Posts Tagged ‘vegetable crop nutrients’

Magnesium Deficiencies in Vegetables

Friday, June 25th, 2010

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

I have recently looked at a field of pickling cucumbers with areas that showed symptoms of magnesium deficiency. Magnesium (Mg) is considered a secondary macroelement and is essential for plant growth. It is a component of chlorophyll, the green pigment that captures light energy in photosynthesis. The chlorophyll molecule has a porphyrin ring with a magnesium atom at the center. Therefore, deficiencies of magnesium will result in reduced chlorophyll production and yellowing of plants.

In most vegetable crops, magnesium deficiency commonly first appears as yellow or white areas between the veins of older leaves. As the deficiency progresses, the yellowed areas may turn into dead spots. Older leaves in plants may also have a purple or bronze appearance and leaf tips and margins may brown and die. The plants may be stunted and have an overall yellow appearance. Symptoms are most severe on older leaves because magnesium is a mobile element in plants and will be scavenged from older leaves and transported to new growth.

In Delaware, magnesium deficiencies are most commonly found in sandy, acid soils with a pH below 5.4. Therefore, magnesium deficiencies are commonly not field wide, but will be in areas of a field with depressed pH such as “sand hills” that have been excessively leached. Often a whole field pH will be in an acceptable range so it is critical to check the soil pH in affected areas. Tissue tests should be considered to confirm the magnesium deficiency.

Excessive levels of potassium can also induce magnesium deficiency where available magnesium levels are low to moderate to begin with.

Commonly, magnesium is applied to soils with dolomitic limestone (Hi-Mag lime). Sulfate of potash and magnesia (K-Mag, Sul-Po-Mag) is a naturally mined mineral deposit that can also be applied to add magnesium to soils. Other magnesium sources include magnesium sulfate (same as Epson Salts), magnesium oxide (basic slag), and magnesium chloride.

To correct a deficiency in growing vegetables, soluble magnesium sources should be used. Foliar applications are effective but must be applied in a dilute solution to avoid salt injury. Spray 20 lbs of a soluble magnesium source (20 lbs of magnesium sulfate for example) in 100 gallons of water per acre (10 lbs in 50 gallons or 5 lbs in 25 gallons). Dry broadcasts of 15-25 lbs of actual magnesium per acre, irrigated in, or fertigation with similar amounts from soluble sources will also be effective. Sidedress applications may also be effective at 15-20 lbs of actual magnesium per acre. For drip irrigated vegetables, soluble magnesium fertilizers can be applied through the drip system.

Mangesium deficiencies corrected early enough in the growing season will often result in little yield loss. However, it is critical to target affected fields with corrective liming for future crops in the rotation. Variable rate liming may be considered and is recommended where there is excessive variability in pH in a field.

If pH is below 5.2 and vegetables are still small, dolomitic limestone may be broadcast over the top and cultivated in to correct pH related problems. This should be coupled with a foliar magnesium application to more quickly address the magnesium deficiency.

In vine crops, low pH may also be a causal factor for manganese toxicities and you may see both magnesium deficiency and manganese toxicity in the same field. Manganese toxicity symptoms in melons will initially show up as small yellow spots on upper leaf surfaces. On lower leaf surfaces you will see dead spots with water soaked rings around these dead spots. As the deficiency worsens, these leaf areas will turn brown and die. In watermelons, manganese toxicity will show up as black speckling n the lower leaf surfaces and extensive vein browning. However, manganese toxicity is not common in watermelon. For a review of manganese toxicity in cantaloupes refer to an article by Jerry Brust in the WCU archives Volume 14, Issue 15, July 7, 2006.

Fertigating Drip Irrigated Vegetables

Thursday, June 17th, 2010

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

Fertigation is the term used when soluble fertilizer sources are delivered through the irrigation system to crops. Drip irrigation is an ideal means to fertigate and to deliver mineral nutrients to vegetables during the growing season. Nutrients are carried with the irrigation water right to the root zone where they can be efficiently taken up by vegetable plants.

There are several strategies for fertigating vegetable plants. One strategy is to split fertigation so that crop nutrient needs, after preplant fertilizers are accounted for, are delivered in 4-5 applications just prior to critical growth stages. For example, for fruiting vegetables, the first fertilizer application through the drip system would be done after planting when plants have become established, the next prior to rapid vegetative growth, the next at flowering or early fruit formation, and the last during fruit expansion. For crops that have long fruiting and harvest periods, an additional application would be made after first harvest to encourage continued production.

Other strategies use weekly applications or applications of fertilizers through the drip system every time the crop is irrigated. In these systems, smaller amounts of fertilizers are applied each time and rates are increased as plants get larger. This requires a somewhat higher level of management.

For general vegetable fertigation through the drip, a 1-1-1 N-P2O5-K2O ratio soluble fertilizer (such as 20-20-20) is recommended. Where phosphorus (P) levels are very high, lower P ratios are appropriate (such as a 21-5-20). In some vegetables, only nitrogen (N) sources will be needed if soil fertility (P and K) are high. Soluble potassium nitrate and calcium nitrate are often used in combination in crops such as tomatoes and plasticulture strawberries to provide N, K (potassium), and Ca (Calcium).

Fully soluble fertilizers must be used for fertigation. Those in dry form must be mixed with water until they fully dissolve to create a concentrated stock solution. Those already in liquid form should be checked to make sure there has been no salting out of nutrients during storage – if salting out has occurred, you will need to make sure the fertilizer re-dissolves by agitation prior to use. It is important to know how much fertilizer is contained in these liquid stock solutions to match to injection rates.

A good quality fertilizer injector matched to the flow rate of your drip system is important to deliver the fertilizer the length of each bed uniformly in the field. Run the drip system to fill the drip tubes and come to steady pressure, start injecting, and then continue injecting using an injection rate that matches the irrigation period. You may then run the irrigation for a short period after fertigation to flush the lines. It is important not to over-irrigate as nutrients may be moved out of the root zone (especially N). Fertigation rates should be base on a mulched acre – that is only the amount of ground covered by plastic mulch.

For more information on fertigation go to our Commercial Vegetable Production Recommendation guide http://ag.udel.edu/extension/vegprogram/pdf/CIrrigation.pdf starting on page C-5.

Calcium Disorders

Friday, June 4th, 2010

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

I have recently looked at both cabbage and spinach fields showing signs of calcium deficiency. With that in mind, the following is information taken from a 2008 article that I wrote on calcium deficiencies.

Calcium disorders in field crops are not common because calcium dominates exchange sites in soils and is therefore rarely deficient in corn, soybeans, and small grains if they have been properly limed. However, a number of calcium disorders can occur in vegetable and fruit crops, even in well limed soils. Some of these disorders include:

● Blossom end rot in tomatoes, peppers, and eggplants
● Blossom end rot in watermelons
● Watercore and glassiness in melons
● Internal leaf tipburn in cabbage
● Leaf tipburn and curd defects in cauliflower
● Internal browning of Brussels sprouts
● Leaf tipburn in spinach
● Leaf tipburn in lettuce
● Leaf tipburn and deformity in strawberry
● Internal browning, hollowheart, storage disorders, and poor skin set in potatoes
● Cavity spot in carrots
● Bitter pit, cork spot, cracking, internal brownspot, and water core in apples
● Hypocotyl necrosis in beans and other legumes
● Meristem death or distortion of new growth from meristems in many plants (cupped leaves)

Calcium is taken up in quantity from the by plants from the undifferentiated area right behind the root tip. Once in the root, it moves in the xylem (water conducting vessels) and is distributed in the plant. Much of this movement in the xylem occurs by exchange. Calcium is attracted to the xylem wall and must be displaced by another ion (another calcium or other cation). This process is driven by transpiration and subsequent water movement through the xylem. Therefore, calcium movement is relatively slow compared to other nutrients that move easily in the transpiration flow. Calcium is not translocated in the phloem (plant food transport system) so it cannot move from one area of the plant to another.

Calcium has many roles in the plant from root growth control, to cell membrane function, to stomatal regulation. The main function that leads to the disorders listed above is in the formation of plant structure. Calcium is component of cell walls and the middle lamella that cements plant cells together. Calcium provides cross linkages in the pectin-polysaccharide matrix and adds to the structural strength of plant tissues. When insufficient calcium is present, plant tissues do not form properly and they may appear deformed and in severe cases may become necrotic – tissues may die or collapse.

There are a number of reasons why vegetables and fruits are more susceptible to calcium disorders. Because calcium moves slowly through exchange in the xylem and is dependent upon water flow, disruptions in that flow can lead to localized deficiencies in calcium. Plant organs with low transpiration rates or that are rapidly expanding such as fruits and storage roots often do not receive enough calcium to support that growth. Growing tips and meristematic areas that are rapidly laying down new cells are also at risk for calcium deficiencies when water flow is interrupted. High humidity, drought, flooding (leading to roots shutting down), root injury, compaction, and root diseases can therefore lead to calcium disorders by the reduction of water flow and calcium exchange and movement in the xylem.

Competition from other cations such as magnesium (Mg2+), ammonium (NH4+), and potassium (K+) can also affect calcium (Ca2+) uptake and movement. In low pH soils, aluminum can interfere with calcium uptake and lead to deficiencies.

Control of calcium disorders starts with proper liming. This provides soil calcium and raises the pH to eliminate the effect of aluminum. The most important factors to control calcium disorders are to supply a steady rate of water (through a good irrigation program), limit root damage (such as root pruning by cultivation), provide a rooting area for plant that is free from compaction and waterlogging, and create a healthy soil environment that limits root disease potential. Above ground, planting at spacings that allow for good air movement around the plant will also help. Control fertilizer programs to limit competition between calcium and other ions (use nitrate forms of nitrogen instead of ammonium forms for example). In addition, choose varieties that are less susceptible to these calcium disorders (varieties with very large or very long fruit are more susceptible to calcium deficiencies).

In cases where calcium is low but where you do not want to increase the pH (as is the case with scab susceptible potatoes or with blueberries), gypsum (calcium sulfate) can be applied to supply additional calcium to the crop. Rates of gypsum application are from 500 to 1000 lbs/acre commonly.

If a calcium deficiency is evident or suspected, calcium nitrate applications often will help. For crops grown with drip irrigation, the calcium nitrate can be put on through the drip system. For other crops, calcium nitrate can be spun on, watered in, sidedressed, or applied through fertigation with overhead irrigation. Calcium nitrate is very water soluble so uncoated forms are used for fertigation and coated forms are used when it is being spun on. Calcium nitrate is 15.5% nitrogen so application rates will largely depend on how much nitrogen can be applied to the crop at any time. Generally 200-400 lbs/acre of calcium nitrate are used.

Sidedressed calcium has been shown to have positive effects on root crops such as potatoes, particularly in sandy soils. Forms that have been used include calcium sulfate (gypsum) and calcium nitrate.

There have been mixed results with foliar application of calcium and these applications should be considered a supplement to help limit these disorders and not a correction for calcium deficiencies and good soil and water management. As stated before, calcium movement is limited so it will be difficult to get calcium to where it is needed by foliar sprays except when applied to active meristematic tissue. However, calcium sprays have been effective in improving quality and eliminating calcium disorders in apples.

On Farm Adoption of Soil Health Practices as a Part of Integrated Pest Management for Vegetables

Thursday, April 29th, 2010

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

Vegetable crops are susceptible to a number of soil borne pests. In particular, soil borne diseases such as Fusarium in vine crops and solanaceous crops, Rhizoctonia in bean crops, Sclerotinia in bean crops, Verticilium in solanaceous crops, Pythium in most vegetables, and Phytophthora in vine crops, solanaceous crops, and bean crops have major impacts on the productivity of important vegetable crops in Delaware . Nematodes are also an issue on many vegetable crops in the state (root knot on many vegetables, lesion on potatoes as examples). Current control practices include fumigation and the use of soil applied fungicides or nematicides. However, the most effective control is the use of long rotations with non-host crops. This has been the standard extension recommendation to reduce the economic impact of these diseases and reduce the need for chemical controls. Long rotations are difficult to achieve on many farms due to land limitations. This problem has been worsened by the pressure of development and the decrease in farmland for rotations.

Insects and slugs in soils are also issues in many vegetable crops that affect overall plant heath. Often, as with the case of seed corn maggots, cutworms, and slugs; practices that are recommended to improve soil health – such as use of cover crops and no-till planting in vegetable production – can increase problems with these pests. This limits the adoption of these practices.

A major limitation to overall plant health in Delaware vegetables is soil compaction. Not only does compaction directly affect root growth, it also leads to increased problems with soil borne diseases as compacted soils often stay wetter for longer periods. In processing vegetables, harvest schedules often necessitate the use of heavy equipment and trucks when soils are wet, leading to extensive, deep, and long term compaction problems. Soil health is compromised and performance of future crops is affected.

Weed competition is another important issue in vegetable production. Management of the weed seed bank in soils can be a challenge and should be considered in an overall soil health program. Many vegetable crops have limited herbicides available to manage weeds and there has been an increase in the number of herbicide resistant weeds. Notably, herbicide resistant pigweed is a major problem in Delaware vegetable production. Adoption of soil health building practices such as the combination of cover crops with no-till and strip-till vegetable production is often limited by these weed control challenges. Weed control in organic vegetable production has also been a major problem limiting organic acreage.

Overall vegetable plant health is greatly affected by plant nutrition and soil fertility. Delaware requires nutrient management plans on all farms. Part of this planning process is how to incorporate best management practices to reduce nitrogen (N) and phosphorus (P) losses. Manure and compost use needs to be balanced against vegetable crop requirements and existing soil levels (for P) often limiting applications. On the other hand, practices benefiting soil such as the use of winter cover crops are encouraged as they can recycle nutrients (N primarily) that would be otherwise lost. While not directly related to management of a specific pest, decisions related to soil fertility and nutrient management will have implications for soil health in general, and therefore needs to be included in soil health and IPM program for vegetables.

Water management is directly tied to practices that influence soil health and vegetable crop performance. This includes ways to increase water holding capacities of soils; and at the same time, how to maintain good drainage and aeration. Improving soil water holding capacities will reduce vegetable plant stress and improve overall plant health. In addition, water management is linked closely with many soil borne pests, particularly in relation to drainage, with disease organisms such as Phytophthora being a major problem with vegetables in poorly drained soils in Delaware.

There has been a renewal of interest in soil health in relation to vegetable production in Delaware especially where tight rotations are an issue. Fortunately, there has been considerable research related to cover crops, green manures, compost, organic matter, and rotations in the past 10 years and there is ongoing research in the region on the effect of different rotations and species to improve soil health and reduce soil borne pests. Vegetable cropping systems that incorporate cover crops with no-till production or that have limited tillage have also been studied in the region. Other research in the region on cover crops that can reduce soil compaction is of great interest. There has been research on soil nutrient and water management for vegetable crops but limited work on how these areas intersect with soil health. Delaware vegetable growers, large and small, can benefit by adopting specific practices that improve soil health and by incorporating these practices into an integrated pest management programs for their farms.

Starting in 2009 and continuing into 2010, there has been a coordinated educational effort in Delaware on soil health as a part of integrated pest management for vegetables with field demonstrations, classroom sessions, publications, and on-farm training sessions. This initial effort emphasized how to evaluate overall soil health on a farm, the use of compost, general cover crop and green manure crop use, and biofumigant cover crops use.

We will be continuing these extension programs on soil health as a part of vegetable integrated pest management (IPM) programs over the next 3 years. In this effort we will expand educational areas to include soil disease management, soil insect and slug management, soil compaction management, weed management, soil fertility and nutrient management, and soil water management as part of an overall soil health and IPM program for vegetable crops. The emphasis will be on the on-farm adoption of soil health practices.

We are seeking 20 vegetable farmers as potential cooperators, targeting farms that are experiencing soil health problems and that have tight vegetable rotations. Plans to improve soil health in problem fields will be developed working with each farmer. This will include recommendations for rotations, cover crop use, green manure use, compost use, manure use, other organic matter additions, biofumigant crop use, and tillage practices. These prescriptive plans will be implemented by each farmer and the effectiveness of suggested actions will be evaluated at the end of 3 years by using soil health assessment tests (before the program starts and after 3 years). The economics of using these methods will also be assessed.

Delaware growers interested in participating in this program should contact Gordon Johnson, Extension Vegetable and Fruit Specialist, University of Delaware gcjohn@udel.edu, (302) 856-7303 office or (302) 545-2397 cell.

Late Summer and Fall Cover Crops for Vegetable Ground

Friday, August 28th, 2009

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

Vegetable growers should make plans to put in late summer or fall cover crops after summer vegetables are harvested. Cover crops help to maintain organic matter, recycle nutrients, reduce compaction, and maintain overall soil health. These benefits far outweigh the cost of establishing the cover crops.

The following are some cover crops to consider:

Winter Annual Legumes
These cover crops will produce significant biomass (organic matter) and, at the same time, provide nitrogen for the following crop through biological nitrogen fixation: hairy vetch, crimson clover, field peas (winter peas). Hairy vetch makes an excellent mulch for no-tilling vegetables into. Plant by September 30.

Small Grains
These winter annual grasses will provide significant biomass, recycle nutrients (especially nitrogen), and produce excellent mulch for no-tilling vegetables in the spring: rye, triticale, wheat, barley, winter oats. Spring oats can be used where you want to get fall cover but need the crop to winter kill for early spring vegetable crops. Plant by the end of October.

Mustard Family Cover Crops
These include both fully hardy overwintering species and species that will winter kill. They provide significant organic matter, recycle nitrogen, can reduce compaction, and offer the potential for biofumigation. Plant by September 15. Included are:

Rapeseed and Canola – overwinter and are good biofumigants

Forage Radish, Oilseed Radish, and Daikon Radish – very good for reducing compaction in soils; forage radish winter kills, oilseed radish is more hardy

Mustards (brown and yellow mustards as well as garden mustard) – offer good biofumigant potential; half hardy

Turnips (forage and garden types) – good biomass production; half hardy

Kale (forage and garden types) – winter hardy; good biomass production

Hybrid Forage Brassicas (such as ‘Typhon’) – these are hybrid crosses of two or more species that will produce excellent fall growth and some will overwinter

Annual Ryegrass
This winter annual grass offers easy establishment, even when overseeded, and puts on significant fall and spring biomass. It scavenges nitrogen and is a quick decomposer in spring. Plant by October 15.

For seeding rates, contact you County Extension Office.

It is often advantageous to plant several of these cover crops together and most will mix well. Use the planting deadline for the species that has to be planted the earliest. Reduce the rate of each component in the mix by 1/3 to ½. I particularly like a rye-hairy vetch-crimson clover mix.

Nitrogen Deficiency in Sweet Corn

Friday, July 17th, 2009

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

Residual effects of wet weather in May and early June continue to be evident in vegetable crops across the state. Nitrogen deficiency is the most common nutrient related disorder being found. This was a difficult year to determine how much additional nitrogen was going to be needed at sidedressing. Leaching rains and waterlogged fields with high levels of denitrification complicated the issue as up to 60% of N applied preplant or at planting was lost, N mineralization from organic matter and manure additions was reduced, and any N that was mineralized was subject to further losses with heavy rains. In addition, in wet areas, corn roots were not functioning properly and N uptake was limited.

I recently looked at several fields of sweet corn with nitrogen deficiency that was severe enough to reduce ear number and size. Nitrogen deficiencies in sweet corn result in an overall pale color with lower leaves becoming yellow from the tips in a V pattern. In severe N deficiencies, these V shaped areas will become necrotic and entire leaves may dry up. These fields showed these classic N deficiency symptoms. A useful tool to determine the extent of N deficiencies is the chlorophyll meter which measures how “green” the plant is. In these N deficient field areas, chlorophyll meter readings were 25-50% lower than surrounding corn that was not showing any symptoms. The following are some pictures:

nitrogen deficient sweet corn 

Nitrogen deficiency showing up as a yellowing of lower leaves in a V pattern from the tip backward

severly nitrogen deficient sweet corn 

More severe N deficiency with necrotic V area on leaf and dead lower leaves

nitrogen deficient sweet corn with chlorophyll meter reading 

Chlorophyll meter reading on the ear leaf of a N deficient plant. The reading on this plant was more that 50% lower than plants without N deficiency symptoms in an adjacent field.

In the fields examined, the most severe problems were in very sandy areas and low spots. These are areas where the most N loss would be expected. Other field areas were not heavily affected and appeared normal with chlorophyll meter readings above 50. One grower reported that they used a Pre-Sidedress Nitrogen Test (PSNT) in areas that had received manure and values indicated that no additional N was needed. While the PSNT is a valuable tool to manage nitrogen in sweet corn, any recommendations should take into account weather at and after the time of sampling. Low PSNT values may result from heavy rains just prior to sampling (it is recommended to wait several days after heavy rains to take samples for PSNT’s). High PSNT values (>21 ppm) would indicate no additional N is needed. However, heavy leaching rains, waterlogging, and cold weather still could still render the crop N deficient even with these high values.

chlorophyll meter reading for sweet corn with adequate nitrogen

Chlorophyll meter reading on the ear leaf of a fully green plant that was not showing N deficiency. We would expect readings in the 50s or low 60s. Values lower than 50 would indicate a N deficiency.

Nitrogen Deficiencies in Lima Beans

Friday, July 10th, 2009

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

Both early planted processing baby lima beans and market garden pole lima beans are showing signs of nitrogen deficiency now across the state, even with adequate N being applied. This is likely a result of excessive N leaching during the wet spring periods in May and early June in many fields, especially on very sandy soils. Pole lima beans commonly show N deficiencies later in mid-summer as pod set and development progresses; however this year, yellow plants are being seen much earlier.

Severe N deficiency in lima beans will be seen as an overall yellowing of plants with lower leaves often dropping off as N is mobilized from the oldest leaves to support the new growth at growing tips. Less severe N deficiency will be seen as a lighter green color than normal with lowest leaves most affected. Tissue tests can be used to confirm N deficiencies. Take the uppermost fully expanded leaves to send off for analysis. There are other potential causes for yellowing in lima beans including low pH leading to magnesium deficiencies and excessively high pH leading to micronutrient deficiencies, most commonly manganese.

It is important to apply additional N as soon as possible in N deficient lima beans. General recommendations are to sidedress 30-40 lbs of N, 3-5 weeks after emergence in baby lima beans and at early pod set in pole lima beans. In fields that have suffered heavy leaching losses, this may need to be increased slightly. However, remember that too much N can lead to excessive vine growth and delay flowering and pod set. Deficient pole lima beans will need an early sidedress application now and will likely need an additional sidedressing of N in August during pod development.

Salt Injury from Starter Fertilizer

Friday, June 5th, 2009

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

I recently looked at several snap bean plantings with symptoms of salt injury from starter fertilizer. Leaves had large areas that were dried from the margin inward, other areas were light green and showing signs of dessication. Symptoms were field wide and did not show up until after the plants had germinated and emerged. It was likely that fertilizer salts had moved toward the seedlings with water from rain and irrigation in high enough concentration to cause the injury. The grower had changed starter fertilizer to a higher analysis from previous years. Other crops were not affected. Caution should be taken with starter fertilizers, especially in crops that are sensitive to salts, such as beans. Choose low salt index fertilizers and limit the total amount of nitrogen and potassium (a general guideline is no more than 80 lbs total of N + K). Adjust fertilizer applicators to deliver the band no closer than 2″ to the seed and 2″ deep. If higher amounts of starter are required, move the fertilizer band farther from the seed.

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

Friday, June 6th, 2008

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.