Posts Tagged ‘nitrogen’

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).

Nitrogen Fertilization and Irrigated Soybean Production

Friday, August 24th, 2012

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

A number of people have been asking about applying nitrogen (N) fertilizer to irrigated soybeans so I thought I would make a few comments about the practice in case they might help you in making the decision as to whether to spend the money in hopes of getting a yield response.

To begin with, I have tried this practice on both full-season and double-cropped soybeans at one time or another. I’ve tried applications of N at both 25 and 50 lbs N/acre at R2 (full flower) and R4 (full pod) and for double-cropped soybean I’ve tried these rates broadcast at planting. I never got a significant response to the treatments although for double-cropped soybeans I was close to seeing an increase in early plant height and pod set. For yield, the treatments were all within a bushel or two of each other.

That being said, I should point out that significant responses have sometimes been reported from down South but only when the N was applied through an irrigation system (for the reports I’ve seen) and when both boron (B) and N were applied in combination. At the time of the research that I conducted, we did not have the capability to apply N to my plots through the irrigation system. I had to apply the N with a back-pack CO2 sprayer while walking through the soybeans. I did have the studies irrigated immediately after applying the treatments to minimize the chance of foliar burn. I remember hearing from the southern researchers that they felt that the leaf damage caused by walking on N or applying N with a ground rig would negate the slight yield response that they were able to obtain using fertigation. I also did not apply B along with the N which may have also reduced the chances of obtaining a positive yield response since B is important in sugar transport and in helping flower set.

I understand that some people suggesting that N should be applied to irrigated soybeans are suggesting the inclusion of sulfur (S) (probably as ammonium sulfate) along with the N. This makes some sense from a biological point-of-view in that the plant requires enough S to make the S-containing amino acids required for protein synthesis. However by the time soybeans reach the full bloom or full pod stage, the root system has reached or will soon reach its deepest penetration of the soil. Even the sandy soils in Sussex County, Delaware, were found to have large quantities of S (typically 300 to 500 lbs S/acre) stored in the clay lenses found in the 1 to 2 foot depth of soil and soybean roots should be able to tap into this S reserve by reproductive stage.

Let me summarize below some of my thoughts on trying to increase soybean yields with N fertilizer.

● If soybeans already have matured to the full seed stage (R6) where a full size seed is found in a pod at one of the four upper most nodes with a fully expanded leaf, it is much too late for N application to increase yield potential in my opinion. All the research that I’ve seen involves the application of N at full bloom (R2) to full pod (R4) stage.

● I doubt that the addition of S as ammonium sulfate is going to increase your chance of obtaining any return on your investment since soybeans are very likely to have more than an adequate supply of S available by this time of year due to root growth. An exception would be where there is a root restricting compaction layer in the top 12 inches of soil but in this case the chance that fertilizer will improve yield is very low.

● If your expected yield potential is not at least 60 to 70 bushels/acre, N fertilization will not help. Next year, try using either the liquid seed Bradyrhizobia inoculants or some of the new graphite soybean inoculants since the new strains available can really help increase your yield potential.

● If you still plan to apply N fertilizer to your soybean crop, be sure to add about 0.5 pound of boron per acre. The data I’ve seen where a yield response was obtained with late season (R2 or R4) N application were always where B had been included with the N.

● I would suggest limiting any N application to no more than 30 lb N/acre since levels higher than this have often been shown to reduce the nitrogen fixing activity of the soybean nodules. If this occurs, you’ll be trading dollars essentially since the nodules will either stop N fixation or reduce fixation to a degree where the plant will need the N you apply just to produce the original crop’s yield potential. Some studies with early season manure applications have shown yield reductions because the crop ran out of N during the reproductive stages and had to reinitiate nodulation because the crop ran out of available N.

● Do not consider N applications on non-irrigated soybeans. Keep in mind that in the case of a dryland soybean crop, the overall limiting factor is water availability not nutrient availability.

● Also if your field has a history of soybean cyst nematode (SCN) infestations, do not add N fertilizer since SCN will be your yield limiting factor not N or S or B fertilizer.

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.

Getting Your Pastures Off to a Fast Start

Friday, March 2nd, 2012

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

This year, with very high hay prices and short supplies, there is a need for early pasture for grazing to stretch tight budgets and short hay supply. One of the few ways to stimulate growth in pasture is the application of nitrogen (N) at, or just before, pasture spring greenup. Even when N was applied in the early to mid-fall period to stimulate root system expansion and provide pasture grasses with stored N for early spring regrowth, an additional application of N just at greenup can be useful in promoting early pasturage.

A question often asked is whether it’s economical and safe to use granular urea on pastures at this time of year. To answer the economics in the question you need to understand what happens when urea is applied over top of a pasture. If conditions are favorable, urea applied to a pasture can react with water from the soil or vegetation and the ever present enzyme, urease, to convert into ammonium carbonate. Ammonium carbonate is a very unstable form of fertilizer N that breaks down spontaneously into ammonium (NH4+) or ammonia gas (NH3), if the pH is alkaline, water, and carbon dioxide. The ammonium then is either taken up by plants, or it attaches to the cation exchange sites on clay and soil organic matter, or is acted on by the nitrifying bacteria to become nitrate (NO3-). If conditions favor it staying ammonia, this is lost to the atmosphere and effectively raises your cost per pound of N. Urea frequently has the lowest cost per pound of N but if much N loss occurs the savings will be eliminated.

Conditions that favor ammonia loss, besides the presence of plant material that provides the urease enzyme, include warm temperatures (especially 70°F. and higher), high humidity or a moist soil surface, and high soil pH where the prill or urea granule rests on the soil. On Delaware soils where the pH is often maintained between 5.5 and 6.5 for pastures and where air and soil temperatures are cool to cold at this time of year, the loss of N from urea fertilizer is minimal. In fact when I worked in the Deep South, pastures or hay fields were fertilized with urea rather than ammonium nitrate all the way into April as long as the temperatures did not warm up into the mid to upper 70s. Through March at least in Delaware, fertilization with urea should be the most cost effective way to provide N for pastures since losses will be minimal.

What about animal health concerns? Since urea, like other fertilizers, is a salt, animals can become ill if they gain access to bags of urea fertilizer and consume too much of it. As long as the applicator practices safe handling and storage principles and ensures that the fertilizer is evenly spread without large clods, animal safety should be ensured. For those that prefer to err on the side of more caution, we suggest that they keep animals off a fertilized field until it has received from ¼ to ½ inch of rainfall. Rainfall or irrigation water will move the urea quickly into the soil eliminating any concerns for animal health; and, at the same time, will reduce or eliminate the concern with ammonia volatilization.

Another way to get pastures off to a fast start, which also plays into the above health concern, is to keep animals off pastures early in the greenup period to promote more growth. As an analogy, think of a tiny little tomato seedling. It can double in size a number of times but until it reaches a critical size the doubling amounts to only a very small increase in dry weight of the plant. Pastures that are grazed even before the permanent grasses green up in the spring will produce little useable forage compared with a pasture that is fertilized and then allowed to grow to a height of 3 to 4 inches before being lightly grazed, rested a couple of weeks and then grazed again. If the grazing animals are removed when 3 inches of pasture remains, recovery and the pounds of dry matter produced per day will be much greater than that of a pasture kept constantly at a grazed height of 0.5 to 1 inch. It may mean using more hay initially but once the pasture reaches that 3 to 4 inch height, it often will produce more feed per day than your animals will consume.

Once you begin grazing a pasture, the best thing you can do to promote growth is to practice rotational grazing where you allow animals on a subdivision of your pasture for a short period, usually no more than 3 to 5 days at most, and then remove the animals to another subdivision while the plants in the recently grazed subdivision rest and recover and renew growth.

Another suggestion is to take that soil test sample you’ve been meaning to get and send it in for analysis. Soil tests should be taken at least every three years and as often as every year at the same time of year each time. The soil test will help you decide if you need to correct a pH problem or apply nutrients to relieve any nutrient deficiencies. If the pasture soil pH level has declined below 6.0, an application of lime will help both grasses and legumes grow better.

I mentioned N fertilization earlier. How much N should you apply? This does depend a bit on the pasture you are fertilizing and your goal for that pasture. Where you either have too much legume (clover) or where you have so little clover that is isn’t contributing N to the surrounding grass, an application of about 100 lb urea per acre (this is about 46 lb N/acre) will stimulate grass growth helping to reduce the percentage legume in the pasture or will replace the N lacking when legumes are grown with grasses. This rate should be enough to jump start the pasture grasses without a risk of overfertilization and risking damage to the environment. On pastures where maintaining legume presence is important, you should apply only half the rate of urea (50 lb urea per acre). At this rate of N, the legume can continue growing and will not slough off the bacteria nodules that help the legume by fixing atmospheric N (N2 gas) in a plant available form.

For First Split Nitrogen Applications on Wheat, Is Price per Pound of N the Right Criteria to Use?

Friday, March 2nd, 2012

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

There’s a debate going on as to what the best source of nitrogen (N) is for broadcasting over winter wheat or barley at this time of year. The question arises because it is known that urea can volatilize to ammonia (NH3), a gas, and be lost to the atmosphere because the enzyme urease, which helps break down urea, is present in all organic matter. What a lot of people overlook is the speed of this conversion which is affected by the soil pH, soil and air temperature, and moisture conditions. When temperatures are relatively low, below 70°F, and soil temperatures remain well below 50º, the activity of the enzyme is significantly reduced. Another factor involved is soil acidity or pH. When the soil surrounding the urea particle is acidic (pH<7.0), there are available hydrogen ions (H+) that can quickly react with ammonia to form an ammonium ion (NH4+). An ammonium ion is a cation that can occupy a place on the cation exchange sites in clay and soil organic matter and be held for plant absorption. The conversion of urea into ammonium bicarbonate and a hydroxyl (OH-) and then into ammonia (or an ammonium ion if an H+ is available), carbon dioxide, and water raises the localize soil pH and increases the likelihood that some of the N will volatilize off as ammonia.

In general, we found in the Deep South that you more economically apply urea to pastures or wheat fields in the early spring and often into mid-spring with only minor losses of N as ammonia. Since the soil temperature in Delaware soils seldom reaches the 50º F. level until well into April and we often have long periods of cool rainy weather in the spring, the choice of fertilizer to use on small grains is most likely best decided by economics rather than concern over just how much might be lost through volatilization. The most likely choices of fertilizer products are a urea ammonium nitrate solution (UAN) and granular urea. Since UAN does contain half urea and half ammonium nitrate, the small percentage N loss from ammonia volatilization is not likely to impact the economics between the two fertilizers very much. Growers should evaluate available fertilizers and choose the most economic fertilizer based on the cost per pound of N plus the expected application cost and the availability of the fertilizer through their usual dealer rather than arbitrarily sticking with what they’ve used in the past or what their dealer prefers to sell them.

First Split Nitrogen Application to Small Grain

Friday, March 2nd, 2012

Richard Taylor, Extension Agronomist; rtaylor@udel.edu

Many wheat and barley fields that were not fertilized with manure or poultry litter last fall have looked poor for much of the winter. Even those fields that received some fall nitrogen (N) fertilizer but as commercial fertilizer have been yellow since mid-winter or earlier. Recently, the date when spring fertilizer can be applied was moved to earlier in February to help those with wheat that was showing signs of N stress and was trying to start spring growth. Since then, some cooler weather has settled over the region and slowed greenup in small grains.

For those who have not yet applied the first shot of N to their wheat, it is time to apply it. In fields that did receive fall manure or litter, the need is not as critical. In wheat, we have often seen about a 5 to 7 bu/acre yield increase when N applications are split into early greenup and then just before or at Feekes growth stage 5 when the first node is visible or can be felt above the soil surface. In work that Bob Uniatowski and I conducted a number of years ago, this response to splitting N applications was fairly consistent across locations and years. The largest response to a split application comes when significant rainfall occurs between the two splits causing some of the applied N to be loss either through leaching or denitrification. We also found that if all N was applied at one time, the early application date was the best choice although if the wheat had adequate tiller numbers in early spring even a Feekes growth stage 5 single application could produce excellent yield potential.

For those who may be growing barley, we did find that this crop is very sensitive to the rate of N applied because the straw strength seemed to be most affected at high rates of N. Whenever we applied much more than about 80 pounds of N per acre, we started seeding significant lodging which can not only make harvest difficult but can cause yield reductions, especially when lodging occurs early in the grain fill period.

PSNT, Yellow Corn, and Sidedressing

Friday, May 29th, 2009

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

Corn sidedressing has begun throughout Delaware. One important tool for nitrogen management in corn is the pre-sidedress nitrogen test (PSNT). The PSNT is based on the concept that amounts of plant-available N for the growing season can be predicted by taking samples from the surface foot of soil when corn plants are 6-12 inches tall. The timing of sampling is important because it allows you to detect any unexpected losses early in the growing season. It is also important because you are sampling just before the corn begins rapid uptake of N.

Fields that have had applications of animal manures or organic wastes and received less than 50 lbs of N as a starter and/or preplant are good candidates for the PSNT. On irrigated fields where farmers have the option of adding N later in the growing season, other methods, such as use of the leaf chlorophyll meter (LCM) may be more appropriate. Fields where more than 50 lbs of N has been applied preplant and/or as starter or fields receiving no manure are not appropriate for the PSNT.

Sample collection and handling are extremely important. Multiple cores, 15-30 per sample, should be collected to a depth of 12 inches to represent a uniform area of a field (in terms of soil type, management, etc.) no larger than about 20 acres. Be careful not to sample bands of previously applied fertilizer or injected manures! The cores should be mixed and quickly air-dried by spreading them on paper in a warm area. Samples may also be refrigerated until it is possible to dry them.

The PSNT level will provide an estimate of the likelihood of seeing a response to additional nitrogen fertilizer. Fields with PSNT values 21 ppm or higher are unlikely to benefit from additional nitrogen fertilizer, and the higher the value the less likely the need for supplemental nitrogen. The problem arises when PSNT values are less than 21 ppm. PSNT values below this level may or may not respond to additional nitrogen fertilizer, but the stock recommendation would be that they do require more nitrogen. There can be sites that have a low PSNT value but show no response to nitrogen fertilization. Bottomline: if the PSNT values are above 21 ppm, adequate nitrogen should be available for this year’s corn crop. If it is less than 15 ppm, the normal nitrogen rate should be applied. Between 15 and 21 ppm, other factors should be considered before reduction of the normal nitrogen rate.

In Delaware, Conservation Districts in Kent and Sussex Counties offer PSNT testing free of charge and will take the samples. A number of different laboratories, including many private laboratories and the University of Delaware Soil Testing Laboratory, can analyze these samples and provide guidelines for interpreting the results.

Many corn fields are showing patches of yellow plants. This is most likely due to a temporary nitrogen deficiency. On sandy soils, heavy rains may have leached nitrate deeper than the depth of the corn roots. On wet soil areas, nitrogen losses due to denitrification may have occurred. In compacted soils with broadcast N but no starter N, roots may not have expanded sufficiently to pick up enough nitrogen.

The key in all cases is to sidedress as soon as possible with additional N. Nitrogen rates may need to be increased from what was originally planned in areas with heavy leaching or denitrification. One common question is whether or not knifing nitrogen is better than dribbling nitrogen. When using nitrogen solutions (such as 30% UAN), there are slight increases in efficiency by knifing the N into the root zone and limiting volatilization losses of ammonia from surface applied urea. This loss of ammonia from urea in dribbled on UAN nitrogen is generally minimal because there is limited contact with the urease enzyme on the soil surface. One additional advantage of knifing in nitrogen is that it can open up compacted soils and improve aeration to a limited degree. Knifing is required when using anhydrous ammonia and the slot needs to be closed to avoid ammonia loss to the air.

Information used in this article came in part from Dr. Dave Hansen, Extension Nutrient Management Specialist, UD; the University of Maryland; and the Ohio State University.

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.