Boron in Vegetable Crops Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Boron has important roles in vegetable plants. It is needed for protein synthesis, development of cell walls, carbohydrate metabolism, sugar translocation, hormone regulation, pollen grain germination and pollen tube growth, fruit set, and seed development. Boron is mobile and readily leached in sandy soils and regular additions are necessary for many vegetables, but only in small amounts.

Boron (B) is a micronutrient required in very small amounts and there is a narrow range of safety when applying boron as toxicities can occur if too much is applied.

Vegetables vary considerably in their B requirements.

High B requirement crops include broccoli, cabbage, cauliflower, beets, spinach, turnips, and rutabaga. Apply 3 lbs/a of B for these crops.

Medium B requirement crops include asparagus, carrots, cucumbers, eggplants, leeks, muskmelons, okra, onions, parsnips, radishes, squash, strawberries, sweet corn, tomatoes, and potatoes. Apply 2 lbs/a of B for these crops.

Low B requirement crops include peppers and sweet potatoes. Apply 1 lb/a of B for these crops.

Very low B requirement crops include beans and peas. No additional boron is usually needed for these crops (snap beans actually are very sensitive to high B levels which will cause toxicities).

Boron deficiency symptoms in plants include the death of growing points resulting in a stunted or rosette appearance; leaves with a yellowish or reddish cast, and in members of the cabbage family most boron deficient cole crops develop cracked and corky stems, petioles and midribs. The stems of broccoli, cabbage and cauliflower can be hollow and are sometimes discolored. Cauliflower curds become brown and leaves may roll and curl, while cabbage heads may be small and yellow. Of all the cole crops, cauliflower is the most sensitive to boron deficiencies.

It is recommended in broccoli and kale to apply 3 pounds of boron (B) per acre in mixed fertilizer prior to planting. In Brussels sprouts, cabbage, collards and cauliflower, boron and molybdenum are recommended. Apply 3 pounds of boron (B) per acre and 0.2 pound molybdenum (Mo) applied as 0.5 pound sodium molybdate per acre with broadcast fertilizer.

Boron may also be applied as a foliar treatment to cole crops if soil applications were not made. The recommended rate is 0.2-0.3 lb/acre of actual boron (1.0 to 1.5 lbs of Solubor 20.5%) in sufficient water (30 or more gallons) for coverage. Apply foliar boron prior to heading of cole crops.

Boron toxicity is common in western states where boron levels in soils or irrigation water are high. In the east, we do not have high boron soils or high levels in irrigation water. In addition, boron leaches readily from soils. Boron toxicities therefore occur only when excess boron is applied in fertilizers. The margin of safety for boron application is small and excess application or improper blending in fertilizers may lead to toxicities – deficiencies show up at 1 ppm and toxicities appear at 5 ppm of available boron in the soil (leaf tissue levels between 20 and 100 ppm are sufficient with tissue levels over 200 ppm being excessive leading to toxicities).

The vegetable crops most sensitive to excess boron are beans, particularly snap beans. Boron is generally not recommended for snap bean production and boron should never be included in starter fertilizer for snap beans. Boron toxicity often occurs where starter fertilizer containing boron for other crops, such as corn, is applied to snap beans.

Boron toxicity in beans commonly appears as yellowing in unifoliate leaves with burning of leaf edges and yellowing of leaf edges of the older trifoliate leaves that can progress to edge burn. In severe cases, plants will develop a scorched appearance and leaves may prematurely drop off.

Monitoring Nutrient Status in Vegetable Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist;

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 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 at: 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.


Yellowing in Peas

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Pea harvest is nearing and we are seeing yellowing and poor growth in many pea fields due to wet conditions. Peas do not perform well in soils that are worked when they are too wet or when they receive heavy rainfall after planting. Compaction and crusting over will lead to poor emergence and reduced growth. This is evident in many Delmarva pea fields in 2019.

Recently, heavy rains have caused some pea fields or parts of fields to turn yellow, particularly were there was compacted soil or poor drainage. Peas are effective at fixing nitrogen; however, we normally apply 40-80 lbs/a of fertilizer nitrogen (N) prior to planting thus reducing N fixation contributions from Rhizobium nodules on the roots. With the frequent rainfall, some fields have remained saturated and denitrification has occurred, reducing available N from the initial fertilizer application. In addition, root function and Rhizobium nodulation is further impaired in saturated soils, thus limiting any potential N fixation contributions.

In pea fields that have had a past history of root rot, we have the potential to see problems in 2019. According to the Crop Profile for Peas in Delaware: “Aphanomyces root rot, or common root rot, is one of the most destructive diseases of peas. It occurs in most pea producing regions of the U.S., including the Mid-Atlantic. In the Northeast, average annual yield loss to this disease is about 10%, though losses in individual fields may be up to 100%. Wet soil conditions and poor drainage are associated with higher rates of infection. The disease is most damaging in years when a cool, wet spring is followed by an early, warm summer with low rainfall.”

Good pea growth and development.

Yellowing in peas in wet soils

Magnesium Deficiencies in Vegetables Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist;

We have seen several cases of magnesium deficiency in vegetables already in 2019. Magnesium (Mg) is considered a secondary macro-element 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 in situations where available magnesium levels are low to moderate to begin with.

Magnesium deficiency in sweet corn.

Magnesium deficiency in tomato.

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.

Magnesium 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.

Foliar magnesium levels for most vegetables at mid growth should be in the 0.3 – 0.6 % range (leaves).

Low pH and Nutrient Deficiencies in Vegetables

Gordon Johnson, Extension Vegetable & Fruit Specialist;

As soil pH drops, availability of magnesium and calcium declines while manganese availability increases, often to toxic levels. Below pH of 5.2, the chemistry of the soil changes and aluminum is released into the soil solution at increasing levels, further acidifying the soil. This free aluminum also is very harmful to plant roots because aluminum interferes with calcium, can bind with phosphorus, and can interfere with cell expansion at root tips, effectively stopping root tip development. Most of the active mineral nutrient uptake occurs in the region just behind the root tips. Without further root tip growth, nutrient uptake will become limited. Effective rooting volume is also reduced, thus placing the plant under additional stress. In severe cases, plants can die. During fruit formation, there will be increased incidence of blossom end rot in tomatoes and peppers in plastic beds with low pH.

The following are minimum pHs for various vegetable crops:

Crop Min. pH
Cucumbers, cantaloupes, squash, pumpkins 5.8
Watermelons 5.5
Tomatoes, peppers, and eggplant 5.8
Cole crops (broccoli, cabbage, cauliflower, Brussels sprouts, kale, collards) 6.0
Spinach, beets, chard 6.0
Snap beans and lima beans 5.8
Sweet corn 5.8
Peas 6.0
Potatoes (scab resistant) 5.5
Carrots 5.5
Sweet potatoes 5.5
Onions 5.8

Below these pH levels, crop performance will be affected, and yields will be reduced. Lime should be applied immediately if soil pH has dropped to these values. Target pHs for vegetable crops can be found in Table B1 in the 2019 Mid-Atlantic Commercial Vegetable Production Recommendations:

In the eastern US, soil pH will drop naturally due to the 45+ inches of rainfall received. In addition, if ammonium and urea containing nitrogen fertilizers are used, they will also lower pH. Ammonium nitrogen is also released from organic nutrient sources. Ammonium will convert to nitrate in the soil, a process called nitrification, and will release hydrogen (H+) ions, thus dropping the pH. As a general rule, lime should be applied to raise the pH every 3 years. After very wet years such as 2018, pH will drop more than normal or dryer years.

Plastic Mulched Beds and pH
Each year we see problems with vegetable crops related to low pH in plastic mulched beds. A common scenario is a field with sandy soil (loamy sand, sandy loam) that has not been limed in the last 2 years. The starting pH of beds in this situation will usually be 5.5-6.0. Granular or liquid nitrogen fertilizers applied prior to or at bed formation and nitrogen fertilizers applied through the drip irrigation system during fertigation will commonly consist of ammonium sulfate, urea, ammonium nitrate or UAN (urea-ammonium nitrate) solutions. All of these fertilizers are acidifying because the ammonium which they contain (urea releases ammonium nitrogen as it reacts with the soil). As a result, pH in the plastic mulched beds gets progressively lower throughout the growing season. Beds with a starting pH of 5.5 can drop down into the 4s. The largest drops in pH will be in the wetted area around the drip emitter and drier areas of the bed will have a higher pH.

It is also possible to have low pH under plastic in organic production systems depending on the rate and type of organic material being applied. For example, blood meal used to supply nitrogen in organic systems is very soil acidifying.

Managing plastic mulched bed pH starts with making sure that fields are limed the fall before beds are to be made. Spring applications can also be made to the area, but full lime reaction should not be expected.

If marginal pHs are encountered after plastic is laid (below 5.8), manage fertilizer programs so that large pH drops do not occur. Consideration should be given to eliminating ammonium or urea containing fertilizers and switching to calcium nitrate and potassium nitrate sources for fertigation. Both these fertilizers cause a basic reaction in soils because plant roots excrete hydroxides and carbonates as they take up the nitrate. There are few other materials that can be used to raise the soil pH through the drip system once plastic is laid. One option is potassium carbonate which is alkaline and thus will raise the pH. It is fully soluble and can be made in liquid forms. Potassium hydroxide is another fertilizer that has a basic reaction and that can go through the drip system.

Liquid lime products with ultrafine ground limestone can also go through a drip system. Recently, a finely ground (< 0.5 micron) liquid limestone-based product (Top Flow 130; Omya, Oftringen, Switzerland) was developed for agriculture use to be injected through drip irrigation tubing. It shows some promise but does not replace liming because it only affects the area around emitters about 4 inches.

Notes from the Annual Meeting of the American Society for Horticultural Sciences (ASHS)

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Each year, the ASHS has an annual meeting bringing together scientists working with specialty crops (vegetables, fruits, ornamentals). This year the meeting is in Washington DC. The following are some notes from sessions I have attended over the last 2 days that have relevance to our Delmarva growers.

  • Sweet corn planted into selected biodegradable black plastic mulches were shown to provide equal weed control, production, and earliness to standard black polyethylene mulch and eliminate mulch disposal costs.
  • Pepper production under biodegradable plastic mulch was equivalent to standard black plastic mulch again eliminating the need for mulch disposal.
  • Low rate compost application in potato (1 ton/a) reduced nitrogen needs and improved quality and yield in potato production.
  • Reduced curing temperatures and time of curing as well as delayed vine termination (mowing just before digging) reduced internal defects in ‘Covington’ sweet potato
  • Using white or reflective mulch did not improve broccoli production compared to black plastic mulch (we have a similar study currently in Delaware)
  • Progress is being made in breeding beets for lower levels of geosmin, the compound that gives beets the earthy taste.
  • Grafting tomatoes onto certain vigorous rootstocks can improve yield in high tunnel production, even in the absence of soil-borne disease.
  • From Matt Kleinhenz at Ohio State University “Commercial microbe-containing crop biostimulants are advertised to maintain or enhance crop growth. More than two-hundred such products ranging in composition (e.g., bacterial, fungal, both; cfu/ml) are currently available. To date, outcomes from standard statistical approaches common in product evaluations, variety trials, and cultural management comparisons show that significant increases in yield or quality have been rare, regardless of inoculation parameters or experimental conditions.”
  • A multistate project is underway to see if there are long term benefits to the “soil balancing” philosophy of soil management — specifically, balancing percentages and ratios of calcium, magnesium, and potassium through applications of lime, gypsum, and other materials to improve soil physics (tilth) and biology and, thereby, crop yield and quality and weed control. Past, shorter-term studies have shown no benefits to soil balancing but some growers and crop advisors disagree. This multi-state research aims at answering claims that University research on soil balancing has not been long term and thus is biased.
  • Recently, a finely ground (<0.5 micron) liquid limestone-based product (Top Flow 130; Omya, Oftringen, Switzerland) was developed for agriculture use to be injected through drip irrigation tubing. Research by Tim Coolong in Georgia showed that Top Flow 130 could be used to adjust pH in a plasticulture system, but that the effects would occur within a zone of 4 inches on each side of the drip irrigation tubing. This may be useful for situations where pH has dropped below 5.2 in plasticulture beds.
  • UV blocking plastic in high tunnel covers were shown to reduce Japanese beetle activity greatly in high tunnel raspberry production.

Foliar Fertilization of Vegetable Crops Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist;

I recently looked a several vegetable plantings that showed severe damage from foliar fertilizers. The extended cloudy weather set up conditions where the plants were more susceptible to salt injury (thinner leaves with less developed waxy cuticles). With plant injury in mind, I thought it would be good to revisit the use of foliar fertilizers in vegetable crops.

Growers will apply most (>90%) of their plant nutrients for vegetable crops as soil applications (preplant, sidedressed, fertigated) based on soil tests and crop nitrogen needs.

To monitor vegetable nutrient status during the growing season, tissue testing is recommended just prior to critical growth stages. Growers can then add fertilizers to maintain adequate nutrient levels during the growing season or correct nutrient levels that are deficient or dropping.

Foliar fertilization is one tool to maintain or enhance plant nutritional status during the growing season. Often quick effects are seen and deficiencies can be corrected before yield or quality losses occur. Foliar fertilization also allows for multiple application timings post planting. In addition, there is reduced concern for nutrient loss, tie up, or fixation when compared to soil applications.

However, foliar fertilization has limitations. There is the potential to injure plants with fertilizer salts, application amounts are limited (only small amounts can be taken up through leaves at one time), multiple applications are often necessary (increasing application costs) and foliar applications are not always effective, depending on the nutrient targeted and plant growth stage.

Where foliar fertilization does have a good fit is for deficiency prevention or correction, particularly when root system function is impaired. This commonly occurs when there is extended rainy weather and soils are waterlogged. Foliar fertilization is also necessary when soil conditions, such as low pH, causes the tie up of nutrients so that soil uptake is limited. Foliar fertilization can also be used to target growth stages for improved vegetable nutrition thus improving color, appearance, quality, and yield.

Foliar fertilizers are applied as liquid solutions of water and the dissolved fertilizers in ion or small molecule form. Foliar nutrient entrance is mostly through the waxy cuticle, the protective layer that covers the epidermal cells of leaves. Research has shown that there is limited entrance through the stomata. While the waxy cuticle serves to control water loss from leaf surfaces, it does contain very small pores that allows some water and small solute molecules to enter the underlying leaf cells. These pores are lined with negative charges. Fertilizer nutrients in cation form or with neutral charges enter most readily through these channels: this includes ammonium, potassium, magnesium, and urea (NH4+, K+, Mg++, CH4N2O respectively). In contrast, negatively charged nutrients (phosphate-P, sulfate-S, molybdate-Mo) are much slower to move through the cuticle (they must be paired with a cation). Movement through the cuticle is also dependent on molecular size, nutrient concentration, time the nutrient is in solution on the leaf, whether the nutrient is in ionic or chelated form (complexed with an organic molecule), and the thickness of the leaf cuticle.

Another factor in foliar fertilizer effectiveness is what happens once the nutrient enters the leaf area. Some smaller molecules or those with less of a charge are readily transported in the vascular system to other areas of the plant (NH4+, K+, Mg++, Urea). Other larger molecules and more strongly positive charged nutrients stay near where they enter because they bind to the walls of cells in intercellular areas that contain negative charges. Tightly held nutrients include Calcium, Manganese, Iron, Zinc, and Copper (Ca++, Mn++, Fe++, Zn++, Cu++). Therefore, when applied as foliar fertilizers, calcium does not move much once it enters plant tissue, the negatively charged nutrients such as phosphorus and sulfur are very slow to enter the plant, and iron, manganese, copper, and zinc are slow entering and do not mobilize once in the plant.

The following is a list of the major plant nutrients that are effective as foliar applications, fertilizer forms best used for foliar applications, and recommended rates;

  • Foliar applications of nitrogen (N) can benefit most vegetables if the plant is low in N. Urea forms of N are the most effective; methylene ureas and triazones are effective with less injury potential; and ammonium sulfate is also effective. Recommended rates are 1-10 lbs per acre.
  • Foliar potassium (K) is used on fruiting vegetables such as tomatoes and melons. Best sources are potassium sulfate or potassium nitrate. Recommended rate is 4 lbs/a of K.
  • Foliar magnesium (Mg) is used on tomatoes, melons, and beans commonly. The best source is magnesium sulfate and recommended rates are 0.5-2 lbs/a of Mg.
  • Foliar calcium is often recommended, but because it moves very little, it must be applied at proper growth stages to be effective. For example, for reducing blossom end rot in tomato or pepper fruits, foliar calcium must be applied when fruits are very small. Best sources for foliar calcium are calcium nitrate (10-15 lbs/a), calcium chloride (5-8 lbs/a) and some chelated Ca products (manufacturers recommendations).
  • Iron (Fe), manganese (Mn), or zinc (Zn) are best applied foliarly as sulfate forms. Rates are: Fe, Mn, 1-2 lbs/a, and Zn ¼ lb/a. While these metal micronutrients are not mobile, foliar applications are very effective at correcting local deficiencies in leaves.
  • The other micronutrient that can be effective as a foliar application is boron. Boron in the Solubor form is often recommended at 0.1 to 0.25 lbs/a for mustard family crops such as cabbage as a foliar application. Boron is very toxic to plants if applied in excess so applying at correct rates is critical.

For foliar fertilizers to be most effective they should remain on leaves or other targeted plant tissue in liquid form as long as possible. Urea and ammonium nitrogen forms, potassium, and magnesium are normally absorbed within 12 hours. All other nutrients may take several days of wetting and rewetting to be absorbed. Therefore, it is recommended that foliar fertilizers be applied at dusk or early evening when dew is on the leaves, in high volume water, and using smaller droplets to cover more of the leaf. Applications should also be made when temperatures are moderate and wind is low. While foliar fertilizers are sometimes applied with pesticides, for best effectiveness and reduced phytotoxicity potential it is recommended that they be applied alone. Use only soluble grade fertilizers for foliar applications (many are already provided in liquid form) and adjust water pH so it is slightly acidic.

Foliar fertilizers are most effective when applied to younger leaves and fruits. Research has shown that as leaves or fruits age, cuticles thicken, and these thicker cuticles absorb significantly lower amounts of nutrients such as potassium. However, younger plant tissue is also the most susceptible to potential fertilizer burn.

Because foliar fertilizers are in salt forms they can damage plant tissue if applied at rates that are too high. Generally, a 0.5-2% fertilizer solution is recommended. Certain vegetables are more sensitive to fertilizer salt injury than others. Vegetables with large leaves with thinner cuticles (such as muskmelons) have greater risk of salt injury when compared to crops, such as cabbage, that have thick cuticles. Apply foliar fertilizers at recommended rates and dilutions for each specific vegetable crop.

In addition, some fertilizer sources are much more likely to cause injury than others. In the past this was given as the salt index for a fertilizer, the lower the salt index the less osmotic stress the fertilizer would place on the plant tissue. A better index would be the osmolality values for the fertilizer material. For foliar nitrogen materials, osmolality values (mmol/kg) for common N sources are as follows: Urea = 1018, UAN-28 = 1439, Ammonium sulfate = 2314, Potassium nitrate = 3434. This shows that potassium nitrate has over 3x the osmotic stress potential compared to urea when applied as a foliar fertilizer. This means that potassium nitrate has much more potential to cause salt injury to plants than urea and must be used at lower rates.

Many of Our Watermelon Fields May be Low in Sulfur

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

For the last 3 out of 5 years (yes I know, but I get busy with other things) I have been looking at whether or not adding extra sulfur to watermelon is worth it. I was asked a similar question from a couple of Eastern Shore growers awhile back and I said I was not sure and I’d look into it. So this is going to be a quick summary of the results for the three trials, two over on the Eastern Shore and one on the Western Shore. Seeded watermelon was used in 2 years of the study with seedless watermelon used in one year of the study. The set-up is pretty straight forward: soil samples were taken to see what nutrients were needed. Based on this we added the recommended amount of sulfur (we used different soil testing labs and used multi samples with similar results and recs). The average amount of sulfur to add was between 15 and 25 lbs/a for the 3 trials. Once sulfur and the other nutrients were added the treatments were: 0, 10, 20, 30 and 40 lbs per acre of extra sulfur being added to the plots. Petiole samples were taken and notes on first flowering, first female flowers, % fruit set, etc. were noted. To save time and because this year’s results were equivalent to the first 2 years of the trial I’ll only show this season’s results. Crimson Sweet was the cultivar used for 2016.

Yields of watermelon were significantly greater in the 20 and 30 extra pounds of sulfur compared with the control (Fig. 1). Over the 3-years of trials there was no significant difference between 20 and 30 lbs. and very little difference between 30 and 40 lbs. So an average addition of 20-30 lbs. is a good starting place.


Figure 1. Yield of watermelon with added amounts of sulfur (lbs/a)

Usually there was no difference in the percent sugar content of the watermelons among any of the sulfur treatments, although for 2016 only the 30 lbs of extra S was significantly greater than the control (Fig. 2). There were no other differences in any of the other measurements between treatments. After two trial years I quit using the 40 lbs of sulfur treatment because that level of sulfur never was much different from the 30 lbs. treatment. I will need to do this study at additional locations for a number of years to be more confident of these results.


Figure 2. Percent sugars in watermelon fruit with differing levels of sulfur.

The reason I am talking about it now is because of what else I did this past year that I should have started 4 years ago. This year I randomly took petiole samples from the watermelon fields. The one thing most of them had in common was a deficiency in sulfur (Fig. 3).

There may have been other deficiencies such as with phosphorous or manganese or nitrogen, etc. but only sulfur was consistently found to be deficient in 52% of the samples with an additional 23% being on the low end of “low”. This was not a big survey (27 fields total; 65% from the Western Shore, 35% from the Eastern Shore) and it was done during a strange weather year and S levels may have been abnormally low. Nevertheless the results of the survey amazed me, so I decided to talk about my sulfur study a little early. For now what growers should do next year, if they are not already doing it, in their watermelon fields is take petiole samples a couple of weeks before first harvest to see where they stand.

I will take more petiole samples from watermelon fields in the coming seasons and repeat the sulfur-additions studies to see if the results hold up. I will talk about the results of the 3-year study in more detail at some of the winter meetings that are coming up. If anyone has any suggestions as to anything else I need to look at in these studies please let me know.


Figure 3. Two examples of petiole nutrient sample results taken from 2 different watermelon

Correcting Nutrient Deficiencies in Vegetable Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist;

As the season progresses, growers and consultants will use tissue tests to determine the nutrient status of vegetable crops and take corrective actions if necessary. As a rule, if levels are in the adequate range or are high no corrective action is needed. If levels have dropped to near deficient levels or are in the deficient category then additional mineral nutrients will need to be added. Critical tissue test values for many vegetables can be found in the 2016 Mid-Atlantic Commercial Vegetable Recommendations. The following are some guidelines for correcting low or deficient levels from tissue tests in vegetables.

If tissue results are low or deficient for Nitrogen (N) apply additional nitrogen as a sidedressing or through fertigation:

Watermelon, muskmelons, mixed melons: 40 lbs/a N
Cucumbers, squash: 20 lbs/a N
Tomatoes, peppers: 40-60 lbs/a N
Eggplant: 30 lbs/a N
White potato: 40 lbs/a N before tubers start to size
Cole crops, greens: 30-40 lbs/a N
Sweet corn: 40-80 lbs/a N
Beans: 20 lbs/a N

Additional nitrogen may be needed for extended harvest in some crops such as watermelons. Use non-acidic forms of nitrogen for blossom end rot sensitive vegetables such as tomato or pepper (calcium or potassium nitrate is recommended).

Foliar applications of N can benefit most vegetables if the plant is low in N. Urea forms of N are the most effective; methylene ureas and triazones are effective with less injury potential; and ammonium sulfate is also effective. Recommended rates are 1-10 lbs per acre N in sufficient water to have less than 2% salt solution. Multiple applications will be necessary to correct deficiencies, or combine with a soil application.

If tissue test results are low or deficient for potassium (K) apply additional K as a sidedressing or through fertigation. Note that fruiting vegetables often have low K levels in tissue tests if fruit loads are heavy and first harvest often brings them back in balance.

Watermelon, muskmelons, mixed melons: 40 lbs/a K
Cucumbers, squash: 20 lbs/a K
Tomatoes, peppers: 40-80 lbs/a K
Eggplant: 40 lbs/a K
White potato: 40 lbs/a K
Cole crops, greens: 30-40 lbs/a K
Sweet corn: 40-80 lbs/a K
Beans: 40-80 lbs/a K

Foliar sprays of potassium nitrate or sulfate (4 lbs/a K foliar) may be useful on tomatoes and melons.

If tissue test results are low or deficient for Phosphorus (P), apply an additional 20-40 lbs/a P for all crops as a sidedressing or through fertigation. Note that areas with high levels of calcium or magnesium in irrigation water can have problem with P precipitates clogging drip irrigation emitters and water may need to be acidified to prevent this.

If tissue test results are low or deficient for magnesium (Mg) apply 15-25 lbs of Mg as a sidedressing or through fertigation. Another option is to apply 2-3 applications foliarly (2-4 lb Mg/A) for sensitive crops such as tomatoes or melons.

For vegetable crops low or deficient in calcium (Ca), foliar applications of 2-4 lb Ca/A. Calcium chloride at the rate of 5-10 lb per 100 gallons per acre or calcium nitrate at the rate of 10-15 lb per

100 gallons per acre is recommended for fruiting vegetables (tomatoes, peppers, eggplant). Calcium chelates are also available. For potatoes, sidedress gypsum (calcium sulfate) at a rate of 500 lbs/a.

For vegetables low or deficient in sulfur (S) apply 20 lbs/A S as a sidedressing or through fertigation.

Ammonium sulfate and ammonium thiosulfate are effective ways to add both N and S at the same time. Gypsum is an inexpensive material to use to provide S.

For micronutrient metals (Iron – Fe, Manganese – Mn, Zinc – Zn) foliar application is often the most effective way to correct low or deficient levels. Suggested rates are: Fe, Mn, 1-2 lbs/a, and Zn ¼ lb/a.

The other micronutrient that can be effective as a foliar application is boron. Boron in the Solubor form is often recommended at 0.1 to 0.25 lbs/a for mustard family crops such as cabbage as a foliar application. Boron is very toxic to plants if applied in excess so applying at correct rates is critical. Do not use boron on bean crops.

Controlled Release vs Slow Release Fertilizer Products in Vegetable Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist;

There has been considerable work on controlled release fertilizer over the years and many of the different technologies have shown potential for use with vegetable crops. Controlled released fertilizer is most useful with nutrients that are subject to leaching losses, particularly nitrogen.

Controlled released fertilizers should not be confused with slow release fertilizers. With slow released fertilizers, release pattern over time is not easily predicted and may be affected by moisture, temperature, and microbial activity. Historically slow released fertilizers have included organic sources that require decomposition and mineralization such as manures, composts, waste products from plant or animal sources, and plant residues.

There are also slow release mineral based fertilizers such as magnesium ammonium phosphate (an N, P and Mg source), clinoptilolite (a natural Zeolite that can be reacted with NH4 which is held tightly and reacts as a slow release N source), rock phosphate (a P source), greensand (a K source), limestone (Ca and Mg source), glass frits (fritted trace elements in a fused glass form), and elemental sulfur. These materials are released upon weathering and dissolution (sulfur is converted to sulfate by microbial action).

An older technology that produces slow release N fertilizer is when urea is combined with an aldehyde. These fertilizers are in liquid or dry forms and include UreaForm (UF), Methylene Urea (MU), Triazone, and IBDU. Longevity depends on length of chemical chain and microbial activity needed to release the N. An exception is IBDU where N is released by hydrolysis at a slow rate and granule size controls longevity. Liquid forms of these products are very useful for foliar fertilization as they have much less injury potential than salt based N sources.

Another older slow release fertilizer technology is sulfur coated urea. This technology was developed at the TVA and commercialized in the 70s. Liquid sulfur coats urea prill then hardens (prills are often then coated with wax. Breakdown and release of the N in sulfur coated urea is by both physical and microbial action and the size of coat determines the release rate.

In contrast to slow release fertilizers, controlled released fertilizers have a predictable release pattern over time that is commonly temperature based. Controlled release fertilizers that are currently used are based on diffusion coatings (polymer and resin coated products). These coatings include thermoset resins, where a fertilizer prill is surrounded by a hardened shell from resins added in multiple layers (such as Original Osmocote), thermoplastics where the prill surrounded by plastic shell with additives to create pores or wicks (such as Nutricote); and reactive layer coatings where a thin polyurethane shell is produced when 2 chemicals react as they are sprayed on the fertilizer prill (such as ESN).

Polymer coatings can be used on most fertilizers and are common in the nursery and greenhouse industries with complete fertilizer products applied to potted plants. Coated product technologies have advanced over the years to give more precise release properties. However, release will still be dependent on the type of coating, the thickness of the coating, as well as temperature and moisture. Controlled release fertilizers are commonly rated as to how long they take to release nutrients in days (70 day, 90 day, 120 day formulations for example). They can also be mixed with a small amount of regular soluble fertilizer to give an initial nutrient charge.

Coast of reactive layer coated urea has decreased over the last decade and this provides an economical opportunity to provide controlled released nitrogen to vegetable crops. These fertilizers increase plant nitrogen-use efficiency by reducing N applied. Use of these products also eliminates need to sidedress or fertigate, giving fuel and time savings. From an environmental perspective there is reduced nitrate contamination from leaching due to the release pattern of the coated fertilizer.

Controlled released fertilizer research in Delaware on vegetable crops has shown equivalent yields to conventional fertilizer in multiple application with the controlled release fertilizer placed before planting (thus eliminating applications). In some cases reduced rates were needed and there was reduced N leaching. Controlled released fertilizers were tested on squash, melons, watermelons, tomatoes, peppers, and strawberries with good results.

Research on the use of ESN polymer coated urea by Cornell on Long Island showed that reduced rates of N could be used on potatoes and sweet corn with equivalent yields.  Recommendations from these studies were not to use straight controlled release but to blend with conventional N sources to provide 75-80% of the total N as controlled release. Overall they were able to reduce total N rates/acre by up to 20% in these vegetable crops.

Because controlled released N fertilizers are more expensive, for the economics to work out they need to be used at 10-25% reduced rates. Research has shown that this is possible on many vegetable crops.