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Calculating Fertilizer Applications

It can feel overwhelming to calculate fertilizer needs for each of the many crops and fields on your farm. Every year we help growers, both new and seasoned, to make these calculations and, if you follow these steps, the process should get easier over time. Our recommendation is always to make these calculations to determine what you should add in an “ideal” setting (if fertilizer was free and easily available and you had all the time in the world), and then figure out what you can realistically add to your fields, based on what is available and economical. Keep records on what you apply so that you can make educated changes to your applications if you see nutrient deficiencies in your crops. Here are the steps to follow, with details below:

  1. Work from a soil test.
  2. Find nutrient recommendations for your crop.
  3. Calculate nitrogen credits.
  • Contributions from previous applications of organic amendments, like compost and manure
  • Cover crop nitrogen contributions
  • Soil organic matter contributions
  • Contributions from sod plowdown
  1. Choose your fertilizer.
  2. Calculate fertilizer needed to meet N needs.
  3. Calculate how much P and K that fertilizer will add.
  4. Calculate how much additional fertilizer you need to reach your other nutrient needs.

 

  1. Work from a soil test.

A typical soil test will report macro- and micronutrient levels and soil pH. Some labs will automatically report soil organic matter (SOM) levels, and at other labs (including the UMass Soil Lab) you need to specifically request for SOM to be tested for. On UMass Soil Lab test results, in addition to reporting the soil nutrient level and optimum range of each nutrient in parts per million, macronutrient (phosphorous, potassium, calcium, and magnesium) levels are reported as very low, low, optimum, or above optimum. Because nitrogen is so mobile and ephemeral in soils, nitrogen is not routinely tested for in soil tests, and instead, nitrogen applications are made solely based on crop need. To test plant available nitrate during the growing season, you can take a pre-sidedress nitrate test (PSNT). We recommend having your soil tested at one of the New England state soil labs, as the tests run at these labs are specific to New England soils and provide nutrient recommendations that are backed by vigorous research results.

The example soil test results we'll work from are below. These results show that P levels in this field are optimum and K levels are low.
A photo of an example of soil test results from the UMass Soil Lab.

  1. Find nutrient recommendations for your crop.

If you indicated a crop on your soil test submission, you will receive nutrient application recommendations based on the crop need and the soil test results. For commercial vegetable growers, those recommendations will be in lbs/acre; for home gardeners, they will be lbs/100 sq. ft. If you indicate what crop you’re growing on your soil test form, you’ll receive nutrient recommendations for that crop on your results. If you don’t indicate a crop, you can easily look it up in the crop sections of the New England Vegetable Management Guide. In this example, we’ll make a fertilizer plan for broccoli, so the nutrient recommendations will be from the Cabbage, Broccoli, Cauliflower, and other Brassica Crops section.
A screenshot of the nutrient recommendations for brassica crops from the New England Vegetable Management Guide
For most crops, these tables are split between up-front application (broadcast and incorporate) and sidedressing a certain number of weeks after seeding or transplanting. All of the P and K that a crop needs can be put down up front, before planting, as P and K will stay in the soil and remain available for when the crop needs them. Depending on the N source, N applications can be split between pre-plant and sidedressing. Inorganic forms of N (e.g. urea) will leach quickly from soil, so any N you put down pre-plant that your crop doesn’t take up relatively quickly will leach out, along with the money you spent on it! Organic forms of N are released slowly by microbial activity throughout the season, so if you are using an organic form of N, you can apply all N up front, along with P and K.

Broccoli N recommendations are 100 lbs/A pre-plant and 60 lbs/A four weeks after transplant (circled in yellow). We’ll do our calculations using Kreher’s 5-4-3 chicken manure, which is an organic amendment that will release N slowly, so we will apply the full 160 lbs/A up front.

P and K recommendations are based on your soil test recommendations. In our example soil test, our P levels are optimum, so the recommendation is to add 50 lbs P/A (circled in red). New England state soil tests are based on a “build and maintain” model, which recommends you add nutrients even when the level in the soil is optimum, so that soil reserves are not depleted. In our example, K levels are low, so we should add 125 lbs K/A (circled in blue).

  1. Calculate nitrogen credits.

Manure, compost, some cover crops, soil organic matter, and sod all contain N that can be credited towards your total N needs for the season. The recommendation entered at the top of Table 1 is the preplant nutrient recommendation from step 2, and the following paragraphs will explain how the credits are calculated:

Table 1. Nutrient credit calculations

Nutrient Recommendation

(From NE Veg Mgmt Guide)

N

160 lbs/A

P

50 lbs/A

K

125 lbs/A

Credits
Manure - - -
Compost - - -
Cover crop 10 - -
Soil organic matter 40 - -
sod - - -
total credits 50 - -
nutrients needed 110 lbs N/A 50 lbs P/A 125 lbs K/A

Manure and compost: Like soil, manures and composts can be analyzed by a soil testing lab to determine the nutrient content, pH, C:N ratio, and other important characteristics. Compost and manure analysis is available through the Penn State and UMaine soil testing labs. If testing your organic amendment is not possible, approximations can be found in Table 2 at the end of this article.

Cover crops: When a cover crop is tilled in, N stored in the leaves and roots is released and much of it will be available for your summer and fall crops. The maximum amount of N is released 4-6 weeks after incorporation. In the spring, the soil doesn’t warm up fast enough for the microbes to release the N before the main crop needs it, so don’t include N credits for early spring crops. Sod also contributes some N when it’s plowed under to start production in a new field. Below is a table with some estimations of N contributions from certain cover crops. These numbers can vary widely, based on the quality of the cover crop stand and time of incorporation. If you’re interested in learning more, the SARE publication Managing Cover Crops Profitably is a great resource.

Soil organic matter: Soil organic matter (SOM) contains N that becomes available for plant uptake slowly throughout the season as it is released through microbial activity. The general rule of thumb in MA is: credit yourself 10-20 lbs/A of plant-available N per 1% SOM, capped at 4% or 40 lbs/A. The amount of N released from SOM increases with soil temperature, so numbers may vary between New England states.

Example credits, entered into Table 1 above:

  • Buckwheat cover crop: 10 lbs N/A (see Table 3, below)
  • Soil organic matter: Our example field has 4.4% SOM, but the SOM credit is maxed out at 4%, so will credit ourselves with 40 lbs/A N from soil organic matter.

After subtracting our nitrogen credits from our total nitrogen needs, we now need to apply only 110 lbs N/A.

Table 2. Nitrogen Credits from Manure Applied Before Planting

Type of manure

Dry Matter

Total N

NH4-N

Organic N

P2O5

K2O

 

 

------------------------- lbs/1,000 gallons  -----------------

Dairy, liquid

<5%

12-16

4.9

7.3

4.8

15.1

Dairy, slurry

5%-10%

22.3

7.6

14.7

8.9

22.0

 

 

-----------------------------  lbs/ton  -------------------------

Dairy, semi-solid

10%-20%

8.5

1.8

6.7

4.1

6.1

Dairy, solid

>20%

5-12

1.4

10.9

8.1

10.0

Beef (paved lot)

29%

14

5

9

9

13

Swine (hoop barn)

40%

26

6

20

15

18

Sheep

25%

23

n/a

n/a

8

20

Poultry, layer

41%

16-37

18

19

55

32

Poultry, broiler

69%

75

15

60

27

33

Horse

20%

12

n/a

n/a

5

9

Source: New England Vegetable Management Guide. Adapted from Nutrient Recommendations for Field Crops in Vermont (2018). Dairy manure values are from Vermont samples analyzed by University of Maine, 2012-2016, others are adapted from University of Nebraska-Lincoln NebGuide G 1335 and Penn State Agronomy Guide (2016). Values do not include bedded pack. Manures vary greatly, so obtaining a manure analysis is always best practice. n/a = data not available.

Table 3. Nitrogen contributions from select cover crops.

Cover Crop

Lbs N per acre

Rye*

25

Oat, Leonard*

10

Hairy vetch, spring incorp.

40-70

Hairy vetch, mid-summer incorp.

90-200

Red clover, spring incorp.

40-70

Red clover, mid-summer incorp.

70-150

Alsike clover

90

Sweetclover

90-170

Sudangrass

25

Field Peas

90-150

Buckwheat

10

Berseem clover

75-220

Sod

20-40

*young rye/oats only. As rye matures, C:N ratio increases and the N release rate becomes neutral. Mature rye/oats can tie up N.

Compiled by Becky Maden, UVM Extension

  1. Choose your fertilizer.

This choice can be made based on many different factors, including:

  • What fertilizer you already have on hand
  • Price
  • Application equipment (some materials are easier to spread with certain types of spreaders)
  • Availability of materials from local distributers
  • What materials meet your nutrient needs most closely

We’ll use Kreher’s 5-4-3 chicken manure for this example.

  1. Calculate how much fertilizer you need to apply to meet your pre-plant N needs.

We need to apply 110 lbs/A N (from Table 1) and our fertilizer is 5% N: 110 lbs/A nitrogen ÷ 0.05 = 2,200 lbs 5-4-3.

  1. Calculate how much P and K that amount of fertilizer will add.

Kreher’s 5-4-3 is 4% P and 3% K.

  • 2,200 lbs/A of 5-4-3 x 0.04 = 88 lbs/A P
    Total P needed (from Table 1) = 50 lbs/A. Our application of 5-4-3 will apply 84 lbs/A of P, so we don’t need to apply additional P.
  • 2,200 lbs/A of 5-4-3 x 0.03 = 66 lbs/A K
    Total K needed (from Table 1) = 125 lbs/A.
    125 lbs/A – 66 lbs/A = 59 lbs/A K
    Our application of 5-4-3 will apply 66 lbs/A of K, so we need to put down an additional 59 lbs/A to reach our total K need.
  1. Calculate how much additional fertilizer you need to reach your other nutrient needs.

We’ll use potash (0-0-50) to meet our remaining K needs.

 59 lbs/A potassium ÷ 0.5 = 118 lbs/A potash

Now we have application rates for both of our fertilizers:

  • 2,200 lbs/A Krehers 5-4-3
  • 118 lbs/A potash

And you’re ready to brew a big pot of coffee and do the same math for your remaining fields and crops!

 

Author: 
Genevieve Higgins
Last Updated: 
April 16