Soil Test and Agricultural Management Practices Survey

of the Clear Lake Watershed

 

Antonio Mallarino and Jeremy Klatt

 

 

A.        Introduction

 

This work was a component of the project Clear Lake Restoration Diagnostic and Feasibility Study lead by Dr. John Downing (Department of Animal Ecology), which included other components and co-investigators.  Objectives and methods of the overall project, and how the component fits with the overall project objectives were described in the original project.  The objective of this component was to survey surface soil test values, with emphasis on phosphorus (P), and nutrient management practices within the agricultural area of the watershed.  This information is a key aspect that should help understand and assess the role of nutrient levels and management practices on the nutrient load to the lake and its water quality status.  This knowledge will help determining alternatives for improving the water quality of the lake.

 

B.        Methods

 

The work included a survey of nutrient levels in soils that involved extensive soil sampling and a survey of relevant soil and crop management practices.  Plans for both surveys were developed during summer and early fall of 1999 in cooperation with personnel of the CLEAR Project, in particular with Mr. Ric Zarwell, who was CLEAR Project Coordinator until December 1999.  Efforts until early September were directed toward developing the plan for the survey and to request permission from landowners to collect soil samples from their fields.  This part proved to be more difficult than expected, and finally permission was denied for approximately 20% of the fields.  Except for the small agricultural area at the extreme southeast corner of the watershed, however, the information collected provides a good representation of nutrient status and agricultural practices in the watershed.

 

1.  Management Practices Survey.  A two-page questionnaire was mailed or delivered in person to all landowners or operators from whose fields soil samples were collected in order to obtain information of agricultural practices that are occurring in the watershed.  The questionnaire requested information about crop rotation, tillage system, and rates of application and type of P fertilizer, nitrogen (N) fertilizer and manure used during the last five years.  Sixty-six percent of the surveyed farmers responded, which represents approximately 60% of the land in the watershed.  This success rate is typical of these types of surveys (when no payment is included), and this high rate was achieved only after repeated telephone calls and some personal visits.

 

2.  Soil Nutrients Survey.  The soil sampling strategy was planned based on available information on field borders, soil types, current crop and planned fall fertilizer management.  Consideration of the objectives, characteristics of the watershed, previous information of areas from where seemingly much of the P load to the lake could be derived from and the available budget were key aspects for deciding among several soil sampling alternatives.  The sampling plan chosen was a systematic stratified sampling method, in which predominant soil map units of each field were sampled following a point mapping and sampling strategy.  In a first step, approximately 500 small sampling areas (points) about 0.2-acre in size were identified within the predominant soil map units within each field on the basis of digitized and georeferenced soil survey maps, georeferenced maps of field borders, surface drainage, aerial photographs and visual observations of the watershed.  Geographical Information Systems (GIS) techniques and ArcView computer software were used to produce several layers of information.  A second step involved a field verification and modification (when necessary) of the location of the sampling points through use of hand-held Global Positioning Systems (GPS) receivers and field observations.  The original number of 500 desirable sampling points had to be reduced to approximately 340 due to several reasons.  These included lack of permission to sample several fields, that a small number of fields could not be sampled because contrary to our request the farmers applied P fertilizer immediately after harvest and before samples could be collected, and because of budget limitations.  In a third step, one composite soil sample made up of 15 to 20 soil cores were collected from the top 6 inches of soil of each sampling area.  This is the sampling depth recommended by ISU for agronomic soil tests.  The GIS map in Figure 1 shows the sample points location.

 

All soil samples were dried, ground to pass a 2-mm screen, and analyzed in duplicates by several agronomic soil tests supported by Iowa State University.  These included analyses for Bray-1 P (which will be referred to as the Bray test hereon), Olsen (or sodium bicarbonate) P, Mehlich-3 P (which will be referred to as M3 test hereon), ammonium acetate-extractable K, Ca, and Mg, soil pH and organic matter.  The Bray test is the most commonly used P test by soil test laboratories and farmers in Iowa, and the Olsen P and M3 P tests provide a different assessment of soil P, mainly in high-pH soils.  In addition, all soil samples were analyzed by the two environmental P tests most commonly used in ongoing research across the United States.  These type of tests are the object of intensive research in Iowa and other states because they could provide a different assessment of the potential impact of soil P levels on P losses with water runoff or through the soil profile.  Agronomic soil P tests were developed and calibrated to predict P sufficiency for crops, not necessarily to predict P losses to water resources.  The tests chosen were the iron oxide-impregnated paper P test (which will be referred to as the Iron-oxide test hereon) and the deionized water extraction test (which will be referred to as the Water test hereon).  The procedures used are not described here, but information can be found in published articles.  Procedures followed for all agronomic tests were those described in the North Central Region Publication 221 (Brown, 1999), which includes agronomic soil test procedures recommended by the North Central Region Committee for Soil Testing and Plant Analysis (NCR-13).  Procedures followed for the two environmental P tests were those described in the Southern Cooperative Series Bulletin 396 (Pierzynski, 2000), which includes procedures for a variety of P tests recommended by the national committee Minimizing Phosphorus Losses from Agriculture - Information Exchange Group (SERA-17/IEG).

 

When soil-test P values were summarized into GIS maps, values of agronomic soil P tests were grouped into the agronomic soil test interpretation classes used by Iowa State University.  Two modifications were made, however, to better describe the results.  One was that the Very Low and Low classes were merged because very few fields tested Very Low and little distinction can be made between these two classes for the purpose of environmental conservation.  The other modification was to split the Very High class into two classes to better describe the high-testing soils.  The Optimum class (16 to 20 ppm for the Bray or M3 tests and 10 to 14 ppm for the Olsen test) defines soil test values for which only maintenance fertilization is recommended to account for P removal in harvested products.  No P fertilizer is recommended for the High or very High classes by Iowa State University.

 

3.  Data Management Procedures.  The procedure used to summarize the soil P and field history survey information was to summarize the information for various sub-watersheds delineated according to surface drainage patterns.  Regression and correlation techniques were used to study how the various soil test extractants assessed soil P.

 

C.        Summary Results

 

1.  Field Management Practices.  Summaries of relevant field history are shown in Table 1 and several figures.  The Clear Lake watershed is dominated by row-crop agriculture.  Eighty-nine percent of the acreage surveyed was in a corn and soybean rotation (Fig. 2).  Of the remaining 11% of the land, 7% was being cropped for corn continuously and the remaining land was either set aside as part of the Conservation Reserve Program (CRP) or under alfalfa or pasture.

 

The most common tillage practice in the watershed is a tillage system that includes chisel plowing in combination with disking and/or field cultivation.  Forty-eight percent of the land was managed with this tillage system (Fig. 2).  The remaining fields under row-crop production were split between no-till (21%), ridge-till (18%), moldboard plow (7%), and those that include V ripping (6%).  From a soil conservation perspective, the moldboard plow system is the most prone to produce soil erosion and the no-till system usually produces the lowest soil erosion.  The other tillage systems usually rank intermediate and their impact on soil erosion is highly dependant on the residue cover.

 

The most common P application method in the watershed is to incorporate the fertilizer into the soil in the fall by plowing, disking, or injecting.  Forty-four percent of the land had P applied in this way.  The next two most common methods were broadcast application in the fall without incorporation (13%) and injection or incorporation of the fertilizer into the soil in the spring (4%).  Fall application with incorporation is the preferred method for P application from an environmental perspective as most runoff events occur in the spring.  Injecting or incorporating is preferred over leaving it on the soil surface because this reduces the risk of P loss with surface runoff.   When a no-till system is used, however, the advantage of incorporating P fertilizer or manure by injecting into the soil over a surface application with good conditions (other than application to frozen, water saturated, or snow-covered ground) is not clear, however.  Commonly used injection equipment cause significant soil disturbance, which may increase soil erosion and total P loads to surface water resources.  Soil loss (and loss of P attached to soil particles) is very small for no-till or pastureland that receives broadcast fertilizer or manure applications but loss of dissolved P can be high when soil conditions are adverse.

 

On average, during the last five years P fertilizer was applied 2.2 times.  When P fertilizer is applied, it is typically applied at a rate of about 65 lb P2O5/acre.  For comparison, Iowa State University recommends 70 lb P2O5/acre be applied once every two years to soils testing in the optimum range in a corn and soybean rotation.  The Optimum range for the Bray or M3 soil P tests is 16-20 ppm.  Consideration of the average M3 soil test P value of 40 ppm and the average application rate of 65 lb P2O5/acre may suggest that the watershed is being over fertilized, which is typical of most agricultural regions of Iowa.  However, a close look at specific areas of the watershed shows that some fields testing much higher than the average are also being fertilized.  These observations suggest that many farmers in the watershed are managing their land using recommended rates of application and that only a few are over-fertilizing their fields.  This aspect will be visited further when results of the soil sampling are discussed.

 

Only 4% of the land received fall-applied N.  In the last several years there has been a push to reduce fall application of N on agricultural lands.  Nitrogen, unlike P, is very mobile in the soil and under the right conditions can be lost with surface runoff or leach through the soil profile into sub-surface drains or to groundwater supplies.  The most common application method was broadcasting without incorporation in the spring (50%), followed by spring sidedressing (26%), and injection (11%).  The remaining land had no N applied (8%).  On average in the last five years, farmers have applied N 2.4 times to their fields.  Farmers, for the most part only apply N before corn due to the ability of soybeans and forage legumes to fix atmospheric N.  When applied, it is typically applied as anhydrous ammonia at a rate of about 130 lb N/acre.

 

Our survey showed that 18% of the farmland had received manure in the previous five years.  The responses in most cases provided insufficient data concerning rates of manure application.  Manure often can be overapplied to certain areas in order to dispose of it, which creates high nutrient levels in concentrated spots.  The data actually shows, however, very similar average soil P tests for manured fields and nonmanured fields in this watershed.  The average M3 soil test P value of manured soils was 34 ppm while nonmanured soils averaged 40 ppm.  Manure is also thought to create more variability within a field but our data suggests very similar variation of P in manured and nonmanured soils.  Our numbers showed higher organic matter percent and calcium in fields where manure was frequently applied.

 

2.  Soils of the Watershed.  The Clear Lake watershed is dominated by loam and silty-clay loam textured soils under a predominately agricultural setting.  The soil survey map is shown in Figure 5.  Background chemical analyses of the collected samples that are useful to describe the fields include organic matter, and extractable K, Ca, and Mg.  Mineral soils predominate and have an average organic matter content of 5.7%.  According to the digital soil survey maps, these soils typically have a cation-exchange capacity ranging from 0-65

(meq/100 gm) with an average of about 22 (meq/100 gm).  The average pH in the watershed was 6.6.  Calcium and Mg concentrations averaged 3357 ppm and 380 ppm respectively.  The K values, which ranged from 49 ppm to 502 ppm and averaged 170 ppm, indicate that there is extensive but very uneven K fertilization occurring in the watershed.  The optimum range for K for corn and soybeans as recommended by Iowa State University is 90-130 ppm.  The landscape forms, soil types, and drainage patterns, which have strong influence on nutrient delivery to the lake were considered together with soil-test P data in others components of the project.

 

3.  Soil Test Phosphorus.

a.  Soil P assessment by five soil P tests.  There were differences in amounts of P extracted by the five soil P tests used.  Differences or similarities between tests and reasons for any difference should be discussed before interpreting distribution of soil-test P across the watershed.  Average amounts of P extracted by the five methods are shown in Table 2.  The M3 soil test extracted the highest amount of P followed very closely by the Bray test in most samples (except in those with high pH).  Iowa research has shown that this test usually extracts similar to or only slightly higher amounts of P than the Bray test, except when soils have high pH and calcium carbonate content.  This result was confirmed in this project because the Bray extracted much less soil P than the M3 in soils with pH 7.2 or higher.  The Olsen and Iron-oxide test extracted very similar amounts of P that were less than the amount extracted by Bray or M3 tests.  The Water P test extracted the lowest amount of P.  All soil tests used with agronomic or environmental purposes extract only a small proportion of the total soil P, which includes the P fraction assumed to be better related to soil P sufficiency for crops (for the agronomic tests) or to potential loss of P dissolved in water (for the environmental tests).  Current Iowa research is developing field calibrations for these tests and others in relation to potential loss of P to water resources.  The ongoing research has not determined clear differences between the tests, and this is the main reason these five tests were chosen for this survey.

 

Three of the five soil P tests used were highly correlated across all samples collected (Table 3).  The correlation coefficients between the M3, Olsen, and Iron-oxide P tests were very high and ranged from 0.96 to 0.97 across samples.  Correlations involving the Bray and Water P tests were variable and usually lower.  Correlations coefficients for the Bray test, the most commonly used agronomic P test in Iowa, were the lowest and ranged from 0.88 with the Water test to 0.92 with the M3 test.  Correlations coefficients for the Water test (an environmental P test) were intermediate and variable, and ranged from 0.91 with the Bray test to 0.96 with the Iron-oxide test (which is another environmental P test).  These differences in correlations between tests may suggest differences concerning estimates of P loss from the fields with surface runoff and tile drainage.  While the Bray, M3 and Olsen tests are used for agronomic purposes, the environmental tests would predict better potential P delivery to surface water resources.  The correlation between the Bray soil test and the other tests across all samples was poorer because this test extracted proportionally less P from soils with pH higher than 7.2, which likely were calcareous.  This is a common occurrence observed in Iowa soils.  Of the 332 samples analyzed from the watershed, 109 had pH higher than 7.2 and tended to have the highest exchangeable calcium concentrations.  Correlation coefficients in Table 3 that included only soils with pH less than 7.2 showed much higher correlations for the Bray test.   The intermediate and variable correlations for the Water test either across all soils or for noncalcareous soils suggest that this test extracts slightly different P fractions than other tests from different soils.

 

Overall, the only test that clearly departs from the others was the Bray test in high pH soils.  This result may indicate that the Bray test should not be used across all fields in this watershed for agronomic assessment because it would underestimate sufficiency levels for crops.  Conclusions concerning assessments of the potential P loss to the lake are not so clear cut, however.  The Bray test would underestimate the amount of P tied-up to soil particles that could reach the lake in eroded soil.  However, the presence of calcium carbonate could reduce P that could be released from the soil phase of the soils and that could be transported dissolved in water though surface runoff or subsurface drainage.  If this were the case, the Bray test would provide a more accurate assessment of dissolved P loss.  Theoretical considerations suggest that the two environmental tests would be less affected by soil properties and would provide better assessments of the potential P loss across all conditions.  The soil test correlations suggest, however, that the Iron-oxide, M3, and Olsen tests provided similar environmental assessments which differed slightly with assessments obtained by using the Bray and Water tests.

 

b.  Distribution of soil-test P throughout the watershed.  There was a wide range of soil test P values throughout the watershed.  Data for the individual soil samples collected from each field showed that values for the M3 P test (for example) ranged from 2 to 395 ppm across the watershed while values for the water P test ranged from 1 to 68 ppm (not shown).  These wide ranges of values confirm that there has been extensive P fertilization occurring in the watershed.  Available soil P of most Iowa soils was in the Very Low or Low interpretation classes until approximately the mid 1900s, and were increased to current levels by fertilization and/or manure management practices.  The average M3 P test across the watershed was 40 ppm while the median value was 32 ppm.  The median may be a better value to look at to prevent a few very high soil samples from skewing the data.  The median value is higher but not very much higher than the value considered optimum for agronomic purposes (20 ppm).  Also, although this median value is high, it is not far from the range that could be considered acceptable (16 to 25 ppm) if the usually very high variability in soil tests due to high and largely unavoidable sampling error is considered.  Iowa research shows that a soil test value of about 30 ppm would drop into the Optimum level within two to three years of cropping without P fertilization.

 

The distribution of soil test values for sub-watersheds is shown in several GIS maps.  On a field basis, using the M3 P test as an example, soil P ranged from 6 to 296 ppm across the fields of the watershed and averaged 41 ppm.  Several fields tested very high in P.  Approximately one third of the fields tested Very High by any of the three agronomic soil tests.    The high-testing fields were spread without clear patterns across the watershed.  In spite of lower correlations discussed above for the Bray and Water P tests, the five tests used tended to agree in identifying high-testing fields. 

 

The other approach used for summarizing the soil test information was to calculate averages for sub-sections of the watershed delineated according to surface drainage patterns.  The results of using this approach are shown in the GIS maps in Figures 4 to 9.   This approach provides a slightly different perspective of the distribution of soil P and better idea of the potential impact of current management practices in different parts of the watershed on P loads to the lake and, also is useful to identify sub-watersheds where changes in P or soil conservation practices are most urgently needed.  Continued use of the M3 P test as an example shows that soil P levels ranged from 18 to 65 ppm across all sub-sections of the watershed.  In spite of minor differences between the five P test used, all tests identified high-testing subsections in the western side of the watershed, and also three smaller high-testing subsections bordering the south, southeast, and northeast sides of the lake.  Although these maps must be interpreted with caution because each sub-section was highly heterogeneous, the results suggest the areas that potentially contribute a large proportion of the P loads to the lake.

 

D.        Component Conclusions

 

The survey of soil P status and P management practices of the Clear Lake agricultural watershed was useful to identify areas that may be sources of large P loads to the lake and to identify priority areas where changes in P management practices would be desirable.  Approximately one third of the area of the watershed had soil-test P values that were twice to five times higher than levels needed to maximize crop production.  The highest soil-test P values were found in a very small number of fields that received either P fertilizer or manure, which likely are the source of a major proportion of the P being transported by surface runoff or subsurface drainage to the lake.  There were minor differences between soil tests in describing the distribution of soil P across the watershed.  All tests identified high-testing areas.  The Bray test measured proportionally less P from high pH soils, and its use may mislead producers to apply more P fertilizer than needed to optimize crop production.

 

Phosphorus fertilizers or manure are not incorporated into the soil in approximately 30% of the area, a major proportion of which is under no-till or ridge-till management.  Only approximately 40% of the area surveyed is under no-till or ridge-till tillage systems, which are the most effective in reducing soil erosion and surface water runoff.  The finding that several fields had above-optimum soil P levels for crop production and were managed with conventional tillage do not necessarily mean that these fields are major sources of P loads to the lake.  However, the results strongly suggest that further adoption of commonly recommended best management fertilization and soil conservation practices by some producers would have a major impact in reducing the risk of P delivery from the agricultural area of the watershed.

 

References

 

Brown, J. R. (ed.).  1998.  Recommended soil test procedures for the north central region, North Central Regional Research Publication No. 221 (Revised).

 

Pierzynski, G.M. (ed.).  2000.  Methods of phosphorus analysis for soils, sediments, residuals, and waters.  Southern Cooperative Series Bulletin No. 396

 


              Table 1.  Summary of the field history information.

 

     Item

Acres

% of land

Primary tillage

 

 

     Chisel/Disk

1983

48

     Moldboard plow

291

7

     No-Till

844

21

     Ridge-Till

725

18

     Other or unknown

229

6

 

 

 

Cropping System

 

 

     Alfalfa, pasture, or CRP

195

4

     Continuous corn

266

7

     Corn-soybean rotation

3649

89

 

 

 

Method of P Fertilization

 

 

     Fall injection/incorporation

1766

43

     Spring injection/incorporation

687

17

     Fall left on top

706

17

     Spring left on top

532

13

     No P applied

349

8

     Unknown

70

2

 

 

 

Method of N fertilization

 

 

     Fall injected/incorporated

177

4

     Spring injected/incorporated

447

11

     Spring sidedress

1068

27

     Spring left on top

2069

50

     No N applied

349

8

 

 

 

Manure History

 

 

     No

3330

81

    Yes

780

19

 


Table 2.  Amount of P extracted by five soil P tests and soil pH for 332 soil samples collected

throughout the Clear Lake Watershed.

 

 


Soil P test or pH

Mean

Median

Range

Minimum

Maximum

 

--------------------------- Soil test P (ppm) --------------------------

Olsen

17

13

147

1

148

Bray-1

32

25

371

1

372

Mehlich-3

40

32

393

2

395

Iron oxide

24

20

168

4

172

Water

8

6

67

1

68

 

 

 

 

 

 

Soil pH

6.6

6.5

3.4

4.8

8.2

 

 

 

 

Table 3.  Correlation coefficients between amounts of P extracted by five soil P tests from 332

soil samples collected throughout the Clear Lake Watershed.

 

 


Soil P test

Olsen

Bray-1

Mehlich-3

Iron oxide

 

------------------- Correlation Coefficients -----------------

Entire pH range of 4.8 to 8.2

Bray-1

0.89

 

 

 

Mehlich-3

0.96

0.92

 

 

Iron oxide

0.96

0.88

0.97

 

Water

0.93

0.91

0.95

0.96

 

 

 

 

 

Samples with pH less than 7.2

Bray-1

0.97

 

 

 

Mehlich-3

0.97

0.99

 

 

Iron oxide

0.96

0.96

0.97

 

Water

0.93

0.95

0.95

0.97