Posts Tagged ‘vegetable crop nutrients’

Excess Nitrogen and Vegetables and Fruits

Friday, September 14th, 2012

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

Vegetable crops vary considerably in their needs for nitrogen with crops such as sweet potatoes falling on the low end and tomatoes on the high end. While a lack of nitrogen will definitely limit vegetable productivity, excess nitrogen can also cause production problems.

Excess nitrogen will often delay maturity in crops. This is a particular problem in fruiting vegetables and vegetables with harvested roots and tubers. Too much nitrogen will favor the growth of foliage over flowering and fruiting or formation of storage organs such as tubers and roots. In a crop such as pumpkins, this can result in delaying fruit set so long that the crop will not mature in time for sales. Excess nitrogen can also reduce yields by limiting storage organ formation. Sweet potatoes would be a good example of a crop that will have reduced yields with excess nitrogen.

Excess nitrogen can also cause reductions in the quality of fruits and storage organs both in flavor and physical characteristics. High nitrogen applications can result in lower sugar content, lower acidity, and reduced firmness in fruits and storage organs. It can cause reduction in nutritional content. In leafy green vegetables, it can result in the accumulation of nitrates in the plant tissue to unhealthy levels. High nitrogen can cause reduced volatile production and negatively impact flavor and aroma in vegetables and fruits. Excess nitrogen can increase disorders such as hollow stems of broccoli and reduce storage and keeping qualities of fruits and vegetables.

Excessive production of foliage from high nitrogen applications can also lead to an increase in disease pressure from having a higher proportion of young tissue that is more susceptible to infections, by creating a more humid microclimate favorable for disease development, and by making it more difficult to get good coverage with fungicides.

Excess nitrogen can cause reductions in the levels of other mineral nutrients in plants such as potassium, calcium, and magnesium, often resulting in the development of deficiencies and associated disorders.

Recommended nitrogen rates and timings for most vegetable crops grown in our region can be found in the Commercial Vegetable Production Recommendations (online at http://ag.udel.edu/extension/vegprogram/publications.htm). These recommended rates have been developed over many years of research by our universities. Applications in excess of these recommended rates is justified only under special circumstances (excess rainfall and leaching for example).

Low Plant Tissue Potassium and Calcium

Friday, September 14th, 2012

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

Growers, consultants, and soils laboratories have noted that plant tissue tests on several vegetables (such as watermelon) have been showing lower than expected levels of potassium (K) and calcium (Ca) in plant tissues this year, even though soil levels are high.

There are a number of possible causes for these lower than expected tissue test results. High rates of nitrogen applied to vegetable crops can often reduce the levels of K and Ca in plant tissue. High nitrogen promotes foliage growth and more leaf area. This can have a dilution effect on K and Ca as there is less available proportionally to supply the new leaves.

The use of fertilizers high in ammonium and/or urea (which quickly released ammonium) can cause a temporary suppression of K and Ca uptake because ammonium is a competing cation. This suppression lasts until the ammonium is converted into nitrate in the soil by nitrifying bacteria. In drip irrigated vegetables where Urea Ammonium Nitrate (UAN) solutions are used as the nitrogen source during regular fertigation, this suppression can last throughout much of the season. The use of fertilizers with calcium nitrate and potassium nitrate as the nitrogen source can eliminate this competitive effect.

Very high levels of K fertilization can also reduce Ca uptake and excess magnesium can interfere with both K and Ca uptake.

In addition to dilution effects and cation competition, use of acidifying nitrogen fertilizers such as UAN or ammonium sulfate will drop the soil pH. When soil pH drops below 5.3, root function can be negatively affected, which will further limit K and Ca uptake. This can occur if soil pH is marginal to begin the season. It is common practice to lime fields on a 3 year rotation throughout the region. In the third year before the next liming, many fields fall into this marginal pH category.

Lower than normal K and Ca in leaf tissues can also be related to high temperatures and plant stress. In periods with extreme high temperatures, plant stomates close earlier in the day, transpiration is reduced, and K and Ca uptake are reduced because less water is being taken up by the plant.

Managing plant tissue K and Ca requires balancing fertilization. Where high nitrogen rates are being used to push high production, additional K should also be added in equal or higher amounts than nitrogen (1:1 to 1:2 ratio). This is particularly true for fruiting crops such as tomatoes, peppers, watermelons, and cantaloupes. Additional fertilizer calcium will also be needed for crops susceptible to blossom end rot.

Low pH in Plastic Mulched Beds

Friday, July 20th, 2012

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

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). Ammonium will convert to nitrate in the soil, a process called nitrification, and will release hydrogen (H+) ions, thus dropping the pH. 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.

As 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.

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. Manage fertilizer programs so that large pH drops do not occur. This means switching some or all of the nitrogen program to nitrate sources – calcium nitrate and potassium nitrate would be examples.

If marginal pHs are encountered after plastic is laid (below 5.8), 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. Liquid lime products with ultrafine ground limestone can also go through a drip system; however, getting enough material into the soil to affect the pH will be difficult and expensive and agitation of supply tanks will be necessary.

Potassium and Nitrogen Fertilization of Fruiting Vegetables

Friday, June 1st, 2012

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

Many fruiting vegetable crops are receiving additional nitrogen and potassium applications as sidedressings or as fertigation through drip irrigation systems at this time. Specific nitrogen and potassium recommendations can be found in the commercial vegetable production recommendations for Delaware which are online at http://ag.udel.edu/extension/vegprogram/publications.htm.

Balancing nitrogen and potassium properly is critical for high yields and good quality in fruiting vegetables. Growers understand the critical role of nitrogen for plant growth. Potassium is equally important for many vegetable crops such as tomatoes, cantaloupes, and watermelons which benefit from additional applications of potassium, even if soil potassium levels are high. High rates of nitrogen can be utilized by the plant and transformed into high yield only in the presence of high potassium levels.

Although potassium does not form part of the structure of vegetable plant, it is important for regulating sugar production, translocation of proteins and sugars, water balance, cell turgor, and stomatal activity. Potassium improves the quality of fruits by maintaining desirable sugar to acid ratio and improving the ripening of fruits.

The “take home” message is that nitrogen should be balanced with potassium during the cropping season with sidedressing or fertigation in fruiting vegetable crops. A 1:1 or 1:2 ratio of nitrogen to potassium should be used depending on the crop.

Tissue Testing and Petiole Sap Testing for Vegetables

Friday, May 18th, 2012

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

Recommended fertility programs for vegetable crops are given in the Commercial Vegetable Production Recommendations publication for Delaware and surrounding states. See http://ag.udel.edu/extension/vegprogram/publications.htm 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.

Tissue testing involves taking samples from the plant at various times during the growth period, most commonly leaves, 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.

When taking tissue samples specific procedures should be followed to obtain reliable results. The following are recommendations from the University of Florida.

“The sample is a whole leaf sample and it should not contain any root or stem material. For sweet corn or onions, the leaf is removed just above the attachment point to the stalk or bulb. For compound leaves (carrots, peas, tomatoes, etc.), the whole leaf includes the main petiole, all the leaflets and their petioliules. For heading vegetables, it is most practical to take the outermost whole wrapper leaf. When sampling particularly young plants, the whole above-ground portion of the plant may be sampled.”

Most commonly the most recently matured leaves (MRML) are used for analyses. Most-recently-matured leaves (MRML) are leaves that have essentially ceased to expand and have turned from a juvenile light-green color to a darker-green color.

“A proper leaf sample should consist of about 25 to 100 individual leaves. The same leaf (i.e., physiological age and position) should be removed from each sampled plant. Plants damaged by pests, diseases, or chemicals should be avoided when trying to monitor the nutrient status of the crop. Individual plants, even side-by-side, may have a considerably different nutrient status. Therefore, by sampling a sufficiently large number of plants, the error due to this variability can be minimized. More accuracy in determining the actual nutrient status is derived from a larger sample size.”

“Samples are often contaminated by fungicides, nutrient sprays, soil, or dust. Data obtained from contaminated leaf samples will be misleading. Decontamination of some dust or soil is best accomplished by quickly rinsing in a dilute non-phosphate detergent solution (2%) followed by two distilled water rinses. Tap water should not be used because it can be high in certain nutrients such as Ca, Fe, Mg, or S. Leaf samples should be washed quickly to minimize the leaching of certain nutrients (especially K) from the leaves.”

“Following rinsing, the sample should be blotted dry with absorbent paper. The samples should be air-dried for several hours before shipment. If a plant analysis mailing kit is not available, the samples should be wrapped in fresh absorbent paper and placed in a large envelope (plastic bags must not be used). The sample should be shipped or delivered immediately to the soil and plant analysis laboratory. An air-dried sample, if loosely packed to avoid rotting, will last two to three days before decomposition begins.”

“If the samples must be held for any length of time before shipping, they should be dried at 150°F in a ventilated oven (leave the door ajar) until dry weight is constant. Once dried, the sample can be placed in a plant analysis mailing kit or a large envelope. This ensures the integrity of the sample until shipping is possible.”

Petiole sap testing is useful for monitoring nitrogen and potassium and can give very quick results with the use of portable meters. The following are guidelines for petiole sap testing from the University of Florida:

“For sap testing, petioles collected from most recently matured leaves (MRML) are used for analyses. Most-recently-matured leaves (MRML) are leaves that have essentially ceased to expand and have turned from a juvenile light-green color to a darker-green color. A random sample of a minimum of 25 petioles should be collected from each “management unit” or “irrigation zone.” Management units larger than 20 acres should be subdivided into 20-acre blocks. Leaves with obvious defects or with diseases should be avoided. Sampling should be done on a uniform basis for time of day (best between 10 AM and 2 PM), and for interval after rainfall or fertilization.”

“Whole leaves are collected from the plant and the leaf blade tissue and leaflets are then stripped from the petiole. A petiole of several inches in length remains. Petioles are chopped into about one-half inch segments. If analysis is not to be conducted immediately in the field, then whole petioles should be packed with ice and analyzed within a few hours of collecting. Given more extreme environmental field conditions (high temperature and bright sun), more dependable results are obtained by making measurement in the lab or office than outdoors.”

“Chopped petiole pieces are mixed and a random subsample (about ¼ cup) is crushed in a garlic press, lemon press, or hydraulic press (obtainable from HACH Co., Table 4). Expressed sap is collected in a small beaker or juice glass and stirred.”

Follow the instructions for the specific meter you are using to analyze the sap. If sap has too high of concentration of nitrate or potassium for your meter, then you will need to dilute the sap to conduct the test.

Information on tissue testing and petiole sap testing for vegetables including tables with recommended levels at different growth stages can be found at this site http://edis.ifas.ufl.edu/ep081.

The following are recommended values for watermelons: 

Plant Tissue Macronutrient Ranges for Watermelons at Different Growth Stages

Crop Plant Part Time of Sampling Status - – - – - – - – - – % – - — – - – - -
N P K Ca Mg S
Watermelon MRM* leaf Vining before flowering 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
Toxic (>) - - - - - -
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

*MRM – most recently matured leaf with petiole.

Petiole Sap Nitrate and Potassium Concentration Ranges for Watermelon

Crop Stage of Growth Fresh Petiole Sap Concentration (ppm)
K NO3-N
Watermelon Vines 6-inches in length

Fruits 2-inches in length

Fruits one-half mature

At first harvest

4000 to 5000

4000 to 5000

3500 to 4000

3000 to 3500

1200 to 1500

1000 to 1200

800 to 1000

600 to 800

 

Updated Fertilization Recommendations for Drip Irrigated Crops

Thursday, April 19th, 2012

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

Extension specialists in the Mid-Atlantic have updated fertilizer recommendations for drip irrigated plasticulture production of crops in the Commercial Vegetable Production Recommendations. The following are recommendations for watermelons and tomatoes.

Suggested Fertilizer Program Using Trickle Irrigation for Watermelons

Days After Planting

Daily

Cumulative

Nitrogen1

Potash1,2

Nitrogen1

Potash1,2

——————–lbs/A——————–

Preplant3

25

50

0-14

1.0

1.0

39

64

15-28

1.5

1.5

60

85

29-56

2.0

2.0

116

141

57-78

1.5

1.5

137

166

79-93

1.0

1.0

150

175

1Adjust rates accordingly if you apply more or less preplant nitrogen and potash.
2Base overall application rate on soil test recommendations.
3Applied under plastic mulch to effective bed area using modified broadcast method. Adjust as needed.
Note: recommendations are based on 8 foot bed centers. If beds are narrower, fertilizer rates per acre should be adjusted proportionally. Drive rows should not be used in acreage calculations.

Suggested Fertigation Schedule – Fresh Market Tomatoes

Days After Planting

Daily

Cumulative

Nitrogen1

Potash1,2

Nitrogen1

Potash1,2

——————–lbs/A——————–

Preplant3

50

125

0-14

0.5

0.5

57

132

15-28

0.7

0.7

67

142

29-42

1.0

1.0

81

156

43-56

1.5

1.5

102

177

57-77

2.2

2.2

148

223

78-98

2.5

2.5

201

276

1Adjust rates accordingly if you apply more or less preplant nitrogen and potash.
2Base overall application rate on soil test recommendations.
3Applied under plastic mulch to effective bed area using modified broadcast method. Adjust as needed.
Note: recommendations are based on 6 foot bed centers. If beds are narrower, fertilizer rates per acre should be adjusted proportionally. Drive rows should not be used in acreage calculations.

Additional recommendations can be found in the Recommendation which is also online at this site: http://ag.udel.edu/extension/vegprogram/publications.htm.

Nitrogen Fertilization After Flooding

Friday, August 26th, 2011

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

With heavy rain expected this weekend, flooding can be expected in some vegetable fields. In addition to nitrate leaching, there is potential for significant denitrification losses in saturated soils.

In fields that have been waterlogged for several days, nitrogen fertilization will be critical to helping plants recover.

Root growth and function is impared by flooding and therefore nitrogen uptake will be limited. N levels in soils will be low due to leaching and denitrification. Nitrogen should be applied as soon as soils have drained. Foliar N fertilization may be of great benefit after flooding as root systems recover.

In flooding studies at the University of Florida byYuncong Li, Renuka Rao and Stewart Reed, they tested several N fertilizers both as dry applications and foliar applications for their effectiveness in recovering flood-damaged vegetable crops and found that potassium nitrate performed the best, urea the second best, and calcium nitrate the third best. Liquid urea-ammonium nitrate solutions should perform similarly to urea as a foliar application and as a sidedressing where crops are still small enough to get equipment through. Limit foliar applications to less than 3% total salt solutions. In plasticulture vegetables that have been flooded, foliar applications will be necessary until the beds have dried out enough to allow for fertigation through the drip system.

Flooding and Vegetables

Friday, August 26th, 2011

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

There is still considerable acreage of watermelons, sweet corn, pumpkins, beans, cabbage, potatoes, and other fresh market vegetable crops in the field on Delmarva. On the processing side, the majority of lima beans have yet to be harvested and there are significant acres of pickles, snap beans, and other processing crops in the field. Many of these crops will be at risk in the coming days due to hurricane Irene.

A late summer hurricane or tropical storm with both wind damage and excess rain can cause major issues in vegetable crops, most notably:

● Damage due to flooded soils in all vegetable crops

● Increased disease incidence in all vegetable crops

● Lodging damage in crops like sweet corn

Other articles will address diseases in with excess rainfall. I will focus on flooding effects on the physiology of vegetable plants.

Flooded and Waterlogged Soils
In flooded soils, the oxygen concentration drops to near zero within 24 hours because water replaces most of air in the soil pore space. Oxygen diffuses much more slowly in water filled pores than in open pores. Roots need oxygen to respire and have normal cell activity. When any remaining oxygen is used up by the roots in flooded or waterlogged soils, they will cease to function normally. Therefore, mineral nutrient uptake and water uptake are reduced or stopped in flooded conditions (plants will often wilt in flooded conditions because roots have shut down). There is also a buildup of ethylene in flooded soils, the plant hormone that in excess amounts can cause leaf drop and premature senescence.

In general, if flooding or waterlogging lasts for less than 48 hours, most vegetable crops can recover. Longer periods will lead to high amounts of root death and lower chances of recovery.

While there has not been much research on flooding effects on vegetables, the following are some physiological effects that have been documented:

● Oxygen starvation in root crops such as potatoes will lead to cell death in tubers and storage roots. This will appear as dark or discolored areas in the tubers or roots. In carrots and other crops where the tap root is harvested, the tap root will often die leading to the formation of unmarketable fibrous roots.

● Lack of root function and movement of water and calcium in the plant will lead to calcium related disorders in plants; most notably you will have a higher incidence of blossom end rot in tomatoes, peppers, watermelons, and several other susceptible crops.

● Leaching and denitrification losses of nitrogen and limited nitrogen uptake in flooded soils will lead to nitrogen deficiencies across most vegetable crops.

● In bean crops, flooding or waterlogging has shown to decrease flower production and increase flower and young fruit abscission or abortion.

● Ethylene buildup in saturated soil conditions can cause leaf drop, flower drop, fruit drop, or early plant decline in many vegetable crops.

Recovering from Flooding or Waterlogging
The most important thing that you can do to aid in vegetable crop recovery after floods or waterlogging is to open up the soil by cultivating (in crops that still small enough to be cultivated) as soon as you can get back into the field. This allows for oxygen to enter the soil more rapidly. Nutritionally, sidedress with 50 lbs of N where possible.

In fields that are still wet, consider foliar applications of nutrients. According to Steve Rieners at Cornell “Use a low salt liquid fertilizer to supply 4 to 5 lb nitrogen, 1 lb phosphate (P2O5) and 1 lb potash (K2O) per acre. Since nitrogen is the key nutrient to supply, spraying with urea ammonium nitrate (28 % N solution) alone can be helpful. These can be sprayed by aerial or ground application. Use 5 to 20 gallons of water per acre. The higher gallons per acre generally provide better coverage”. As with all foliar applications, keep total salt concentrations to less than 3% solutions to avoid foliage burn.

Cover Crops for Vegetable Rotations Revisited

Friday, August 12th, 2011

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

August is here and it is time to consider late summer and fall cover crop options for vegetable rotations. Cover crop planting windows vary with crop and timely planting is essential to achieve the desired results. Here are some reasons to consider using cover crops in vegetable rotations:

Return organic matter to the soil. Vegetable rotations are tillage intensive and organic matter is oxidized at a high rate. Cover crops help to maintain organic matter levels in the soil, a critical component of soil health and productivity.

Provide winter cover. By having a crop (including roots) growing on a field in the winter you recycle plant nutrients (especially nitrogen), reduce leaching losses of nitrogen, reduce erosion by wind and water, and reduce surface compaction and the effects of heavy rainfall on bare soils. Cover crops also compete with winter annual weeds and can help reduce weed pressure in the spring.

Reduce certain diseases and other pests. Cover crops help to maintain soil organic matter. Residue from cover crops can help increase the diversity of soil organisms and reduce soil borne disease pressure. Some cover crops may also help to suppress certain soil borne pests, such as nematodes, by releasing compounds that affect these pests upon decomposition.

Provide nitrogen for the following crop. Leguminous cover crops, such as hairy vetch or crimson clover, can provide significant amounts of nitrogen, especially for late spring planted vegetables.

Improve soil physical properties. Cover crops help to maintain or improve soil physical properties and reduce compaction. Roots of cover crops and incorporated cover crop residue will help improve drainage, water holding capacity, aeration, and tilth.

There are many cover crop options for late summer or fall planting, including:

Small Grains
Rye is often used as a winter cover as it is very cold hardy and deep rooted. It has the added advantage of being tall and strips can be left the following spring to provide windbreaks in crops such as watermelons. Rye makes very good surface mulch for roll-kill or plant through no-till systems for crops such as pumpkins. It also can be planted later (up to early November) and still provide adequate winter cover. Wheat, barley, and triticale are also planted as winter cover crops by vegetable producers.

Spring oats may also be used as a cover crop and can produce significant growth if planted in late August or early September. It has the advantage of winter killing in most years, thus making it easier to manage for early spring crops such as peas or cabbage. All the small grain cover crops will make more cover with some nitrogen application or the use of manure.

To get full advantage of small grain cover crops, use full seeding rates and plant early enough to get some fall tillering. Drilling is preferred to broadcast or aerial seeding.

Ryegrasses
Both perennial and annual ryegrasses also make good winter cover crops. They are quick growing in the fall and can be planted from late August through October. If allowed to grow in the spring, ryegrasses can add significant organic matter to the soil when turned under, but avoid letting them go to seed.

Winter Annual Legumes
Hairy vetch, crimson clover, field peas, subterranean clover, and other clovers are excellent cover crops and can provide significant nitrogen for vegetable crops that follow. Hairy vetch works very well in no-till vegetable systems where it is allowed to go up to flowering and then is killed by herbicides or with a roller-crimper. It is a common system for planting pumpkins in the region but also works well for late plantings of other vine crops, tomatoes and peppers. Hairy vetch, crimson clover and subterranean clover can provide from 80 to well over 100 pounds of nitrogen equivalent. Remember to inoculate the seeds of these crops with the proper Rhizobial inoculants for that particular legume. All of these legume species should be planted as early as possible – from the last week in August through the end of September to get adequate fall growth. These crops need to be established at least 4 weeks before a killing frost.

Brassica Species
There has been an increase in interest in the use of certain Brassica species as cover crops for vegetable rotations.

Rapeseed has been used as a winter cover and has shown some promise in reducing levels of certain nematode in the soil. To take advantage of the biofumigation properties of rapeseed you plant the crop in late summer, allow the plant to develop until early next spring and then till it under before it goes to seed. It is the leaves that break down to release the fumigant-like chemical. Mow rapeseed using a flail mower and plow down the residue immediately. Never mow down more area than can be plowed under within two hours. Note: Mowing injures the plants and initiates a process releasing nematicidal chemicals into the soil. Failure to incorporate mowed plant material into the soil quickly, allows much of these available toxicants to escape by volatilization.

Turnips and mustards can be used for fall cover but not all varieties and species will winter over into the spring. Several mustard species have biofumigation potential and a succession rotation of an August planting of biofumigant mustards that are tilled under in October followed by small grain can significantly reduce diseases for spring planted vegetables that follow.

More recent research in the region has been with forage radish. It produces a giant tap root that acts like a bio-drill, opening up channels in the soil and reducing compaction. When planted in late summer, it will produce a large amount of growth and will smother any winter annual weeds. It will then winter kill leaving a very mellow, weed-free seedbed. It is an ideal cover crop for systems with early spring planted vegetables such as peas.

Oilseed radish is similar to forage radish but has a less significant root. It also winter kills.

Brassicas must be planted early – mid-August through mid-September – for best effect.

Mixtures
Mixtures of rye with winter legume cover crops (such as hairy vetch) have been successful and offer the advantage, in no-till systems, of having a more rapidly decomposing material with the longer residual rye as a mulch.

Balancing Growth and Fruiting

Friday, July 9th, 2010

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

As fruiting vegetables reach full size, we often get called to look at problems with fruit set. Poor fruit set, blossom drop, and fruit abortion can be caused by many factors including heat stress, water stress, insect and mite damage, chemical injury, lack of pollinating insects, and nutrient deficiencies to name few. Excessive vegetative growth is another common reason for poor fruit set or delayed fruit set.

Excessive vegetative growth commonly occurs when plants are grown under high fertility conditions and are being heavily watered. Under these conditions, flowering and fruiting is delayed as plants continue to put on new growth. High plant populations or close plant spacing can further compound the problem.

In this vegetative growth mode, the hormone balance is such that flowering and fruiting is limited. This will not change until growth slows and the hormone balance shifts, signaling flowering.

In addition, as the area covered by leaves and stems increases, so does shading within crop canopies. New growth starts to shade out older growth. Unfortunately, the stem nodes where flowering and fruiting is occurring is often so shaded that nearby leaves cannot produce enough photosynthate to support these flowers and fruits.

Heavy dense canopies can also reduce the effectiveness of pollinating insects as flowers are more difficult to access.

Another problem with excessive vegetative growth is creating a high humidity environment in the canopy that favors plant diseases. Sclerotinia (white mold), Phytophthora, Pythium, Septoria, and Botrytis are examples of diseases that can become problems in dense canopies. Often, these diseases will infect flowers and young fruit, causing them to drop.

We commonly see delayed fruiting, poor fruit quality, and reduced fruiting in vine crops with excessive growth. This includes watermelons, cantaloupes, and pumpkins. This is especially true of robust vining varieties. In contrast, bush and semi-bush forms of cucurbits such as some summer squashes and winter squashes have fewer problems with overgrowth. Tomatoes are also susceptible to excessive plant growth, reducing fruit set and quality. Snap beans and lima beans with excessive foliage are at high risk for diseases infecting pods, causing pod drop. Cucumbers with excessive foliage also are high risk for fruit diseases.

The goal in producing these fruiting vegetables is to balance vegetative and reproductive growth. This is done by managing fertility, especially nitrogen (N), to have enough nutrients available to grow a healthy plant but not to promote excessive growth. Nitrogen fertilization rates in the commercial vegetable production recommendations guide for Delaware should be consulted. In addition, nitrogen release from manures or other organic sources needs to be considered in overall N applications. We often see over-fertilization problems in fields with a history of heavy manure use or with high organic matter where N is being released from these sources as they mineralize.

Water should be managed so that plants are not over-irrigated. You certainly should not let plants become so water stressed that growth is affected. However, low levels of water stress actually can promote reproductive development in many vegetables.

Crop type, plant type, and plant morphology should also be considered prior to planting. Varieties that produce heavy growth (long vines or stems, many branches, large leaves, etc.) need to be spaced further apart to avoid overcrowding. Some vigorous varieties will even require reduction in fertilizer to control growth. Also consider how each crop responds to high fertility and heavy irrigation. Crops such as sweet corn, summer squash, and peppers normally do not have problems with excessive growth and will yield well under those conditions. In contrast, tomatoes, watermelons, cantaloupes, and lima beans will have yield reductions if fertility and irrigation are not managed well.