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ORGANIC FERTILIZERS NITROGEN AVAILABILITY CALCULATOR FOR IRRIGATED FIELDS

Welcome

Organic and sustainable producers rely on cover crops, soil organic matter, composts, manures, and the addition of fertilizers to supply important plant nutrients for good plant health and maximum yield. Similar to conventional fertilizers, fertilizer grades (N-P2O5-K2O) are listed on organic fertilizer bags and can be determined for manures, but these organic products are very different from their inorganic fertilizer counterparts. As their name would suggest, nutrients in organic fertilizers are in their organic form, meaning they must be transformed by microorganisms in the soil before becoming plant available inorganic forms. For organic fertilizers, how much and when the nitrogen will actually become plant available is dependent on the particular fertilizer and the conditions in the field at the time of application. Please check out our Research Tab in the red bar above for more information on nitrogen availability, the development of the calculator, and helpful resources.

What to Expect from the Organic Fertilizers Nitrogen Availability Calculator

This calculator will predict how much and when nitrogen will be plant available from applied organic fertilizers and poultry litters by predicting nitrogen mineralization for irrigated fields. The calculator takes into account the initial inorganic nitrogen in the material (what's immediately available), individual rates of mineralization, and the soil temperature (5-year averages) from the closest weather station to predict plant available nitrogen over the season. Additionally, the calculator will provide the TOTAL amounts of nitrogen (N), phosphorus (P), and potassium (K) that was applied with the fertilizers used. This does NOT necessarily mean that these amounts are all plant available but it will help avoid over-applications. Results are given on a pound-per-acre basis.

Instructions

Click the calculator tab located in the red bar above to start. You will be led through questions about your farm location (for weather data), farming practices, and fertilizer applications.

Contact

For questions about how to use the calculator, contact your local County Extension Agent.

Calculator

Please answer the questions below and click "Next Page" when complete.

Background

To choose the closest weather station, what county is your farm located in?
(OR Choose from interactive map.)

What is the crop?
What is the planting date? mm/dd/yyyy
What are the estimated days to harvest?
What is your target nitrogen rate (lbs N/acre)?

(See crop recommendation sheet for specific crops if unknown here.)

 lbs N/acre
Do you plant cover crops?
Please enter your nitrogen credit for our cover crop calculator or an estimate from our bulletin. lbs N/acre

Please answer the questions below and click "Next Page" when complete.

Your Organic Fertilizer

Choose an ORGANIC fertilizer

Choose OTHER if not on the list.
What is the grade of your fertilizer? - - N-P2O5-K2O
Application Rate?
Date Applied? mm/dd/yyyy
Do you apply a SECOND ORGANIC fertilizer?

Choose OTHER if not on the list.
What is the grade of your fertilizer? - - N-P2O5-K2O
Application Rate?
Date Applied? mm/dd/yyyy
Do you apply a THIRD ORGANIC fertilizer?

Choose OTHER if not on the list.
What is the grade of your fertilizer? - - N-P2O5-K2O
Application Rate?
Date Applied? mm/dd/yyyy

Please answer the questions below and click "Next Page" when complete.

Your Poultry Litter Application

Do you apply POULTRY LITTER?
What is the grade of your fertilizer?

(If unknown, the program will use typical values found in poultry litter.)

 - - N-P2O5-K2O
Application Rate?
Date Applied? mm/dd/yyyy
Was it incorporated?

Please answer the questions below and click "Next Page" when complete.

Your Inorganic Fertilizer

Do you apply INORGANIC fertilizer?

Examples include Chilean Nitrate (NaNO3), rock phosphate, sulfate of potash.

What is the grade of your fertilizer? - - N-P2O5-K2O
Application Rate?
Date Applied? mm/dd/yyyy
Do you apply a SECOND INORGANIC fertilizer?
What is the grade of your fertilizer? - - N-P2O5-K2O
Application Rate?
Date Applied? mm/dd/yyyy
Do you apply a THIRD INORGANIC fertilizer?
What is the grade of your fertilizer? - - N-P2O5-K2O
Application Rate?
Date Applied? mm/dd/yyyy

Is this information Correct? If not, click a data point to edit it.

        Organic and Poultry Fertilizer Calculations

        Fertilizer Applied
        (lbs/A)         		Water Content
        (WC)         		Total N Pool of
        mineralizable N
        (N0)         		Rate constant
        of mineralization
        (k)         		Initial
        Inorganic N
        g N/kg

        Inorganic Fertilizer Calculations

        Fertilizer Applied N
        (lbs/A)

        Constants for this site

        Bulk Density (BD)1.5
        Drained Upper Limit (DUL)
        Drained Upper Limit / BD
        Air Dry (AD)
        Air Dry / BD
        Saturation water content (g/g) (Sat)

        Daily data

        Weather Station Data
        Day Temperature
        °C         		Moisture
        %         		Sw Temperature
        Factor
        (TFac)         		Moisture
        Factor
        (MFac)

        TOTAL NUTRIENTS APPLIED

        This table represents the TOTAL amount of nutrients applied to your plot, not necessarily what is plant available (see the Instructions and Research tabs for details). Organic fertilizers and materials must undergo mineralization to become plant available (below).

        TotalLbs/acreLbs/1,000 ft2
        N
        P2O5
        K2O
        EXAMPLE OUTPUT

        PLANT AVAILABLE NITROGEN

        Based on your fertilizer selection and the predicted weather on your field, pounds of N will become plant available from the time of application to your selected date.

        This graph will give you an idea about when the N will be released and your target N goal. To determine how much available N is available at a given time, follow the vertical line up from the date to the plotted curve. Each N fertilizer is shown as well as the cumulative N release from the combination of fertilizer products.

        This graph may help you adjust your N fertilizer at planting or at side dress.

        Understanding Nitrogen Availability from Organic Sources

        Organic fertilizers are derived from a wide variety of products. These products (such as feather meal, poultry manure, or vegetable matter) can range from 3 to 15% total nitrogen. Unlike inorganic fertilizers (example Chilean nitrate or ammonium nitrate), the nitrogen in organic products is tied up in organic forms (such as amino acids or proteins) that is not immediately available for plant uptake. Organic nitrogen must be degraded by microorganisms in the soil to become plant available inorganic ammonium and nitrate through processes called mineralization and nitrification. How much and when this inorganic N will be available depends on the fertilizer or organic material used and the weather conditions after application. The more complex the organic nitrogen molecule, the more difficult it is to degrade and with mineralization and nitrification dependent on soil microorganisms, the soil temperature and water content determine the health of microbial communities and the rate of decomposition.

        Because the rate of available nitrogen from organic sources is dependent on many variables, it is difficult to synchronize plant-available N release with plant need. Too much N too soon, could lead to nitrogen runoff/leaching, increased pest and disease pressure, or decreased fruiting. Too little N could lead to poor plant health and reduced yield. Either scenario leads to a loss of profits. Matching plant uptake with available nitrogen is important to maximizing yields, reducing costs, and reducing the potential for environmental degradation.

        How the calculator was developed

        This calculator was funded by a Southern Sustainable Agriculture Research and Education (SSARE) grant and was developed using studies conducted in the lab, on research farms, and on private farms from participating farmers.

        Lab Studies

        Lab studies were conducted to determine how much nitrogen would mineralize from organic fertilizers, poultry manure, and composts under "ideal conditions". These ideal conditions (85°F and water content near field capacity) should maximize microbial activity and give estimates of the maximum amount of nitrogen that becomes available from these materials. For our lab studies, we looked at 24 different organic fertilizers, 15 different poultry litters, and 11 different composts to determine the rate and total amount of nitrogen availability. For the organic fertilizers, plant available nitrogen released over 100 days ranged from 25 to 93% of the applied nitrogen. Plant available nitrogen release from poultry litters ranged from 25 to 56% of the total nitrogen. For the composts, many of the materials showed immobilization (net tie up of nitrogen) or very low nitrogen availability (4% of the applied nitrogen). Immobilization occurs when the microorganisms decomposing the material tie up some of the nitrogen, thereby reducing availability to plants. While compost is a great soil amendment that can provide other nutrients and stimulate soil health, it does not provide much as a nitrogen fertilizer and (in general) should not be considered in the nitrogen budget. Below is an example of the pattern of release for some of fertilizers studied for the calculator.


        Field Studies

        To determine how soil water content and temperature affect plant available nitrogen from these materials, field studies were conducted during summer and fall seasons. Studies were carried out at the University of Georgia Durham Horticulture Farm and at four private farms from participating farmers located in the Georgia Piedmont. These studies were used to test the calculator under real world conditions with fluctuating temperatures and soil water contents. Participating farmers chose their own fertilizer and fertilizer application rates to demonstrate differences between practices.

        Helpful Publications and Resources

        UGA Extension Fertilizer Calculator
        http://aesl.ces.uga.edu/soil/fertcalc

        UGA Extension Nitrogen Availability Calculator for Cover Crops
        http://aesl.ces.uga.edu/mineralization

        Crop Recommendations from AESL
        http://aesl.ces.uga.edu/publications/soil/CropSheets.pdf

        Sustainable Agriculture at UGA: Organic Production
        https://sustainagga.caes.uga.edu/systems/organic-production.html

        Southern SARE
        https://www.southernsare.org/

        University of California UC Davis Crop Uptake of Nitrogen
        https://apps1.cdfa.ca.gov/FertilizerResearch/docs/N_Uptake.html