Exercise 4. Determining Manure Application Rates and Phosphorus Build Up Rates
(adapted from Olson and McKenzie, 1998)


Exercise 4a.  Determining Manure Application Rates

Using the soil and manure test results provided, determine the application rate for an 80 bushel barley crop using 360 tons of beef manure.

Total beef manure produced (tons)
Next crop B(1)
Recommended N fertilizer rate (lbs/acre) from soil tests
Total N from manure test (lbs/ton)
= B(1) / C(1)

Manure application rate (tons/acre)
= A / D(1)

Land required to balance N requirements
- barley (80 bu) - _____ x 0 .35 = 
1st year
- -
Total beef manure produced (tons)
Next crop B(2)
Recommended P2O5 fertilizer rate (lbs/acre) from soil tests
Total P2O5 from manure test (lbs/ton)
= B(1) / C(2)

Manure application rate (tons/acre)
= A / D(2)

Land required to balance phosphorus requirements
  barley (80 bu)   _____ x 0 .50 =  available
1st year
* Column D in Exercise 1a.** This factor is the frequency of manure application in years.In this example, manure is applied every other year, so a value of 2 is used. If manure was applied every 5th year, a value of 5 would be used. *** This value will be used in Exercise 4b F
= D(1) – D(2)
Excess manure applied when balancing for N (tons)
= F x C(2)
Excess P2O5 applied (lbs/acre)

= D(1) – {D(2) x 2**}
Excess manure (tons) applied based on application every other year
= H x C(2)
Excess P2O5 (lbs/acre) applied based on application every other year

If the beef manure is only applied every second year, what is the excess P2O5 in lbs/acre that is applied? This, answer (I) , will be used in Exercise 4b.



Exercise 4b. Phosphorus Buildup Calculation (adapted from USDA-NRCS 1999).
The use of livestock manure and organic material as nutrient sources presents a problem for developing a nutrient budget. Nutrients contained in manure are not balanced in the same proportion as crop requirements. While most animal manure has an N-P2O5-K2O ratio of 3-2-3 to 2-1-2, crops require nutrients in a ratio of 8-1-3 to 3-1-2. Balancing nutrients on any one of the major crop elements (N, P, or K) creates either a deficiency or excess in nutrients for the other two.

When phosphorus accumulates in the soil, it becomes an environmental concern. Monitoring the levels of phosphorus in the soil is important to avoid situations of excess P nutrients building up on the landscape and causing detrimental environmental impacts. Excess potassium can cause nutrient imbalance in forage feed rations.

The process used to calculate the phosphorus and soil thresholds is described in paragraphs following the questions.


1.   From (I) in Exercise 4a, enter the amount by which applied P2O5 exceeds crop requirements when manure application is every other year. (A)




2.  Determine the P buildup factor. This is the inverse of the pounds of P2O5 necessary to raise the soil test level 1 ppm. (B)




3.  Multiply amount in line (A) by the soil test buildup factor (B). (C)




4.  The suggested threshold for sensitive soils and landscapes in Alberta (sandy and organic soils; runoff-prone lands) is 200 lbs/acre for elemental P. (D)




5.  Using the current soil test P level, calculate the available ppm of P storage before reaching the sensitive soil threshold level. (E)




6.  Manure application based on a Nitrogen rate, and every other year, will build up the soil test P level to the sensitive threshold level in how many years? (F)





Enter the excess amount of P2O5 over the agronomic crop requirements that will be applied in the Nitrogen based example of exercise 4a.

Excess levels of phosphorus application above the amount required for crop production will build up in the soil and be expressed by higher soil test levels. The rate of buildup depends on the soil type, soil test method, and excess level of P application. As a general guidance rule, it takes somewhere somewhere between 8 to 16 pounds of excess P2O5 to raise the soil test P level 1 pound. This is the same as saying it takes 16 to 32 pounds of P2O5 to raise the soil test P level 1 ppm.

Using a value of 20 pounds of excess P2O5 as an estimate to raise the soil test level 1 ppm, then multiplying the excess phosphorus amount in pounds by 0.05, the P buildup factor (the inverse of 20) would give the increase in soil test P level in ppm. If your province uses a different rate of soil test buildup, use that amount in your calculations. (P buildup factor equals the inverse of the pounds of P2O5 required to raise the soil test P level by 1 ppm.)

The example showed an excess of 50 lbs/acre P2O5. 50 x 0.05 = 2.5 gives the increase in soil test level P (in ppm) for each year that an excess of 50 lbs P2O5 is applied.

Many areas have developed the relationship between soil test levels of P and potential for significant P movement on the landscape. Some jurisdictions have set threshold soil test levels of phosphorus at which either nutrient management should change or management practices should be put in place to control runoff and erosion. Above some soil test P level, there may even be a total restriction of additional P application to the site.

If the threshold soil test value has been developed, enter it here. If no threshold soil test has been developed, a surrogate value can be determined using the agronomic soil test levels suggested for the crop being grown. The basis for using agronomic soil test levels relates to the producers understanding of a high soil test level at which no expected crop yield increase will occur. As a surrogate, five times the soil test P value for the minimum level of the high category can be considered the threshold level. The minimum level of the high soil test category is the breakpoint between a medium (marginal) and high (optimum) soil test level.

As an example, when the soil test level category of high for corn starts at 50 ppm P, a surrogate threshold level would be 5 x 50 ppm, or 250 ppm.

The calculation for time required to raise the current soil test P level to the threshold level follows.

Multiply the excess phosphorus application in pounds per acre by the P buildup factor. The soil test P level will raise this amount per year. Next, subtract the current soil test P level from the P threshold value. This is the amount of soil test value remaining until the threshold is reached. Divide the remaining soil test value by the annual rate of increase in soil test P. This is the number of years that it will take for that field with the current cropping and nutrient budget to reach the threshold level.

If the excess amount of phosphorus being applied each year is 50 lbs/acre and the P buildup factor is 0.05, then the annual increase in soil test P is 50 lbs x 0.05 = 2.5 ppm

If the current soil test level is 70 ppm and the threshold soil test P level is 250 ppm, then the amount of remaining soil test value is 250 - 70 = 180 ppm

The buildup of soil test P to reach the threshold level will take 72 years (180 / 2.5).