USDA Researching New Test to Determine Nitrogen Levels in Soil

USDA Researching New Test to Determine Nitrogen Levels in Soil

Surface soil is sampled in a field in Virginia while in winter cover crop, but that will be planted to corn in the spring. Photo by Alan Franzluebbers, USDA/ARS

Sharon Durham, ARS Office of Communications

Nitrogen is the main nutrient added to cereal crops like corn, which makes them grow faster and stronger. But too much of a good thing could sometimes have negative outcomes. Too much nitrogen can run off with rainwater or leach through to soil and contaminate groundwater. Now, a simple, rapid and reliable test can determine the nitrogen amount in soil.

For corn growers, the current assumption is that corn grain requires 1.2 pounds of nitrogen applied for every bushel produced. This works for some soils, but not exactly for others, as the assumption doesn’t factor in nitrogen from soil organic matter. Knowing the soil’s potential to mineralize nitrogen from organic matter, making it available to plants, would help improve nitrogen fertilizer recommendations, according to U.S. Department of Agriculture (USDA) ecologist Alan Franzluebbers, lead investigator of this research.

A series of experiments published in Soil Science Society of America Journal studied the effectiveness of this quick and inexpensive approach that can tell a farmer prior to the growing season how much nitrogen will be available by testing a soil sample. In the first experiment, Franzluebbers, with Agricultural Research Service’s (ARS) Plant Science Research Unit in Raleigh, North Carolina, and his colleagues illustrated how soil nitrogen mineralization can be predicted with a three-day analysis of soil-test biological activity (STBA).

Soil is not an inert, dead plot of dirt; it contains many living organisms that enhance the soil’s ability to make nutrients available to plants. Insects, bacteria and fungi play a part in making soil valuable for crop production. The STBA measures how much “life” is contained in soil and how much usable nitrogen is in soil.

In the second experiment, Molly Pershing, a graduate student under Dr. Franzluebbers’ guidance, conducted greenhouse trials to determine if higher levels of STBA actually equated to plant uptake of nitrogen from soil. The researchers found that indeed greater STBA was associated with greater plant nitrogen uptake. Greenhouse-grown plants were not supplied any nutrients other than what was present in soil. More than three-fourths of the plant nitrogen uptake was from organic nitrogen that had to be mineralized, which was well predicted by the STBA level.

In the third experiment, Franzluebbers asked farmers to participate in the research. Forty-seven fields were sampled in the spring for STBA. On those fields, different rates of nitrogen fertilizer were applied to test which was most effective in optimizing corn yield. The higher the STBA level—indicating a large amount of “life” in the soil—the lower the need for additional nitrogen. The lower the STBA level, the greater the need for additional nitrogen.

Adding too little nitrogen can lead to a smaller harvest—costing farmers the opportunity to make more money. Adding too much nitrogen costs farmers money in unnecessary input to soil. Applying nitrogen at the correct levels can optimize yield and profit while keeping excess nutrients out of rivers, lakes and groundwater. Using STBA, corn growers now have a preseason test that can more accurately determine the proper amount of nitrogen to apply for economically optimum yield.

Double Cropping Options for a Fall Vegetable Crop

Double Cropping Options for a Fall Vegetable Crop

Fruit and vegetable production on plastic mulch is a substantial investment.  To help justify the high input cost, farmers oftentimes choose to double crop.  This practice can provide a significant amount of additional income for the farm if a good farm management plan is in place, there is a demand for the product, and the weather cooperates.

plastic mulch bed

Plastic mulch beds. Photo Credit: Blake Thaxton.

Field Prep

Spring fruit and vegetable crops are usually grown on black plastic mulch, but fall crops are usually grown on white plastic.  Black plastic mulch helps absorb heat to warm the soil in the late winter and spring.  White plastic mulch helps reflect light to cool the soil in the late summer and fall.  In order to reuse black plastic from the spring, painting the mulch is recommended.  White interior latex paint can be diluted with water and sprayed on the plastic.  At least one study has shown the ratio of paint to water can vary drastically without any significant difference in yield.

Crop residue leftover from the spring crop should be removed to reduce the risk of plant and human pathogens and deter harboring of insect and rodent pests.  If pests are an issue or a potential threat, then the soil can be fumigated before the second crop is planted.  Also, it is important to continue to irrigate the beds during the time between the two crops.  This will ensure a good water distribution throughout the bed when the second crop is planted.


When double cropping, it is important to consider each crop’s fertilizer needs independently.  Never assume that excess fertilizer from the spring crop will be taken up by the fall crop.  Take a soil sample before the second crop is planted to determine nutrient deficiencies.  If phosphorus is required for the second crop, then phosphoric acid can be injected through the drip.

Stringing tomatoes. Photo Credit: UF/IFAS Photo by Tyler Jones UF/IFAS Communication Services

Crop Selection

It is never a good idea to plant members of the same plant family sequentially, such as tomatoes after an eggplant crop, or zucchini after a watermelon crop.  The Vegetable Production Handbook of Florida lists production practices for various crops by plant family.  Recommended fall crops to follow tomatoes include: squash; broccoli; or cabbage.  Recommended fall crops to follow watermelons and other cucurbits include: peppers; tomatoes; or broccoli.  Consider plant spacing when selecting a second crop.  Added holes in the plastic mulch will reduce it’s integrity and promote weed growth.  No matter what crop you choose to plant this fall, make sure you have a good marketing plan in place, with your buyers already lined up.


Foliar Fertilizer Applications for Cotton

Foliar Fertilizer Applications for Cotton

Michael J. Mulvaney UF/IFAS Cropping System Specialist, and Glen Harris UGA Soil Specialist

With the frequent rains this summer, it’s been wet. You may not have been able to get into fields for timely topdress fertilizer applications on your cotton. If you’re still within 60 days of planting, nutrient demand has been low, so you still have an opportunity to apply granular fertilizer, particularly nitrogen (N) and potassium (K). However, some of our early-planted cotton has reached the third week of bloom, and nutrient demand are much higher during this time.  Questions have come in about the possibility of foliar fertilization to address this issue.

Can I apply foliar fertilizer by pivot?

Not effectively, no. The amount of water applied by pivot essentially washes the fertilizer off the leaves, making this essentially a soil-applied fertilizer. And if the soil is saturated, cotton will need to recover somewhat before it can effectively take up foliar applications.

What can I do to determine if my crop is nutrient stressed?

Petiole testing and leaf tissue testing are good ways to track the nutrient status of cotton. However, petiole and tissue testing should be done at intervals throughout the season, so that you can track how the crop is doing. A “one-and-done” tissue test won’t help you track nutrient levels over time, and may provide a snapshot of the field, but without referencing previous tests, these can be of limited value since wide variation in nutrient levels exist among cultivars. In addition, remember that K can be taken up as “luxury consumption,” which can confuse interpretation of tissue K results. That said, during early bloom, I like to see 3.5-4.5% N (35,000-45,000 ppm NO3-N), 1.5-3.0% K (15,000-30,000 ppm), and 20-60 ppm boron (B) in leaf blades from vegetative branches collected representatively from the field. (Vegetative branches are identified as those at nodes before fruiting branches, where the first position of the branch is not reproductive, Fig. 1). If you’re below these values at this time of year, a supplemental foliar application may be appropriate. After blooming, petiole tests are a better indicator of nutrient status in cotton because leaf blade nutrients are shunted to reproductive tissue, and leaves are often damaged by disease.

Figure 1. Vegetative branches are those branches prior to reproductive branches, where the first node on the branch is not a square. Leaves from these branches can be useful for tissue testing prior to boll set, although periodic petiole testing throughout the season is better for tracking nutrient status in cotton. Leaves from vegetative branches are better than those from reproductive branches because there are fewer flowers/reproductive parts that serve as nutrient sinks from leaves. Image from

Can I get enough nutrients on cotton using foliar fertilizer during peak demand?

Foliar fertilization should be considered supplemental fertilization as part of a sound fertility management program. If you applied 1/4 to 1/3 of N and K at planting, along with all of the required P, this is likely enough to hold you over until you can get back into the field. In this case, you may be able to foliar feed until you can get in with ground spreaders.

However, if you didn’t apply any N or K at planting, you are not likely to meet demand during peak production using foliar applications. (Remember, peak K demand in cotton is up to 3 lbs K/ac per day.) This is partly why we recommend applying 1/4to 1/3 of N and K at planting – as a mitigation strategy for late applications due to weather. I am often asked if split applications result in increased yield. The answer is no, not always. The reason we recommend split applications is in large part for cases such as this, where you can’t get in the field on time for topdressing. But if you’ve applied part of your fertility early, you should have enough nutrients to hold you over until you can apply topdress applications. If you need both N and K, potassium nitrate (KNO3) can be applied alone or in combination with urea. (The use of KCl is not recommended because of the high salt index.) The main problem here is application volume and rates. High rates can burn leaves, particularly with urea, and low volumes will not likely apply enough nutrients to make a difference.

What about micronutrient foliar fertilization?

Foliar feeding micronutrients is effective, but deficiencies should be addressed prior to bloom. Indeed, liquid applications (including by pivot) of micros are more effective than granular applications because of the small amounts applied. Suppose you were to spread granular B at 1 lb/ac, prills would end up tens of feet apart, which doesn’t do the plants in between any good. Liquid applications of micro-nutrients ensure that they are uniformly distributed in the field. Identification of micronutrient deficiencies is best accomplished with a tissue test (again, prior to bloom).

Will adjuvants help?

Probably not. Research has shown increased uptake with adjuvants, particularly with K applications, but this does not commonly increase yield. If it makes you sleep better, feel free to use an adjuvant though. It probably won’t hurt anything except your wallet, and even then the damage won’t be too great. Just don’t expect a yield boost.

Take a stand. Just make a recommendation, will you?

If you have a sound fertility management program, additional foliar fertilizer applications are unlikely to help. If you need a rescue treatment for micros, foliar fertilization is a good idea. If you need a rescue treatment for macros (N-P-K), I remain skeptical that foliar applications will help. If you’re managing 3 bale/ac cotton, more attention to fertility and disease will be critical. If you’re managing 1 bale/ac cotton, the extra expense of foliar fertilization won’t be worth it. Response to foliar K is unlikely, if soil test K is adequate or greater than 125 ppm K.

Managing 3 bale/ac cotton requires more attention to detail than managing 1 bale cotton.


Boron Deficiency:  A Rare but Serious Issue in Peanut

Boron Deficiency: A Rare but Serious Issue in Peanut

Figure 1. Boron deficient Tifguard, more than 100 days after planting. Field composed of well drained sand in Jackson County. Confirmed by soil and tissue tests.

Authors: Ethan Carter, UF/IFAS Regional Crop IPM Agent, and Michael Mulvaney, UF/IFAS Cropping Systems Specialist (WFREC)

Now that it’s early June, peanut fields across the Panhandle range in age from freshly planted to nearly 40 days after planting. Within the past two weeks, many areas have had consistent rainfall and overcast days. The recent weather has left fields saturated, complicating late plantings and spray applications. It also presents the opportunity to discuss potential issues associated with a nutrient that leaches like boron.

Boron is highly mobile and can rapidly leach from the root zone. Along with manganese, it is one of the most commonly deficient micronutrients on sandy soils. When tissue testing is done to evaluate boron levels, the entire above-ground portion of plants are collected. The desired range is 20-60 ppm, with less than 20 ppm being a critical level for deficiency and 100 ppm or more being a critical level for toxicity. Deficiency can occur when little or no boron is applied with the crop fertilizer. In our region, typically well to excessively-drained sands are most susceptible to boron deficiency. In peanut, boron deficiency is associated with hollow-heart, which lowers grade. This condition presents itself as discoloration within the kernel. However, severe boron deficiency can present itself in a manner similar to zinc toxicity- with split stems and roots, compacted branch terminals, and few developing pods (Figures 1, 2, and 3).

Figure 2. Split stems and roots on Tifguard plants suffering from boron deficiency in Jackson County. Confirmed by soil and tissue tests.

Figure 3. Split branches in Tifguard suffering from boron deficiency in Jackson County. Confirmed by soil and tissue tests.

When these types of  symptoms are found, a tissue and soil test are important to determine if the field has issues with either boron or zinc.  Boron can be added in with the initial crop fertilizer, or applied with early herbicide and fungicide applications. Foliar applications of 0.5 lb/ac elemental boron can be made during mid-bloom to correct boron deficiency.

Fertility should always be monitored after prolonged rain events or heavy irrigation. Nutrient leaching can result in unexpected field problems. More information regarding boron in peanut can be found in the following articles:

  1. Field Symptoms of Boron Toxicity and Deficiency in Florida Peanuts

  2. Management and Cultural Practices for Peanuts

  3. Boron and Other Minor Elements for Peanuts


Water Source Recommendations for Food Safety

Water Source Recommendations for Food Safety

Supplemental water is necessary for good crop yields in fruit and vegetable production. Water quality is equally as important as water quantity when it comes to fruit and vegetable production. Unfortunately, water can transport harmful microorganisms from adjacent lands or other areas of the farm. The water source and how the water is applied influence the risk for crop contamination to occur.

Water is used for various purposes during production: harvesting, and handling fresh produce, irrigation, cooling, frost protection, as a carrier for fertilizers and pesticides, and for washing tools and harvest containers, hand washing, and drinking.

Washing lettuce. Photo Credit: Cornell University Extension

The FDA’s Food Safety Modernization Act (FSMA) proposed water compliance date is not until 2022, but it will be here before you know it. Water quality is an important component of a Food Safety Plan. A good first step in ensuring compliance with FSMA water quality standards is to evaluate the water sources on the farm.  For more information on compliance dates, please visit the Produce Safety Alliance’s Website.

Water Source

The three common sources of water used on farms are surface water, well water, and municipal water.

Surface water includes ponds, lakes, rivers, and streams. It is at the highest risk for contamination because there is limited control on what flows downstream or from adjacent land. Wild and domestic animals, manure piles, and sewage discharges are all potential sources of contamination in surface waters.

The most common water source for North Florida farms is well water. Well water used for farming is at a moderate risk of becoming contaminated, when compared to surface water (highest risk) and municipal water (lowest risk). Wells are at a higher risk of becoming contaminated when located near flood zones, septic tanks, drainage fields, and manure/compost storage areas. The risk of contamination is further heightened if the well was not constructed properly, or if the casing is cracked. Wells should be properly sited, constructed, and maintained to keep contamination risks lower.

a well pump

A recently installed well pump on a North Florida watermelon farm. Photo Credit: Matt Lollar, University of Florida/IFAS Extension

Well Design and Construction

  • Preliminary Investigation – A preliminary investigation helps determine the design of a well. Existing wells in the area should be checked out to help determine depth and potential capacity. If records for the area aren’t available, then test holes should be drilled to determine the best location for water production.
  • Casing – Casing material should be determined based on site characteristics. The casing needs to extend above the surface water level to reduce contamination risks. The casing is sealed in place with grout. A poor grouting job can also promote contamination. Casing diameter is selected based on well capacity.
  • Well Screen – A commercially designed well screen should be installed to minimize hydraulic head loss. Screen diameter and material should be determined based on the preliminary investigation results. Gravel packing is recommended in some areas.

For more recommendations on well design and construction, please visit the University of Florida/IFAS publication: Design and Construction of Screened Wells for Agricultural Irrigation Systems

Please note that it is important to monitor your well water quality at least twice during each growing season.  A list of FSMA approved water testing methods can be found at Cornell University’s Law School Website.


Utilizing Compost to Boost Crop Productivity

Utilizing Compost to Boost Crop Productivity

Tractor front loaders make turning large amounts of compost possible for farmers. Photo by Turkey Hill Farm.

International Compost Awareness Week is May 6-12 this year. This educational initiative, promoted by the Composting Council Research and Education Foundation, was started in Canada in 1995, and has continued to grow in popularity as communities, businesses, municipalities, schools, and organizations celebrate the benefits of compost and composting. But perhaps the most important people involved in composting are the farmers who produce compost to grow the food we eat.

Compost can be produced and used on the farm as a valuable soil amendment, capable of providing not only a source of slow-release nutrients for crops, but also a way to improve soil structure, increase soil moisture-holding capacity, promote biological activity to enhance plant nutrient availability, suppress weeds, and even help combat some plant diseases.

Farmers can source compostable materials from many businesses, including fish waste from seafood markets. Photo by Turkey Hill Farm.

Although creating on-farm compost can take a lot of time and energy, it can be worth a farmer’s effort, if it keeps soil fertility costs down. One way many farmers produce enough compost to meet their fertility needs is to collect waste products generated by their surrounding community. If a system for collection and transportation can be developed, and non-compostable waste can be excluded, farmers can use waste from grocery stores, restaurants, food processing facilities, breweries, seafood markets, horse stables, dairy operations, and chipped trees collected by power line crews as they clear encroaching tree canopies.

Once a farmer has secured sources for compostable materials, next comes the step of mixing the materials to generate heat, up to 140 degrees Fahrenheit. Fortunately for the farmer, microorganisms do most of the work in the decomposition process. But it is the farmer’s responsibility to provide enough –  and the proper balance of – air, moisture, and nitrogen and carbon-rich food to fuel the aerobic microbial oxidation process. The volume needed to generate favorable composting conditions can be anywhere from about one cubic yard up to 40 cubic yards, depending on these factors.

This is why farmers, who depend on compost to supply a majority of their crops’ nutrient needs, often rely on a dump trailer and tractor front-end loader to move compost ingredients, turn compost piles, and spread the finished product on row beds. With experience, farmers learn the correct ratio of ingredients, proper volume and porosity of their piles, when temperatures plateau and piles need to be turned, and when the compost is finished and ready for use.

Spreading compost on crop rows provides a source of nutrients, improves soil structure, increases soil organic matter content, suppresses weeds, and provides many other benefits. Photo by Turkey Hill Farm.

High quality finished compost typically has an organic matter content of about 50 percent, a carbon to nitrogen ratio of around 20:1, near neutral pH, low soluble salts, and is free of weed seeds and plant phytotoxins. Compost nutrient content by volume is relatively low, and availability can vary greatly depending on soil and climatic conditions, so it is important for the farmer to monitor crop nutrient requirements and use additional amendments as needed. But when compost is used as a long-term strategy for improving soil health and building soil organic matter, its benefits can be appreciated for generations.

Interested in learning more about compost? Leon County Extension is hosting a “Got Compost?” workshop May 8, 6:00 p.m. to 8:00 p.m. EST, in celebration of International Compost Awareness Week. This workshop is tailored more for home-composters, but will also touch upon ways to up-scale compost production and will discuss small farm compost production strategies. To find out more and to register, visit the Leon County Extension Eventbrite Page.

Additionally, the Red Hills Small Farm Alliance – a non-profit organization of over 50 farmers within a 100-mile radius of Tallahassee – is promoting International Compost Awareness Week on its website and Facebook page. If you utilize compost on your farm, upload a short compost video to the Red Hills Small Farm Alliance Facebook Page for a chance to win a $50 gift certificate to the Red Hills Online Market.