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.
(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
*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.