Silage Corn Nutrient Management Guide for Western Oregon

This guide is applicable to fields that receive manure and for those that do not. Recommendations in this guide are adequate to produce high yields where weeds, insects and diseases do not limit crop yield. Soil pH above 5.5 is required for maximum crop yield response to nitrogen and other nutrient inputs.

Silage corn removes approximately the same amounts of N and K, while P removal is lower (Table 1). When applied nutrients are not removed by harvest, they are lost to air or to water or they accumulate in soil. The major pathways for N loss are 1) ammonia loss to the atmosphere during the first days after manure application, and 2) nitrate leached during fall and winter months. About half of the total N applied in manure is in organic form. Some of the organic N is converted to nitrate in the year of application; the remainder is incorporated into soil organic matter. The amount of manure N that is held in soil organic matter for longer than a year is approximately 10–30% of manure N applied. By contrast, P and K are not lost to the atmosphere, and they are relatively insoluble in soil. Therefore, P and K accumulate in topsoil when inputs (manure) exceed nutrients removed in the harvested crop.

On dairy farms, silage corn is often double-cropped with annual ryegrass or a cereal cover crop. This protects soil from erosion and increases annual crop nutrient removal. A fall-seeded annual ryegrass cover crop, when harvested in April with a dry matter yield of 3-ton/acre, removes approximately 115 lb N, 18 lb P and 150 lb K per acre.

Manure also supplies other nutrients that are required for corn, including sulfur (S), calcium (Ca), magnesium (Mg), zinc (Zn) and micronutrients. Usually, when manure has been applied annually, application of these nutrients does not increase silage corn yield.

Soil pH and liming

Corn for silage grows best at a soil pH greater than 5.5. When soil pH is less than 5.5, silage corn yields can be reduced by excess aluminum (Al) or manganese (Mn) in soil solution. Soil pH less than 5.5 can also inhibit root growth to the point that plants appear to be P deficient, exhibiting purple color early in the growing season.

Lime application increases soil pH (reduces soil acidity). To be effective, incorporate lime into the soil by tillage prior to seeding. To determine the need for lime, we recommend testing soil for pH (1:2 soil:water method) and for lime requirement (SMP buffer method or other OSU Extension-approved test method).

The lime requirement test does not measure the actual soil pH in the field; it only serves as an index value for a lime rate recommendation (Table 2). If soil pH in water is greater than 5.5), no lime is required.

Calcium and magnesium

Calcium is an essential plant nutrient. Agricultural lime is calcium carbonate, so it supplies calcium as it increases soil pH. Some sandy soils such as the Newberg series have a soil pH above 5.5 but have less than 5 meq Ca/100 g soil. In such cases, 1-ton lime/acre is recommended to supply Ca, even if the SMP buffer test is above 6.2.

Manure supplies Mg. Soil test values less than 0.8 meq/100g are unusual in manured fields. Apply Mg when soil test indicates that Mg is below 100 ppm (0.8 meq/100 g soil).  When Mg need is identified by a spring soil test, apply magnesium sulfate as a banded starter fertilizer application at 10 to 15 lb Mg/acre at seeding. If magnesium deficiency is identified by a fall soil test and the field will benefit from liming, fall-applied dolomitic lime can be used to supply Mg.

Nitrogen supplied by manure

The most common time to broadcast manure is prior to seeding. Base manure application rate on manure N analyses. Table 3 shows the volume of liquid or slurry manure required to achieve N application rates of 50, 100 and 150 lb/acre. Table 3 is based on manure N analyses. Units shown for manure N analysis in the same row (left side of the table) are equivalent. For example, an N concentration of 2 lb N per thousand gallons equals 240 ppm N. To use Table 3, find a manure analysis value in lb per thousand gallons or ppm in the columns on left. Then choose an application rate (50, 100, or 150 lb/acre) in the row at the top of the table. Find the volume of manure to apply in the body of the table. Calculating Dairy Manure Nutrient Application Rates, EM 8768, provides guidance for calibration of manure application equipment.

The amount of N supplied by manure, as indicated in Table 3, is an approximate value that may vary by + or -30% from the target value. This variability in actual N application is due to manure N analyses variability and the lack of precision in manure application methods. We strongly recommend a midseason soil nitrate test (also referred to as a pre-sidedress nitrate test or PSNT) to assess the balance between soil N supply and crop N requirements.

Most fields with a history of manure application require no starter N fertilizer at planting. Fields that have not received manure for more than three years may require a small amount of N (20–40 lb/acre) at planting time.

Using soil nitrate testing to fine-tune N management

Preplant soil nitrate in May usually underestimates the amount of nitrate available to the crop for the full growing season. Additional nitrate is mineralized after planting, as the soil warms. Therefore, a preplant soil test cannot accurately determine the need for additional N inputs.

Soil nitrate measured at midseason (PSNT, described below) provides the most useful management information for the current year (Tables 4 and 6).

Postharvest soil nitrate tests are not useful for this year’s N management decisions but can provide feedback to use in planning for future years (Table 7).

Midseason soil nitrate testing

The recommended timing for PSNT soil sample collection is at the four- to six-leaf growth stage, before the period of rapid N uptake in corn (Figure 1).

Soil nitrate-N values of 5-15 ppm at planting can increase to over 30 ppm by PSNT sampling time, because of mineralization of soil organic matter to nitrate.

PSNT sampling protocol

Collect PSNT samples when the corn has four to six collared leaves (V4-V6 growth stage). This usually coincides with a plant height of about 12 inches at the center of the whorl.

Sample between rows, away from any fertilizer bands. Avoid irregular areas, such as low spots, field entries or places where manure accumulates.

• Collect at least 15 cores.
• Sample soil to a depth of 12 inches.
• Mix the sample thoroughly in a clean container.
• Fill a soil sample bag with a subsample of the composite soil sample.

The sample should be delivered to the lab immediately. Keep the sample cool until it arrives at the laboratory. Changes in N occur as the soil warms. To avoid shipping delays over the weekend, do not mail samples on Thursday or Friday. Send the sample to a soil testing lab to be analyzed for nitrate-N (NO₃-N). Request results from the laboratory in units of ppm or mg/kg, not lb/acre.

Soils that are high in clay may pose difficulty in sampling to the 12-inch depth at PSNT sampling time. PSNT soil test interpretations (Tables 4 and 6) should be used with caution when soil samples were collected from 0–6 or 0–8 inches, as all PSNT calibration research employed a soil sampling depth of 12 inches. Soil nitrate concentrations generally are highest in the plow layer near the soil surface.

Using PSNT to determine midseason N application rate

Table 4 shows recommended midseason sidedress N inputs (fertilizer or lagoon water) based on PSNT testing. If soil nitrate is below 25 ppm, additional N from fertilizer or lagoon water is likely to increase silage corn yield. Table 3 shows the volume of lagoon water required to supply a target amount of N, based on water or slurry N analysis. If PSNT values are greater than 25 ppm, additional in-season N application as fertilizer or as lagoon water is not recommended.

The following sections provide additional details on interpretation of N monitoring tests.

Pre-sidedress nitrate test, or PSNT

Soil samples are collected for the PSNT when corn has four to six collared leaves (V4–V6 growth stage). Although the main purpose of the PSNT is to determine the need for in-season lagoon water or N fertilizer application (Table 4), it can also serve as an indicator of excess N supply for silage corn. PSNT values above 45 ppm NO₃-N (0–12 inch depth) indicate excess soil N supply relative to corn uptake capacity (Table 6).

Postharvest soil nitrate test, or PHNT

The PHNT measures soil nitrate-N not used by the crop. The PHNT is a look backward in time to evaluate the balance between N supply and crop N uptake. Nitrate accumulates in the soil when the total of applied N plus plant-available N mineralized from soil organic matter exceeds corn N uptake. Although we are inclined to think of this as an annual process being evaluated, it’s important to remember that soil organic matter content and manure application history over the past five to 10 years also influence PHNT results.

Target soil test values listed in Table 7 are based on best professional judgment, as informed by field research studies (Source: Marx; Cogger; See Appendix 1). Target values listed in the table assume near-maximum crop yield, with some late season nitrate release via soil organic matter mineralization in manured soils.

If postharvest soil nitrate-N is above target, consider ways to reduce the N surplus in the future. Evaluate the overall N supply, including the timing and amount of current-season application(s) of N fertilizer, manure or lagoon water. Also consider managing corn and a winter cover crop to maximize crop N removal. See Postharvest Soil Nitrate Testing for Manured Grass and Silage Corn (West of the Cascades), EM 8832, for management options to reduce fall soil nitrate concentrations.

Note that a low PHNT test value (less than 20 ppm NO₃-N) does not indicate that too little N was applied. Continual mineralization of plant-available N from soil organic matter can provide enough N to meet crop need without accumulation of high concentrations of nitrate-N in the soil in the fall. Heavy rainfall or excess irrigation can also leach nitrate below the 12-inch PHNT soil sampling depth.

Cornstalk nitrate test

The cornstalk nitrate-N test is performed at harvest and can be a valuable diagnostic tool to determine the N status of the corn crop. Unlike soil nitrate tests, the stalk nitrate test will identifies whether N supply limited crop yield. If silage yield is low, but the stalk nitrate-N test is adequate, an N deficiency was not the cause of the low yield.

For this test, collect an 8-inch-long section of stalk from 15 plants. Make the lower cut in the stalk about 6 inches above the soil surface. Submit samples to the laboratory immediately. See End-of-Season Corn Stalk Nitrate-Nitrogen Test for Postharvest Evaluation, WSU FS336E, for sample preparation details. A 2017 study (Source: Ketterings) reported that stalks could be stored at room temperature or in a refrigerator for up to eight days after collection without affecting nitrate test values.

Interpretive categories in Table 8 reflect Oregon-based research (Source: Marx). Research findings from Iowa showed a similar target value (3,500 ppm) for cornstalk nitrate in manured fields (Source: Kyverga). Iowa researchers also found a strong correlation between soil nitrate at the six-leaf stage (PSNT) and the cornstalk nitrate test values taken from the same fields at harvest (Source: Balkcom). Oregon-based research confirmed this finding (Appendix 2).

Phosphorus

Manure is an excellent source of phosphorus for silage corn. Phosphorus from manure or lagoon water is 100% plant-available and equivalent to P from mineral fertilizer. When manure is applied at a rate to satisfy the silage corn N requirement, the amount of manure P applied often exceeds the amount of P removed by the crop. On dairy farms, soil test P is an indicator of the long-term balance between P inputs from manure and fertilizer versus crop removal of P via silage harvest.

Table 9 shows that when soil test P exceeds 50 ppm, applied P is not expected to benefit corn yields. Elevated soil P levels are slow to change in response to management actions; it cannot be easily or quickly reduced. Soil test P in western Oregon soils typically declines at a rate of approximately 10–20 ppm per decade in response to forage crop P removal, provided no additional P input is provided. Annual soil testing is not useful to document changes in soil test P in response to management. A big change in soil test P over a few years is likely caused by inconsistency in soil sampling methods or in laboratory analytical techniques.

Corn roots must be colonized by mycorrhizal fungi if corn is to grow normally without turning purple. These mycorrhizal fungi extract nutrients from soil and deliver it to the crop, in exchange for carbohydrates – the products of photosynthesis. On the commercial scale, amending soils with mycorrhizal fungi has not been successful. Rather, crop rotation and reduced tillage practices are the best ways to encourage mycorrhizal fungi. Mycorrhizal fungi will be present in soil if the previous crop was a host. Annual ryegrass is commonly grown in rotation with silage corn on dairies. It is a host of mycorrhizal fungi. Planting corn after fallow provides fewer mycorrhizal fungi for corn root colonization. In research trials conducted in lower mainland British Columbia, minimum tillage was superior to plowing for reducing the incidence of purple color in young corn plants (Source: Bittman). Mycorrhizal fungi grow much like roots and must remain intact to function. Tillage disrupts the fungal network in the soil.

Recommendation: If silage corn turns purple between the three- and six-leaf stages, even when soil test P exceeds 50 ppm, adopt the following management practices in subsequent years:

• Plant corn after a crop that hosts mycorrhizal fungi. Most crops support mycorrhizal fungi. However, crops in the beet and brassica families do not.
• Reduce tillage. Most important, do not plow.
• At planting, apply P fertilizer in a band 2 inches from the seed at a rate of 30 lb P₂O₅/acre.

Potassium

Manured soils usually supply adequate or excess potassium (K). When manure is used as an N source for silage corn, more K is added than the crop can remove. Soil test K is often highest in fields near the barn and in fields that receive the most manure. Potassium has limited mobility in soil and remains near the site of placement. Soil test K is usually highest in the surface foot of soil or to the tillage depth.

If a soil test indicates that K is needed (Table 10), it can be supplied by banding or broadcasting mineral fertilizer (K chloride or K sulfate) or by manure or lagoon water application. Potassium from manure or lagoon water is 100% plant-available and is considered equivalent to K from mineral fertilizer.

Micronutrients

Because western Oregon soils are acidic and contain relatively high amounts of organic matter, metal micronutrients such as Mn, Zn, Cu and Fe are usually adequate for corn production. These micronutrients are more soluble in acidic soils and are thus more available to plants. Manure application also supplies these micronutrients. Micronutrient deficiencies are rare. Corn research trials conducted in western Oregon have not shown economic benefit from fertilization with micronutrients other than Zn.

Commercial soil testing laboratories working under OSU guidelines use a chelate extractant (DTPA) when analyzing soil for metal micronutrients (Zn, Mn, Fe and Cu).

Zinc

It is unusual to observe zinc deficiency on manured soils in western Oregon. Manure is considered an excellent source of plant-available zinc because the organic matter from manure can serve to chelate Zn and maintain it in plant-available form in soil solution. When corn is severely deficient in zinc, the upper leaves are yellow between the veins. Zinc deficiency in corn is likely when the uppermost fully expanded leaves contain less than 25 ppm Zn. Corn plants with a poorly developed root system may exhibit zinc deficiency symptoms, even when there is plenty of plant-available Zn present in soil.

Dairy farms sometimes employ foot baths containing Cu and Zn sulfate to maintain hoof health. Footbath effluent is often mixed with slurry manure or lagoon water, and then applied to fields. 

Although Cu and Zn are plant-essential nutrients, in excess they can injure plants and produce feed with elevated Cu and Zn concentrations. Elevated soil test Cu and Zn have been measured in western Oregon fields where footbath Cu and Zn have been routinely applied (Source: Downing). More than 75% of soil samples collected from western Oregon dairy farms in 2010 had soil test Cu concentrations (DTPA method) considered high (greater than 2 ppm), and 38% of the soil samples had extremely high Cu (greater than 5 ppm). This interpretation is of DTPA Cu and Zn soil tests is based on Soil Test Interpretation Guide, EC 1478. To avoid buildup of Cu and Zn to excessive levels in soil, we recommend regular monitoring of soil test Cu and Zn in fields receiving footbath effluent.

A summary of published research (Source: Reuter) provides general guidance for interpretation of corn leaf Zn and Cu concentrations:

• Typical Zn concentrations in corn leaf tissue are 25–50 ppm.
• Leaf Zn concentration above 100–300 ppm can reduce yields.
• Leaf Zn concentrations greater than 500 ppm can severely reduce yields.
• Leaf Cu concentrations above 20 ppm can reduce yields.
• Leaf Cu above 50 ppm can severely reduce yields.

Appendix 1: Postharvest soil nitrate distribution in soil profiles

The recommended sampling depth for the PHNT is 12 inches. This appendix provides supporting data for the choice of a 12-inch sampling depth. This data was collected from N monitoring trials on Willamette Valley dairies (Source: Marx). At harvest, an auger mounted on a tractor was used to collect soil samples in 12-inch depth increments.

At all field locations, the 0–12-inch depth contained the highest concentration of postharvest soil nitrate in the top 36 inches of the soil (Figure 8). On average, nitrate in the top 12 inches represented 78% of the total nitrate present in the top 36 inches of soil. Postharvest nitrate-N was at or below the target PHNT concentration (20 ppm in 0–12 inches; Table 7) in eight of 26 fields.

Appendix 2: Relationship between midseason and postharvest indicators of N sufficiency

A series of N monitoring trials was conducted on dairy farms producing silage corn in the Willamette Valley in 1993–94 (Source: Marx). Twenty-six replicated field experiments were conducted on 17 dairy farms. Most trials (24 out of 26) were conducted in fields receiving manure application. Dairy farmers applied manure and lagoon water according to their usual crop production practices. Data was collected on crop yield response to N fertilizer (urea) applied at sidedress time (V6 growth stage) to small plots within each field. Soil nitrate was measured near the time of planting, just prior to sidedress N fertilizer application, and at harvest. These trials provided an opportunity to evaluate the relationships among three N monitoring tests: PSNT, PHNT and stalk nitrate concentration at harvest. The guidance provided in the Monitoring soil and crop N section of this publication is based on these field trials.

Silage corn yields demonstrated that in most fields, N supply from manure plus other nonfertilizer N sources (for example, N mineralization from organic matter) was adequate or more than adequate to maximize crop yield. Silage corn yield was increased by sidedress N fertilizer application at four of 26 field sites. At the other 22 sites, there was no statistical difference between silage yield with or without sidedress N fertilizer. When PSNT soil nitrate concentrations at the 0–12-inch depth were greater than 25 ppm, sidedress N fertilizer application did not increase corn yield. Interpretive ranges for the PSNT (Tables 4 and 6) are based on data from this study.

A linear relationship was demonstrated between soil PSNT concentrations (V6 growth stage) and stalk nitrate at harvest for field plots that did not receive N fertilizer application at midseason (after the PSNT soil sample was collected; Figure 9). A soil nitrate-N concentration of 25–30 ppm at the recommended level at PSNT time resulted in a stalk nitrate-N concentration of about 5,000 ppm at harvest (Figure 9). Stalk nitrate-N concentrations of 3,500 to 5,000 ppm are considered adequate (Table 8). When soil nitrate-N at PSNT was considered excessive (greater than 45 ppm; Table 6), stalk nitrate-N was also at a level considered excessive (above 5,000 ppm; Table 8).

Soil nitrate concentrations in fields receiving irrigation from fresh water only demonstrated a strong correlation between midseason (PSNT) and at harvest (PHNT, Figure 10) values. Postharvest soil nitrate concentrations represented two-thirds (64%) of PSNT values (slope of line in Figure 10). A PSNT test value at the recommended concentration (less than 30 ppm; Table 6) was correlated with postharvest soil nitrate-N concentrations of less than 20 ppm (Figure 10).

Fields that received dairy lagoon water applications had some of the highest postharvest soil nitrate values (Figure 10). For these fields, the PSNT was not a reliable indicator of postharvest soil nitrate.

Extension and outreach publications

• Advanced Silage Corn Management: A Production Guide for Coastal British Columbia and the Pacific Northwest. 2004. S. Bittman and C.G. Kowalenko (eds). Pacific Field Corn Association, Agassiz, B.C.
• Applying Lime to Raise Soil pH for Crop Production (Western Oregon), EM 9057.
• Calculating Dairy Manure Nutrient Application Rates, 2015, EM 8768.
• Corn Growth and Development, Iowa State University, 2011, PMR 1009.
• Date, Rate, & Place: The Field Book for Dairy Manure Applicators, 2017, PNW 506.
• End-of-Season Cornstalk Nitrate-Nitrogen Test for Postharvest Evaluation, 2019, WSU Extension Factsheet FS336E.
• Estimating Plant-Available Nitrogen From Manure, 2020, EM 8954. 
• Interpreting Compost Analyses, 2018, EM 9217.
• Manure Application Rates for Forage Production: Western Oregon, 2020, EM8585.
• Monitoring Soil Nutrients Using a Management Unit Approach, 2003, PNW 570.
• Natural Resource Conservation Service Hydrologic Soil Groups.
• Postharvest Soil Nitrate Testing for Manured Grass and Silage Corn (West of the Cascades), 2021, EM 8832.
• Sampling Dairy Manure and Compost for Nutrient Analysis, 2015, PNW 673.
• Soil Acidity in Oregon: Understanding and Using Concepts for Crop Production, EM 9061.
• Soil Test Interpretation Guide, EC 1478.

Research publications

• Balkcom, K. S., A.M. Blackmer, D.J. Hansen,, T.F. Morris, and A.P. Mallarino. 2003. Testing soils and cornstalks to evaluate nitrogen management on the watershed scale. Journal of Environmental Quality, 32(3), 1015-1024.
• Bittman, S., C.G. Kowalenko, D.E. Hunt, T.A. Forge and X. Wu. 2006. Starter phosphorus and broadcast nutrients on corn with contrasting colonization by mycorrhizae. Agronomy Journal, 98:394-401.
• Cogger, C.G., A.I. Bary, T.N. Cramer and D.C. Grusenmeyer. 2000. Soil and plant test calibration to improve nutrient management. In: C.G. Cogger (ed) In: Addendum to final report, Abbotsford-Sumas Aquifer Nitrate Management Project G9500254. August, 2000. OSU Scholars Archive.
• Downing, T. W., K. Stiglbauer, M.J. Gamroth, and J. Hart. 2010. Case study: Use of copper sulfate and zinc sulfate in footbaths on Oregon dairies. The Professional Animal Scientist, 26(3), 332-334.
• Geisseler, D., P.A. Lazicki, G.S. Pettygrove, B. Ludwig, P.A. Bachand and W.R. Horwath. 2012. Nitrogen dynamics in irrigated forage systems fertilized with liquid dairy manure. Agronomy Journal, 104(4), 897-907.
• Hart, J. M., E.S. Marx, N.W. Christensen and J.A. Moore. 1997. Nutrient management strategies. Journal of Dairy Science, 80:2659-2666.
• Jackson, T., and J. Hay. 1965. Factors affecting response of sweet corn to Zn in the Willamette Valley. Proc. 16th Ann. Fert. Conf. Pacific N.W. Salt Lake City, UT. July 13–15.
• Ketterings, Q. M., S. Gami, G. Godwin, E. Hong, K. Orloski, R. Breslauer, and R.R. Mathur,. 2017. Improving sample collection, sample processing, and laboratory analyses for corn stalk nitrate test. Agronomy Journal, 109(5), 2312-2322.
• Kyveryga, P.M., and T.M. Blackmer. 2013. 4R management: Differentiating nitrogen management categories on corn in Iowa. Better Crops with Plant Food, 97: 4-6.
• Marx, E.S. 1995. Evaluation of soil and plant analyses as components of a nitrogen monitoring program for silage corn. Master’s thesis, Oregon State University, Corvallis.
• Moberg, D.P., R.L. Johnson and D.M. Sullivan. 2013. Comparison of disturbed and undisturbed soil core methods to estimate nitrogen-mineralization rates in manured agricultural soils. Communications in Soil Science and Plant Analysis 44:1722-1732.
• Reuter, D., and J.B. Robinson, eds. 1997. Plant Analysis: An Interpretation Manual. CSIRO Publishing. Collingwood, Australia.
• Sullivan, D.M., A.I. Bary, C.G. Cogger, and E.A. Myhre. 1999. Field microplot estimates of soil N mineralization for manured and non-manured soils. Western Nutrient Management Conference. Salt Lake City, UT. March 4–5, 1999.
• Zebarth, B. J., J.W. Paul, O. Schmidt, and R. McDougall. 1996. Influence of the time and rate of liquid-manure application on yield and nitrogen utilization of silage corn in south coastal British Columbia. Canadian Journal of Soil Science 76: 153-164.
• Zebarth, B. J., J.W. Paul, M. Younie and S. Bittman, S. 2001. Fertilizer nitrogen recommendations for silage corn in high-fertility environment based on pre-sidedress soil nitrate test. Communications in Soil Science and Plant Analysis, 32:2721-2739.
• Zhang, H., S. Bittman, D.E. Hunt, and F. Bounaix. 2020. Corn response to long-term manure and fertilizer applications on a preceding perennial forage crop. European Journal of Agronomy, 115, 125990.