Soil Nutrient Management
SOIL 4234

Soil Nutrient Management

Mobility Lab

 SOIL 4234    Laboratory Exercise 1
 

Excel Spread sheet with results.  Finish the calculations in grey to answer the questions. 

Clarification of the Excel File.  I apologize but there are two worksheets on this file.  You should work in the Leaching Worksheet. 

The lab gives results in lb/ac always assuming the sample is 6in deep.  So I back calculated for you to ppm.
It is now your job to convert back to lbs per acre I have done one for you, see if you can figure out the rest.
hint. In 6 in soil.  ppm*2 = lbs/ac so in 3 in soil ppm* 1 = lbs/ac.


 

Nutrient Mobility and Fertilizer Leaching

Overview:

            The degree to which fertilizer materials leach, or wash through the soil, is a concern both from the viewpoint of general society and interests in protecting the environment, and the agricultural community and sustainable food and fiber production.  When fertilizers leach through the soil they may reach groundwater aquifers and degrade the quality of public drinking water sources.  If the nutrients in the drinking water are a health hazard and the level exceeds established standards (e.g. nitrate-N in excess of 10 ppm), then society may wish to limit fertilizer use, or at least have input to how the fertilizer is managed.  The agricultural community is interested in knowing if fertilizers leach because this could decrease the effectiveness in relation to plant growth and crop production.  If the fertilizer leaches easily, more fertilizer may need to be applied to meet the needs of the crop.

            Bray’s nutrient ‘Mobility Concept’ indicates that management of nutrient inputs to correct deficiencies, or project deficiencies, is closely linked to whether the nutrient is mobile in the soil or not.  Fertilizers that are subject to leaching must contain nutrient forms that are relatively mobile in the soil.

 Approach:

            Nutrient Mobility.  High rates of N, P, and K fertilizers are applied in the field to the surface of a relatively level, permeable soil in advance of a normal rainy period or irrigation.  High rates are used to assure nutrient accumulation levels can be detected by common nutrient extraction methods and that the levels will be much higher than that found in unfertilized (control) soil.

            Amounts of each nutrient extracted from fertilized areas are first adjusted by subtracting the amounts found in similar soil layers of the control soil.  A second adjustment may be made for soil-changes to the nutrient that causes it to be “non-extractable”.  The amount found in each layer of soil extracted is summed and the sum divided into the total added to obtain an “adjustment” factor for that nutrient.  This factor is used to adjust the amount of that nutrient found in each layer before comparisons among nutrients are made.  Step-wise calculations are made by completing indicated tables.

            Fertilizer Leaching.  Fertilizer leaching is estimated by calculating changes in the water-soluble forms of the nutrients found in subsoil of fertilized areas.

 Objectives:

1.      Estimate the relative mobility in soil of nitrogen (NH₄⁺ and NO₃^-), phosphorus (H₂PO₄^-, and HPO₄^2-), and potassium (K⁺),when applied to the soil surface of a cultivated field. 

2.      Estimate the percentage of fertilizer added that has leached into the subsoil.

Materials and Methods:

1.      The fertilizer materials are Urea {((NH₂)₂CO) 46-0-0}, monocalcium phosphate (Ca(H₂PO₄)₂, 0-46-0) and potassium chloride (KCl, 0-0-60).

2.      Pre-weighed fertilizers are applied in solution form to the surface of “plots” in a cultivated field at the Stillwater OSU Experiment Station Agronomy Farm at the rate of 1000 lb/acre of N, P₂O₅, and K₂O.

a.      Plots will be defined by the dimensions of an “inner ring”,  4 inches in diameter, inserted about 2” deep into the soil.  A second, or “outer ring” will be similarly placed to enclose the inner ring in its center.

b.      Fertilizer materials will be applied in a volume of solution equivalent to 1 inch of irrigation.  A similar irrigation rate will be applied to the outer ring enclosure.

c.      After one week an additional 5 (lab 1) and 9 (lab 2) inches of water will be applied to the inner and outer enclosure for one replication (lab section) of each treatment.

3.      Two weeks after the initial fertilizer application, surface and subsoil samples will be obtained from each fertilized and control plot.

4.      Soils will be analyzed for plant-available and water soluble forms of the nutrients.

5.      Estimated relative soil-mobility of N, P, and K, will be calculated by performing the indicated calculations, from fertilizer application and soil test values, to complete the following tables.

Results:

            Nutrient Mobility Five or nine inches of water added to leach the fertilizer materials.  Soil test results from field samples have been entered in Table 1.  Perform the indicated calculations to complete Table 1.

Table 1.   Soil test results¹ of surface (0-6”) and subsoil (6-24”) layers of sampled soil.

¹All values in units of lb/acre unless otherwise indicated.

²For each nutrient form and layer, subtract the control value from the fertilized value.

³Divide ‘Fertilized-Control values’ by ‘total added’ and multiply by 100.

⁴Calculated immobility is based on nutrient retention in the surface soil.  Dividing each value for ‘surface, percentage of total applied found’ by the largest value in that row and multiply by 100.  Percent mobility may be assumed to be the difference between 100 and percentage immobility, therefore, subtract the percentage immobility from 100 and recording the value in (     ).

⁵ Calculate mobility by dividing each value for ‘Sub, percentage of total applied found’ by the largest value in that row and multiplying by 100.

⁶Sum the value for mobility calculated from surface soil retention (     ) and that found in the subsoil and divide the result by two.

⁷Divide the largest value into each value, multiply by 10 and round the value to a whole number.

 Fertilizer Leaching.  Record soil test results, as provided by the instructor, and perform the indicated calculations to complete Table 2.

Table 2.   Plant-available¹ nutrient levels from soil test results of subsoil (6-24 inch) from fertilized and control soil.

¹All values in units of lb/acre unless otherwise indicated. P and K extracted by Mehlich-3, Cl by saturated CaSO₄ solution, and N using 1M CaCl₂.

²For each nutrient form, subtract the value for the control from the value for the fertilized area.

³Divide ‘fertilized – control’ by ‘total added’ and multiply by 100.

Questions:

1. The most mobile nutrient was ______________.
2. The second most mobile nutrient was ____________.
3. Identify a chemical characteristic of the two most mobile nutrients that might account for their high mobility and explain why this characteristic is related to high mobility of nutrients in soils.

4. The least mobile nutrient in this soil was _____________.
5. The second least mobile nutrient was _______________.
6. Identify chemical characteristic(s) of the two least mobile nutrients that might account for their low mobility and explain the relationship to low mobility of nutrients in soils.

7. Compare relative nutrient mobility values and percentage of fertilizer leached.   Do these results agree? 

If not, explain the differences.

8. Explain why you believe fertilizer leaching is or is not a serious problem in Oklahoma soils.

9. Describe a condition of this exercise that was not normal for fertilizer use in Oklahoma.

10. What cultural practices (at least two) would you recommend to reduce fertilizer leaching from what was found in this exercise.

Excel