Managing Soil Acidity to Improve
Nutrient Use from Liquid Swine Manure

Stanley J. Henning and Russell K. Doorenbos
Agronomy Department
Iowa State University
Ames, Iowa 50011

Manure management requires an understanding of manure's value as a nutrient source instead of being waste disposal. The soil itself should be an environment that promotes decomposition of manure to release nutrients for plant uptake. At the same time those nutrients should be retained in the soil when they are not needed by plants. In this paper I will report on an experiment designed to compare nutrient supplying capability of liquid swine manure with a commercial fertilizer source. Also, I will report on how a very acid surface soil responds to liming and its effects on nutrient availability and retention.

The research is being conducted at Iowa State University's Armstrong Research Farm located 15 miles southwest of Atlantic, Iowa in Pottawattamie county. The field where the experiment takes place has a long history, possibly 40 years, of corn after corn production. The soil is a Marshall silty clay loam (a fine-silty, mixed, mesic Typic Hapludoll). Soil test values for both phosphorus (P) and potassium (K) were very high when the study began. The surface soil possessed a very low pH, which indicates that it had not been recently limed or possibly never limed. Normally, Marshall soil becomes acid at the surface soil but the subsoil remains only slightly acid. The effective calcium carbonate equivalent (ECCE) requirement to raise soil pH to 6.5 was determined to be 7.5 tons acre^-1. Lime treatments were selected that included a check with nothing applied and four rates intended to achieve soil pH's to 7. Manure or urea is applied to supply varying amounts of nitrogen (N). Nitrogen fertilization and lime are expected to help produce good yields on Marshall soils.

Agricultural limestone (Aglime) from the Atlantic quarry was applied to plots in the spring of 1995. It has an ECCE of about 1,000 lbs. ton^-1. Corn was grown in 1995 without manure or urea treatments; N as a solution was applied to the research area. In 1996 and 1997, manure and urea-fertilizer variables were applied.

An analysis of the liquid swine manure is given in Table 1. The manure was applied through a manifold fitted with 2-inch diameter outlets positioned 30-inch apart. Each outlet receives an orifice that restricts flow to selected rates when the tank is pressurized. A different orifice is used to achieve each application rate. During the two years reported here, manure was broadcast-applied on frozen soil. Urea was broadcast-applied prior to planting in 1996 and between rows when the corn was 2 to 3 feet high in 1997.

Soil samples are taken with a 0.75-inch diameter soil probe to a depth of 12 inches. In 1996 samples were taken in accordance with procedures used for the Late-spring Nitrate-nitrogen test and in 1997 they were obtained following grain harvest.

Analyses were conducted in 1996 and 1997 using standard soil testing procedures to determine soil acidity and soil test phosphorous (P) and potassium (K). In 1996, soil and corn stalk samples were analyzed for chloride (Cl) and nitrate-nitrogen (NO₃-N) with appropriate ion selective electrodes and ion specific electrode (pH/ISE) meter. In 1997, fluoride (F), Cl, NO₃-N, and sulfate-sulfur (SO₄-S) were determined simultaneously in a distilled water extract by ion chromatography.

Grain was harvested with a John Deere 4400 combine from the center 4 rows of 8-row plots. The combine is fitted with weighing apparatus and grain moisture meter. Corn grain samples were collected on the combine and brought to Ames for analysis. A Foss Infratec Near-Infrared (NIR) instrument was used to determine moisture, protein, oil, and starch content and density (a measure of kernel hardness) of the grain.

Soil acidity remains acid with the check (no Aglime applied) having a pH of 4.8. Aglime has increased soil pHs to 5.2, 5.3, 5.8 and 6.2 for applications of 1.7, 5, 15 and 45 ton acre^-1, respectively. Soil test P declined significantly where only urea-N was applied in 1996 and 1997. Where manure-N was applied, soil test P declined at the 45 and 90 lbs. N rates, remained constant at the 135 lb. rate and increased at the 180 lbs. N rate. These values indicate that fertilizer-P will have to be applied with urea-N and where manure-N rates are less than 135 lbs. Although soil test K was determined, extreme variations were noted among plots. The cause of these variations is suspected to be from the previous landowner's practice of field feeding hay to cattle at the site during winter.

Summations of corn grain yield data from 1996 and 1997 are given in Table 2. Corn yields were increased by greater amounts of nitrogen from both urea and manure. Neutralizing soil acidity by liming which increased soil pH also resulted in greater grain yields. Grain composition responded differentially to N-sources from year to year. In 1996, corn receiving N from manure contained about one-half percent less protein than corn receiving N from urea. But in 1997, grain protein contents were nearly equal regardless of N-source and increased nearly one percent as N-rates increased from 45 to 180 lbs. acre^-1. Conversely, grain oil content was greater in corn receiving manure-N in 1996 but equal in 1997.

Efficiency of N to produce corn was of interest in this experiment. We defined N use efficiency as the pounds of grain produced from each pound of N supplied. These calculated efficiencies are presented in Table 3. If corn is assumed to contain 1.25 lbs. of N per bushel, a pound of N would produce 45 pounds of grain. The table suggests that this efficiency is achieved with 135 lbs. of N acre^-1. Examining the 135-lb N rate shows that grain production efficiencies increase with greater Aglime rates or reduced soil acidity. The effect of Aglime on corn production efficiency is even greater at lower N rates. This is probably due to more N being released from soil organic matter where soil is limed. At the greatest N rate, more than an ample amount of N is available from both urea and manure and the effect of Aglime on corn production is not evident. In fact, more protein forms in grain at the greater N rates because of its larger presence.

In 1997, we were able to conduct an analysis of anion concentrations in soil samples collected after harvest. We used an ion chromatography instrument to measure fluoride (F-), Cl-, NO₃-, and SO₄⁼ in a distilled water extract. Although phosphate can be determined with this instrument, it is poorly extracted from soil with distilled water. Fluoride contents ranged from 0.3 to 1.9 ppm and were of no significance. Sulfate-S inceased with manure and Aglime additions. Results from Cl- and NO₃- anion analysis are given in Table 4. Chloride content of soil increased with increasing manure application indicating a significant occurrence of Cl- in swine manure. Chloride is an essential plant nutrient that is required during photosynthesis. Little is known about requirements for Cl- by crops in Iowa. In South Dakota and other western states, wheat has responded to fertilization with Cl-. Nitrate extracted from soil showed significant differences between manure and urea and a response to soil acidity. At the lower rates of N from either source, NO₃- was constant at about 5 ppm N. The content of NO₃- doubled with the highest rate of manure application but tripled or quadrupled with urea. Where soil acidity remained low, NO₃- content was greater than where greater pH's were observed.

In the short time this research has been underway, we have shown that P levels in soil can decline, remain constant or increase depending on rate of manure application. The P obtained from a moderate application achieved P-maintenance in this soil. The same amount of manure also provided nearly optimal N too. Another finding of importance was a reduction of residual NO₃- from manure as compared to N from urea fertilizer.

Assistance provided by the Iowa Limestone Producers Association, Des Moines, IA. Aglime was supplied by Schillberg Construction, Greenfield, IA. Field assistance provided by Armstrong Farm staff of the Wallace Foundation for Rural Research and Development, Lewis, IA.

Table 1. Analysis of liquid swine manure used at ISU Armstrong Farm.

Table 2. Grain yield response to lime and nitrogen sources and rates.

Table 3. Corn production efficiency from lime and nitrogen sources and rates.

Table 4. Post-harvest soil Cl and NO₃-N found in 1997.

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