Introduction

High fertilizer prices are prompting farmers to take a closer look at manure as a fertilizer option for agronomic crops. This factsheet explains how Connecticut and Northeast U.S. farmers and agronomic crop growers can use manure more effectively as a fertilizer source, by estimating nutrient credits, to reduce fertilizer costs and protect crop performance and water quality.

This factsheet uses regional planning values from UVM Extension (2020) for manure nutrient content and first-year N availability. It keeps UConn-specific guidance for manured corn, starter fertilizer, the pre-sidedress soil nitrate test (PSNT), and maximum manure rates.

Nutrient Content of Manure

Manure can supply much or all of the phosphorus (P₂O₅)[1] and potassium (K₂O) that an agronomic crop needs, and it can supply a substantial share of the nitrogen (N). The main limitation is that manure N is less predictable than fertilizer N, especially when manure is surface applied, applied well before the crop needs it, or when weather effects after spreading are not considered.

The most practical approach is to base manure use on a current soil test, a representative manure analysis, and a known application rate. This factsheet uses regional planning values from UVM Extension (2020) for manure nutrient content and first-year N availability. It keeps UConn-specific guidance for manured corn, starter fertilizer, the pre-sidedress soil nitrate test (PSNT), and maximum manure rates.

Start with Three Numbers Before you Buy Fertilizer

Three pieces of information are needed before manure as a fertilizer option can be introduced properly:

1. A current soil test that shows whether the field actually needs lime, P, and K, and that gives a recommended N rate for the target crop. A routine soil test is recommended every one to three years and more often in intensively managed fields, low-testing fields, or after flooding;
2. A manure analysis that shows what the manure can actually supply. UConn Extension and NRCS both recommend testing manure for at least total N, ammonium-N (NH₄-N), total P, and total K, and sampling as close to the time of application as possible because nutrient content varies widely with species, ration, bedding, storage, moisture, and handling;
3. A known application rate. The spreader or tank should be calibrated so the field rate is measured, not guessed. Practical methods for Connecticut farms that do not have truck scales are described in Section 6.

Because manure nutrient content is so variable, a recent manure analysis is always the best basis for nutrient credits. When a recent analysis is not available, regional book values (Table 1) can be used for planning. These are appropriate as planning estimates for Connecticut and the broader Northeast U.S., but they should not replace a current manure analysis for important nutrient-management decisions.

Table 1. Regional planning values for manure nutrient content, as-applied basis.

Note. Dairy values are from UVM Extension (2020), based on Vermont samples analyzed at the University of Maine, 2012–2016. Non-dairy values are adapted from UVM (2020), which in turn draws on the Penn State Agronomy Guide and University of Nebraska-Lincoln NebGuide G1335. Recent operational averages from the DairyOne Forage Lab database (Northeast samples, 2004–2022) fall in a similar range. Variability is large; a current manure analysis is always preferable.

Which Fertilizer Nutrients Does Manure Supply Most Reliably?

Manure does not behave like a balanced fertilizer, even when the total nutrient analysis looks impressive. The three major nutrients behave differently in the field.

Potassium is the easiest to ascertain. Manure K is generally in ionic form and behaves like fertilizer K, so it can be fully credited when planning fertilizer purchases. Phosphorus is usually credited with confidence as well. Manure P and fertilizer P are very similar in building soil P and supplying crop needs, so manure P is treated as equivalent to fertilizer P. One practical note for Connecticut: starter P can speed early corn growth when soils are cold, but in most years, once soil warms above about 65°F, soil bacteria make manure P available to the crop, and many CT farms will not see a measurable yield gain from starter P beyond what manure already supplies.

Nitrogen is more complicated. Manure N is split between an inorganic fraction (mostly ammonium-N), which acts much like fertilizer N if it is protected from loss, and an organic fraction, which becomes available more slowly as soil microbes mineralize it (Bechini & Marino, 2009). This is why many farms get the biggest immediate economic return from manure by crediting P and K first and being more conservative with N.

Manure history also matters. Fields with a long history of annual manure applications stockpile a substantial amount of organic N in the soil, that continues to mineralize for three to five years after applications stop. In one UConn Extension experiment on continuous corn in northwest Connecticut, research plots on a long-term manured field showed no yield response to added N for five years after manure applications were stopped. Long-manured fields often need less fertilizer N than growers expect, even when current manure applications are modest.

Table 2. Practical expectations for fertilizer replacement from manure.

Timing and Placement Can Change Manure N Value Dramatically

For nitrogen, the manure application method often matters as much as the manure analysis. Ammonium-N is essentially equivalent to fertilizer N, if managed properly, but it can volatilize quickly as ammonia gas (it evaporates into the air) when manure is left on the soil surface.

Incorporation by tillage or substantial rainfall (about one-haf inch) moves ammonium-N into contact with soil organic matter and clay, which protects it. Liquid manures that infiltrate the soil quickly also lose less ammonium-N than thicker manures left on the surface.

Research on dairy slurry has shown that application method strongly affects these losses, and regional studies have shown that injection or prompt incorporation conserves more manure N for crop use than a broadcast application left on the surface (Pfluke et al., 2011; Milliron et al., 2019). Manure should therefore be applied as close to crop uptake as practical, and incorporated or injected as soon as feasible. Warm weather, wind, and delay after spreading all increase the chance that ammonium-N will be lost.

Fall-applied manure requires extra caution in the Northeast U.S. It may still fit some forage or cover crop systems, but for annual row crops there is more time for N losses before spring uptake.

Where manure must be applied well ahead of planting, do not assume all of the manure N will still be available the next spring (Milliron et al., 2019). UVM Extension reduces fall ammonium-N availability to roughly 40% of the spring values shown in Table 3 to account for overwinter losses.

Table 3. Spring- or summer-applied manure: ammonium-N availability (%) as a function of manure dry matter and time to incorporation.

Source: UVM Extension (2020), Table 17.

Table 4. Organic-N availability (%) from manure applied in the current and past years.

Source: UVM Extension (2020), Table 19.

Do Not let N-based Manure Rates Quietly Oversupply P and K

A common mistake is to use manure to satisfy the full N requirement of every field, every year. That can work for a while on fields testing low to moderate in soil P, but over time, repeated N-based manure applications often apply more P and K than the crop removes. The result is that fertilizer purchases drop in the short term, but soil test P climbs, manure flexibility declines, and water-quality risk goes up. Long-term research in New York has shown that shifting from N-based to P-based manure management can reduce P buildup, although it may require supplemental sidedress N where manure alone no longer meets crop N demand (Sadeghpour et al., 2017).

In practical terms, fields that already test high in P, are close to surface water, or carry greater runoff risk should not keep receiving manure simply to meet full crop N needs. Those fields are better managed with a lower manure rate based on P removal or overall field risk, and the remaining N can be supplied later through sidedress fertilizer or another lower-P N source. Fields with lower soil test P are usually the best places to capture the full fertilizer value of manure.

UConn also publishes maximum manure application rates per acre. These rates are useful guardrails even when the calculated N credit suggests a higher rate would ‘fit’ the crop's nitrogen need.

Table 5. UConn maximum manure application rates per acre.

Source: UConn Soil Nutrient Analysis Laboratory, Suggested Fertilizer Practices for Agronomic Crops (Griffin et al., 2023).

Worked Example: 8,000 Gallons of Dairy Slurry Per Acre

This example uses dairy slurry at five to ten percent dry matter, to show the logic. Substitute actual manure analysis values whenever possible. P and K credits come directly from the manure analysis or book values. N requires a separate adjustment for ammonium retention and organic-N mineralization. UVM Extension (2020) Tables 17–19 provide those adjustment factors; the simplified versions are reproduced as Tables 3 and 4 in this factsheet.

Table 6. Example calculation for 8,000 gal/acre of dairy slurry (5–10% DM).

Calibration and Recordkeeping

Manure cannot replace fertilizer cost-effectively, unless the application rate is known. Spreader calibration is one of the most important parts of a manure nutrient-management plan. NRCS describes two practical methods: the load-area method and the weight-area (tarp) method. The load-area method is usually the most accurate and works for both liquid and solid manure. The weight-area method is less accurate but practical when a scale is not available and is well-suited to solid or semi-solid manure.

Load-area formulas (when load weight or tank volume and area covered are known):

Tarp (weight-area) method for solid manure on farms without truck scales:

Connecticut farms generally do not have truck scales, and most truck scales are not wide enough to weigh modern spreaders. A practical alternative is to place a tarp of known length, width, and weight on the ground, spread manure across the tarp as normal, then pick the tarp up and weigh it with a hanging scale of sufficient capacity. Subtract the tarp weight from the total to get the manure weight. Use the following calculation:

This calculation requires somewhere to hang the scale with enough height below it for the tarp. A bucket can be hung from the scale and the tarp placed in the bucket to reduce the height needed. A 100-lb hanging scale with a 5 × 8 ft tarp covers application rates up to roughly 54.5 tons per acre, which is well above any realistic field rate.

For liquid manure, the simplest approach is usually to use known tank volume and the area covered rather than a tarp. If a tarp is used, weigh several five gallon buckets of the same manure first to get an average weight per gallon, then convert the tarp weight to gallons for comparison with the target rate in gallons per acre.

Even when farmers do not calculate the fertilizer value precisely every time, it is strongly recommended that they keep records of field, date, manure source, estimated rate, application method, and whether manure was incorporated. Over time, those records help explain why some fields consistently test high or low on the PSNT, why some fields show rising soil test P, and where manure is giving the best economic return.

Practical Step-by-Step Approach

• Soil test fields regularly (every one to three years) and do not assume all acres have the same nutrient needs;
• Analyze manure whenever possible, especially when animal diet, bedding, storage, or dilution changes;
• Use the regional book values in Table 1 only as planning estimates when a recent manure analysis is not available;
• Calibrate the spreader or tanker so the field rate is known, not guessed;
• Credit manure P and K directly. Credit manure N more cautiously using first-year availability (Tables 3 and 4) rather than total N alone;
• Apply manure as close to crop uptake as practical and incorporate promptly;
• Move repeated N-based manure applications away from high-P or runoff-sensitive fields;
• For corn on manured fields, do not apply preplant N and use sidedress N or the PSNT to fine-tune the remaining need;
• Keep field-by-field records so future manure allocation decisions are based on results, not habit.

Manure can replace a large share of purchased fertilizer. For many Connecticut and Northeast U.S. farms, the smartest strategy is not to ask whether manure can replace fertilizer, but to ask how much of each nutrient it can supply on each field without creating new N losses or long-term P problems. That is where the real savings are.

[1] Throughout this factsheet, the elemental forms P and K are used when discussing soil test results, plant uptake, and general concepts, while the oxide forms P₂O₅ and K₂O are used for manure analysis values and fertilizer recommendation rates, since that is how fertilizer is sold and labeled. To convert between forms, multiply P₂O₅ by 0.44 to get P, and K₂O by 0.83 to get K.

Resources

Bechini, L., & Marino, P. (2009). Short-term nitrogen fertilizing value of liquid dairy manures is mainly due to ammonium. Soil Science Society of America Journal, 73(6), 2159–2169. https://doi.org/10.2136/sssaj2008.0217

Beegle, D. B., & Spargo, J. (2016). Soil fertility management. The Agronomy Guide. The Pennsylvania State University.

Griffin, G., Pettinelli, D., & Morris, T. (2023). Suggested fertilizer practices for agronomic crops. UConn Soil Nutrient Analysis Laboratory. https://soiltesting.cahnr.uconn.edu/soil-test-results-for-agronomic-crops/

Milliron, R. A., Karsten, H. D., & Beegle, D. B. (2019). Influence of dairy slurry manure application method, fall application-timing, and winter rye management on nitrogen conservation. Agronomy Journal, 111(3), 995–1009. https://doi.org/10.2134/agronj2017.12.0743

Morris, T. (2003). Soil testing to improve nitrogen management for corn. UConn Cooperative Extension System. https://soiltesting-cahnr.media.uconn.edu/wp-content/uploads/sites/3514/2023/05/Soil-Testing-to-Improve-Nitrogen-Management-for-Corn-1.pdf

Pfluke, P. D., Jokela, W. E., & Bosworth, S. C. (2011). Ammonia volatilization from surface-banded and broadcast application of liquid dairy manure on grass forage. Journal of Environmental Quality, 40(2), 374–382. https://doi.org/10.2134/jeq2010.0102

Sadeghpour, A., Ketterings, Q. M., Godwin, G. S., Czymmek, K. J., & Vermeylen, F. (2017). Agro-environmental consequences of shifting from nitrogen- to phosphorus-based manure management of corn. Soil Science Society of America Journal, 81(5), 1127–1138. https://doi.org/10.2136/sssaj2016.03.0417

Shapiro, C. A., Johnson, L., Schmidt, A. M., & Koelsch, R. (2015). Determining crop available nutrients from manure (NebGuide G1335). University of Nebraska–Lincoln Extension.

University of Vermont Extension. (2020). Nutrient recommendations for field crops in Vermont (BR 1390.3). https://www.uvm.edu/d10-files/documents/2024-05/NutrientRec_BR1390.3_Sept2020.pdf

U.S. Department of Agriculture, Natural Resources Conservation Service. (n.d.). Sampling manure for nutrient management. https://www.nrcs.usda.gov/sites/default/files/2022-10/Sampling_Manure_for_Nutrient_Management_SD-FS-36.pdf

U.S. Department of Agriculture, Natural Resources Conservation Service. (n.d.). Calibrating manure spreader application rates. https://www.nrcs.usda.gov/sites/default/files/2022-10/Calibrating_Manure_Spreader_Application_Rates_SD-FS-43.pdf