Long-Term Soil P and K Trends in
Relation to Nutrient Removal in
Corn-Soybean Rotations
Antonio Mallarino
Iowa State University
Joint Meeting NEC-17, NCERA-13, SERA-6
June 22-25 2008
Basis of Soil P and K Testing Use
 Soil test correlation and calibration
 Interpretation: interpretation categories
 Fertilizer recommendations
 Response-based and often some buildup
for the low-testing categories, those with
high probability of response.
 Maintenance of desirable soil-test levels,
those with low probability of response.
Based on nutrient removal and/or empirical
data.
Removal-Based P and K Maintenance
Example from Iowa
Pounds per unit of yield
Crop
Corn
Corn silage
Corn silage
Soybean
Oat and straw
Oat straw
Wheat
Alfalfa
Red clover
Pm-1688 pub, ISU
Unit of Yield
bu
bu grain equiv.
ton, 65% H2O
bu
bu
ton
bu
ton
ton
P2O5
0.375
0.55
3.50
K2O
0.30
1.25
8.0
0.80
0.40
5.0
0.60
12.5
12.0
1.5
1.0
33.0
0.30
40.0
35.0
Some Relevant Questions
 Are assumed buildup and maintenance
rate appropriate?
 Does maintenance make sense? What
level should be maintained?
 Does removal-based fertilization really
maintain STP and STK levels?
 How do yield, removal, and soil-test
values in the short term and long term?
 What are the impacts of wrong yield and
nutrient concentration estimates?
Long Term Studies in Iowa
 5 trials for P, 12 years each, no-till
 5 trials for K, 10 years each, no-till
 Corn/Soybean rotations, both crops
each year on adjacent identical trials
 3 P rates: 0, 14, and 28 kg P/ha
 3 K rates: 0, 33, and 66 kg P/ha
 Broadcast and band placement methods
 Measured yield from all plots and soil
tests for the control and the high rate
Soils and Properties
Soil Classification
Trial Series
SubGroup
Soil Properties
Clay OM pH
---- % ----
1
Kenyon
2
Webster Typic Endoaquoll
3
Galva
Typic Hapludoll
Typic Hapludoll
Ca
ppm
25 4.0 6.6 2872
32 5.8 6.5 4204
38 4.7 6.2 3488
4
Mahaska Aquertic Argiudoll
29 4.4 5.5 2408
5
Marshall
29 4.0 6.6 2886
Typic Hapludoll
POTASSIUM
Soybean Long-Term Trends
Responsive Sites
Grain K Removal Grain K Concentration
(kg K ha-1)
(g kg-1)
Grain Yield
(Mg ha-1)
4.4
Other Sites
No K
33 kg K ha-1 yr-1
66 kg K ha-1 yr-1
4.0
3.6
3.2
2.8
No K
33 kg K ha-1 yr-1
66 kg K ha-1 yr-1
2.4
2.0
21.5
20.0
18.5
17.0
15.5
14.0
12.5
70
65
60
55
50
45
40
35
94
95
96
97
98
99
OO O1
O2
O3
O4
94
95
96
97
98
Years of Corn-Soybean Rotations
99
OO O1
O2
O3
O4
Corn Long-Term Trends
Grain K Removal Grain K Concentration
(kg K ha-1)
(g kg-1)
Grain Yield
(Mg ha-1)
Responsive Sites
14
13
12
11
10
9
8
7
Other Sites
No K
33 kg K ha-1 yr-1
66 kg K ha-1 yr-1
No K
33 kg K ha-1 yr-1
66 kg K ha-1 yr-1
3.9
3.6
3.3
3.0
2.7
2.4
43
38
33
28
23
18
94
95
96
97
98
99
OO O1
O2
O3
O4
94
95
96
97
Years of Corn-Soybean Rotations
98
99
OO O1
O2
O3
O4
Average Relative Responses
Relative Response (%)
Corn
Soybean
120
Grain Yield
Grain K Concentration
No K
33 kg K ha-1
66 kg K ha-1
115
No K
33 kg K ha-1
66 kg K ha-1
Grain K Removal
Responsive
No K
33 kg K ha-1
66 kg K ha-1
Responsive
110
Responsive
Others
Others
105
Others
100
120
No K
33 kg K ha
No K
-1
33 kg K ha
66 kg K ha-1
115
No K
-1
33 kg K ha-1
66 kg K ha-1
66 kg K ha-1
Responsive
110
Responsive
Responsive
105
Others
Others
100
Others
Soil-Test K Long-Term Trends
225
200
225
NERF
66 kg K/ha/yr
175
175
150
150
125
125
100
Soil-Test K (mg kg-1)
200
275
No K
75
NWRF
225
66 kg K/ha/yr
SERF
66 kg K/ha/yr
200
250
175
225
150
200
125
175
No K
150
125
400
66 kg K/ha/yr
100
No K
75
300
NIRF
No K
100
75
250
SWRF
MEANS
225
66 kg K/ha/yr
350
66 kg K/ha/yr
200
300
175
250
150
200
150
No K
93 94
95 96
97 98 99
00 01 02 03
No K
125
100
93 94
95 96
Year of Corn-Soybean Rotations
97 98 99
00 01 02 03
K Removal & Post-Harvest STK
70
Soybean-Corn
0
STK
50
-20
40
-40
30
K removal
20
70
-60
Corn-Soybean
0
STK
60
50
-20
40
-40
30
K removal
20
-60
94
95
96
97
98
99
Year
OO
O1
O2
O3
Soil-Test K (mg kg-1)
Grain K Removal (kg ha -1)
60
Grain K Concentration and Yield
-1
Grain K Concentration (g kg )
24
SOYBEAN
Responsive sites
Other sites
22
20
18
AVG K
16.6
16
14
12
Avg = 1.2 lb K2O/bu
Current (dry basis) = 1.7 lb K2O/bu
10
1.5
2.0
2.5
3.0
3.5
4.0
-1
Grain Yield (Mg ha )
4.5
5.0
Grain K Concentration and Yield
-1
Grain K Concentration (g kg )
5.0
Responsive sites
Other sites
CORN
4.5
4.0
3.5
AVG K
3.20
3.0
2.5
2.0
Avg = 0.22 lb K2O/bu
Current (dry basis) = 0.35 lb K2O/bu
1.5
4
6
8
10
12
-1
Grain Yield (Mg ha )
14
K Removal and Yield
90
-1
Grain K Removal (kg ha )
80
SOYBEAN
Responsive sites
Other sites
70
60
50
40
Krem = 1.1 + 16.2 Yield
r ² = 0.83
30
20
1.5
2.0
2.5
3.0
3.5
4.0
-1
Grain Yield (Mg ha )
4.5
5.0
K Removal and Yield
60
-1
Grain K Removal (kg ha )
Responsive sites
Other sites
CORN
50
40
30
20
Krem = -5.3 + 3.8 Yield
r ² = 0.70
10
0
4
6
8
10
12
-1
Grain Yield (Mg ha )
14
PHOSPHORUS
P Concentration Soybean Grain Yield
(lb P2O5/bu)
(bu/acre)
P Soybean: Yield Trends
Responsive Sites
Non-Responsive Sites
No P
28-lb rate
56-lb rate
70
60
50
40
30
1.0
0.9
0.8
0.7
0.6
P Removal
(lb P2O5/acre)
60
50
40
30
20
1994
1996
1998
2000
2002
2004
2006
1994
1996
1998
2000
2002
2004
2006
P Corn: Yield Trends
P Concentration
(lb P2O5/bu)
Corn Grain Yield
(bu/acre)
Responsive Sites
220
Non-Responsive Sites
No P
28-lb rate
56-lb rate
200
180
160
140
120
100
0.50
0.45
0.40
0.35
0.30
0.25
P Removal
(lb P2O5/acre)
0.20
80
70
60
50
40
30
1994
1996
1998
2000
2002
2004
2006
1994
1996
1998
2000
2002
2004
2006
P Average Relative Responses
Grain Yield
Soybean (%)
160
P0
-1
14 kg ha
28 kg ha-1
140
120
Grain P Removal
160
160
140
140
Responsive
Sites
Responsive
Sites
Responsive
Sites
120
120
Other Sites
Other Sites
Other Sites
100
160
Corn (%)
Grain P Concentration
P0
-1
14 kg ha
28 kg ha-1
140
100
100
160
160
140
140
Responsive
Sites
Responsive
Sites
120
120
120
Other Sites
Other Sites
Other Sites
100
Responsive
Sites
100
100
Grain P Concentration & Yield
0.8
Grain P Concentration (%)
SOYBEAN
0.7
0.6
Mean = 0.57%
0.5
0.4
0.3
0.2
Avg = 0.78 lb P2O5/bu
Current (dry basis) = 0.92 lb P2O5/bu
Other Sites
Responsive Sites
0.1
1000
2000
3000
4000
Grain Yield (kg/ha)
5000
6000
Grain P Concentration & Yield
Grain P Concentration (%)
0.40
CORN
Other Sites
Responsive Sites
0.35
0.30
0.25
Mean = 0.24%
0.20
0.15
Avg = 0.31 lb P2O5/bu
Current (dry basis) = 0.44 lb P2O5/bu
0.10
2000
4000
6000
8000
10000
12000
Grain Yield (kg/ha)
14000
16000
18000
Grain P Removal and Yield
SOYBEAN
Grain P Removal (kg/ha)
40
Others
Responsive
30
20
10
0
1000
2000
3000
4000
Grain Yield (kg/ha)
5000
6000
Grain P Removal and Yield
50
Grain P Removal (kg/ha)
CORN
40
30
20
Others
Responsive
10
0
2000
4000
6000
8000
10000
12000
Grain Yield (kg/ha)
14000
16000
18000
Soil-Test P Trends Over Time
60
50
NERF
NIRF
NWRF
No P
28 kg P/ha
40
No P
28 kg P/ha
Bray-1 Soil P (mg/kg)
30
20
No P
28 kg P/ha
10
0
60
SERF
50
SWRF
No P
28 kg P/ha
No P
28 kg P/ha
MEAN
No P
28 kg P/ha
40
30
20
10
0
1994
1996
1998
2000
2002
2004
2006
1994
1996
1998
2000
Year
2002
2004
2006
1994
1996
1998
2000
2002
2004
2006
P Removal & Post-Harvest STP
P Removal (lb P2O5/acre)
Soybean-Corn
Bray P
50
0
45
-2
40
-4
35
30
-6
P removal
25
-8
20
-10
55
Corn-Soybean
50
45
0
-2
P removal
40
-4
35
-6
30
-8
Bray P
25
20
-10
1994
1996
1998
2000
2002
2004
Bray P Change (ppm)
55
Cumulative P Removal & Soil P
Averages of Non-Fertilized Plots Across Locations
-100
Cumulative P Removal
0
-2
37 lb P2O5/year decrease
-200
-4
Bray-1 Soil P
0.78 ppm/year decrease
-300
-6
47 lb P2O5/acre/year for
1 ppm/year decrease
-400
-8
-500
-10
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
Year of Cropping without P Application
2004
2005
Bray-1 Soil P Change (ppm)
Cumulative P Removal (lb P2O5/acre)
0
P Removal & Soil P Increase
30
Averages of Fertilized Plots Across Locations
100
25
Bray-1 Soil P
2.2 ppm/year increase
80
20
60
15
40
10
Cumulative Net P Addition
9 lb P2O5/year increase
20
5
0
0
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
Year of Cropping with 56 lb P2O5/acre/year
2004
2005
Bray-1 Soil P Change (ppm)
Cumulative Net P Addition (lb P2O5/acre)
120
Grain Yield and Concentrations
 Crop yields vary greatly across field and
years. We overestimated yield, as most
farmers do with yield goals.
 Grain P and K concentration also vary
across field and years
 Estimates of P and K concentration in grain
were much higher than observed averages.
 Increased up to a certain plateau with the
fertilizer rate and soil-test values
 Grain P and K concentrations were not
correlated with yield level across sites.
Removal and Soil Test Values
 Our overestimation of yield levels and
nutrient concentrations resulted in a
significant overestimation of removal.
 Removal was well reflected on soil-test
values in the long term, but very poorly
or not at all in the short term
 slow P recycling of residue P and K?
 slow equilibria between soil P and K pools?
Implications for Management
 Major errors at estimating P and K
removal is caused by error in grain yield
estimates.
 Removal-based P and K maintenance
based on good yield and concentration
estimates maintain or only slightly
increase soil-test P and K it over time.
 Removal-based maintenance rates must
be based on yield averages over time,
not from year to year changes.