedlings 6 leaf stage

Deficient

<3.0

0.3

2.5

0.5

0.25

0.4

Adequate Range

3.0

0.3

2.5

0.5

0.25

0.4

4.0

0.5

4.0

0.8

0.5

0.6

High

>4.0

0.5

4.0

0.8

0.5

0.6

MRM leaf 30 inches tall

Deficient

<2.5

0.2

2.5

0.5

0.2

0.2

Adequate Range

2.5

0.2

2.5

0.5

0.2

0.2

4.0

0.4

4.0

0.8

0.4

0.4

High

>4.0

0.4

4.0

0.8

0.4

0.4

MRM leaf Just prior to tassel

Deficient

<2.5

0.2

2.0

0.3

0.15

0.2

Adequate Range

2.5

0.2

2.0

0.3

0.15

0.2

4.0

0.4

3.5

0.6

0.4

0.4

High

>4.0

0.4

3.5

0.6

0.4

0.4

MRM leaf (ear leaf) Tasseling

Deficient

<1.5

0.2

1.2

0.3

0.15

0.2

Adequate Range

1.5

0.2

1.2

0.3

0.15

0.2

2.5

0.4

2.0

0.6

0.4

0.4

High

>2.5

0.4

2.0

0.6

0.4

0.4

*most-recently-matured whole leaf plus petiole (MRM leaf) unless otherwise noted

Table 3. Critical (deficiency) values, adequate ranges, and high values for macronutrients for watermelon

Plant Part* Time of Sampling

Status

- – - – - – - – - – - – % – - – - – - – - – - -

N

P

K

Ca

Mg

S

MRM leaf Layby (last cultivation)

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

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

*most-recently-matured whole leaf plus petiole (MRM leaf)

Table 4. Sidedress Nitrogen Recommendations for Sweet Corn Based on the PSNT Soil Test Level and Manure History

PSNT Soil Test Level(ppm NO3-N) Sidedress N Recommendation(lbs/acre)*
Manured Soils
0 to 10 160
11 to 15 120
16 to 20 80
21 to 25 40
greater than 25 0
Non-Manured Soils
0 to 15 160
16 to 20 120
21 to 25 80
26 to 30 40
greater than 30 0

*When 100 lbs. or more of sidedress N are recommended on very light sandy soils, apply half of the sidedress when the corn is 12 inches tall and half when the corn is 18 to 24 inches tall.

The Presidedress Soil Nitrate Test (PSNT) has been developed to assess the nitrate levels in soils just prior to sidedressing in field corn and relate that to expected crop response to nitrogen fertilizer. As soils warm, mineralization of organic matter increases along with nitrification. By measuring nitrate levels prior to sidedressing a “snapshot” of N available from organic sources is obtained. Therefore, the PSNT is used where manures have been applied or leguminous cover crops have been grown and limited fertilizer N has been applied preplant or at planting. This test has been adapted to several vegetable crops such as sweet corn, peppers, and pumpkins. Soil samples are taken about a week prior to normal sidedressing at a depth of 12 inches. They are dried and then tested for nitrate at a laboratory or using a quick testing kit (available from several sources). There is an example for sweet corn from Rutgers University in Table 4.

Other PSNT recommendations for vegetable crops can be found at the Spectrum Analytical website: http://www.spectrumanalytic.com/support/library/ff/Presidedress_Soil_Nitrate_Test_Corn.htm.

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