Mission #2

Soil Plant Nutrient Cycling and Environmental Quality 5813
 

The OSU soil testing lab initiated a program to establish improved P recommendations based on soil test indices. Initially, soil samples were collected from six different locations and samples were subsequently analyzed using two different extracting solutions.  It was found that no other elements were limiting other than phosphorus at the locations sampled.

Farmer	Mehlich III	Bray & Kurtz P-1	Grain Yield	Maximum
	ug/g	ug/g	kg/ha	Yield
1	7	10	1000	4600
2	11	15	1350	4600
3	16	20	2116	4600
4	28	26	3400	4600
5	31	31	3630	4600
6	35	35	3549	4600

1. Assuming that all conditions other than soil test P were the same, determine which procedure would be best for future use in soil testing. To do this, you will need to establish the relationship between the soil test and percent maximum yield using quadratic regression and linear-plateau equations.

A. Determine the critical soil test level for both procedures using quadratic regression, linear-plateau and Cate-Nelson procedures.  The critical level is determined by the joint from linear-plateau and by taking the first derivative of quadratic regression, setting it equal to zero and solving for x.

2. Once the ideal analytical procedure and critical soil test levels have been determined you must now evaluate data obtained at each of these sites where phosphorus fertilizer rate studies were conducted. In each case, N, K and other micronutrients were applied in adequate but not excessive amounts and P was applied as triple superphosphate (0-46-0) at rates of 0, 20, 40, 80, 120, 160 and 200 kg P/ha. In each experiment, P was applied broadcast before planting and was disk incorporated. Results from these experiments were as follows;

Farmer P rate   Grain Yield

                          kg/ha                       kg/ha

____________________________________

1                        0                              915

1                        20                            1350

1                        40                            2212

1                        80                            2715

1                        120                          3210

1                        160                          3399

1                        200                          3601

2                        0                              1500

2                        20                            2450

2                        40                            2850

2                        80                            3250

2                        120                          3550

2                        160                          3590

2                        200                          3570

3                        0                              2316

3                        20                            2860

3                        40                            3340

3                        80                            3690

3                        120                          3604

3                        160                          3606

3                        200                          3601

4                        0                              3180

4                        20                            3360

4                        40                            3450

4                        80                            3598

4                        120                          3502

4                        160                          3622

4                        200                          3518

 

Assuming that the soil test values obtained for each individual farmer are representative of larger populations with the same value;

A. Determine the optimum fertilization rate for each farmer using quadratic regression, linear-plateau and Cate-Nelson procedures.

B. Using the price of wheat at 0.13$/kg and the cost of P/kg applied at 0.50$, what rate of P would you recommend to these same farmers using quadratic regression.

3.  Briefly discuss the results obtained from #1 and #2 in terms of how we make fertilizer recommendations to farmers, and how you might change the sequence of events to improve upon soil test/correlation/calibration/recommendation work.

4.  Based on the information collected, establish a table which will recommend P fertilizer rates using soil test indices (range) and percent sufficiency (see your Soil Fertility Handbook)

5. Why is it valuable to have at least two extremely high rates (beyond that which farmers would commonly apply) in fertilizer rate evaluation studies.

1._________________________________________________________________________

CL - critical soil test level