By Ron Alexander
Biosolids can be both a nutrient source and a long-term soil-building amendment. But incorporating them into a nutrient management plan requires more than choosing a standard application rate. The planner must connect the product analysis to current soil conditions, crop needs, field characteristics, phosphorus restrictions, application equipment and the farmer’s rotation.
Done well, this process brings soil science and practical farm planning together. It accounts for the ways biosolids can support soil function and resilience while translating their nutrient content into a workable, environmentally responsible field plan.
Start with the soil, not the product
Every nutrient recommendation begins with an accurate picture of the field. Soil texture, organic matter, pH, cation exchange capacity and existing nutrient levels influence how a soil holds water and nutrients, how quickly nutrients cycle, and how vulnerable the field may be to runoff or leaching.
Organic matter is central to those functions. Agricultural soils gradually lose organic matter through cropping, erosion and biological decomposition. Adding stabilized organic material can improve aggregation, reduce compaction, increase water-holding capacity, support microbial activity and add exchange sites that retain positively charged nutrients.
Those improvements do not happen overnight. In field agriculture, soil-health gains generally build through appropriate applications over time. That makes biosolids more than a one-season fertilizer purchase: when the site and rate are appropriate, they can contribute to the long-term productive capacity and resilience of the soil.
Understand what biosolids contribute
Biosolids-based products supply nitrogen, phosphorus, potassium and other plant nutrients, but they also add organic carbon and stable organic matter. The nutrient contribution must be credited in the plan, while the soil-amendment value helps explain why a farmer may choose biosolids instead of relying exclusively on soluble commercial fertilizer.
The two roles should not be confused. Soil-health benefits do not justify applying nutrients beyond what the field, crop and governing plan can accommodate. The product must be evaluated as an organic nutrient source first, using a current and representative analysis.
A planner needs both sides of the equation:
- A representative soil analysis for each management unit or field
- A current analysis of the biosolids or other organic material to be applied
For the field, the planner should review phosphorus, potassium, magnesium, calcium, pH, organic matter, cation exchange capacity and the laboratory’s fertilizer recommendations. For the material, the relevant inputs include plant-available nitrogen, organic nitrogen, phosphorus and potassium, along with the treatment or stabilization method used to determine appropriate nitrogen-availability assumptions.
Careful sampling matters. An analysis is only as useful as the sample behind it. Soil samples should represent the management area, and samples of an organic material should account for the variability that can occur within a stockpile or production stream.
Translate the analysis into the nutrient management plan
A biosolids product should be entered into the applicable nutrient-management planning system as an organic nutrient source. This allows the planner to compare the nutrients supplied at a proposed rate with the crop recommendation for a particular field.
That step is not simply data entry. The planner must check that the analysis was entered in the correct units, that the nitrogen availability or mineralization assumption matches the material’s treatment method, and that the resulting nutrient totals are plausible. Unusual outputs should be checked outside the program with an independent calculation. Planning software is a tool, not a substitute for professional judgment or basic math.
For nitrogen, the immediately available fraction of an organic material is different from its total nitrogen content. The assumptions used for anaerobically digested, aerobically digested or lime-stabilized biosolids may differ. The planner should therefore use the approved value for the actual product and treatment process rather than treating all biosolids as interchangeable.
Application rate is a field-specific decision
A typical or familiar rate can be a useful starting point, but it is not automatically the correct rate for a given field. The final recommendation must reflect:
- The nutrient requirements of the planned crop
- Nutrients already available in the soil
- The biosolids analysis and approved availability factors
- Soil phosphorus status and applicable phosphorus-management requirements
- Crop rotation and timing of future nutrient demand
- Soil texture, slope, tillage and proximity to water
- The method and physical limits of the application equipment
Knowing the farmer and the cropping sequence is essential. Biosolids do not necessarily belong on the same field every year. A better strategy may be to place an application ahead of a crop that can make effective use of the supplied nutrients, then rotate applications among suitable lower-phosphorus fields. This can improve nutrient-use efficiency while reducing the risk of repeatedly building soil phosphorus in one location.
The rate written into the plan must also be achievable in the field. At lower rates, spreader design, material moisture, flow characteristics, ground speed and terrain can become limiting. Equipment should be calibrated under realistic conditions. If the equipment cannot deliver the planned rate uniformly, the planner and farmer need to adjust the application approach or revise the recommendation before full-scale spreading.
Phosphorus is often the deciding factor
Phosphorus management is one of the most difficult parts of planning for biosolids and other organic nutrient sources. Planning programs commonly calculate applied phosphorus from the material’s total phosphorus analysis. Yet total phosphorus is not the same as immediately plant-available or mobile phosphorus. Digestion and stabilization can leave a substantial share of phosphorus in less-soluble forms that become available gradually.
Water-extractable phosphorus testing may provide useful information about phosphorus mobility. However, planners must use the methods and values accepted by the applicable nutrient-management program. Emerging science can inform professional judgment and future policy, but it does not permit an unapproved availability value to be substituted in a regulated plan.
In Maryland, fields at or above the applicable soil-phosphorus threshold must be evaluated under the state’s phosphorus-management process. The analysis considers not only the amount applied, but also the field’s soil type, slope, tillage, crop, application method and potential pathways to surface water or groundwater.
That review can lead to a straightforward conclusion: biosolids may be an excellent fit for one field and the wrong fit for another. A high-phosphorus sandy field near shallow groundwater or surface water may call for a different nutrient source and more precise nutrient placement. Lower-phosphorus fields with stronger nutrient-holding capacity may offer a better opportunity to use biosolids productively and responsibly.
Choose fields for agronomic and environmental fit
It is tempting to direct an organic amendment to the poorest field because that field “needs help.” This instinct can be misleading. A coarse, high-phosphorus field next to a sensitive water resource may present more risk than benefit. In some cases, the best biosolids fields are the farm’s better soils—fields with balanced fertility, adequate cation exchange capacity and the ability to hold and release water and nutrients efficiently.
Good field selection therefore considers the whole site:
- Soil test results: Is there room in the nutrient budget, especially for phosphorus?
- Crop demand: Will the crop and rotation use the nutrients on an appropriate timeline?
- Soil properties: Can the soil retain water and nutrients without creating unacceptable loss risk?
- Landscape position: Do slope, drainage, setbacks or proximity to water make the field unsuitable?
- Application practicality: Can the selected equipment apply the material uniformly at the planned rate and time?
If those pieces do not align, the planner should be candid with the farmer and recommend a different field, rate, timing or nutrient source.
A practical planning checklist
Before including biosolids in a nutrient management plan:
- Obtain representative, current soil-test results for each field.
- Obtain a current analysis of the specific biosolids product.
- Confirm the product’s treatment method and the approved nutrient-availability assumptions.
- Enter the product into the planning program using the correct units and values.
- Compare proposed applications with crop nitrogen, phosphorus and potassium needs.
- Check calculated outputs with independent math when a result appears unusual.
- Evaluate soil phosphorus status and complete any required phosphorus-management assessment.
- Match applications to the farmer’s crop rotation rather than defaulting to annual use on the same field.
- Select fields based on nutrient capacity, soil properties and environmental risk—not simply on which field appears most depleted.
- Calibrate application equipment and confirm that the planned rate can be delivered uniformly.
- Document the recommendation clearly and revisit the plan when product analyses, field conditions or cropping decisions change.
The planner connects soil science to farm practice
Biosolids can improve soil function by adding organic matter, supporting biological activity, improving structure and helping soils retain water and nutrients. Those benefits only become meaningful when the product is placed in the right field, at a defensible rate, for the right crop, using equipment that can carry out the recommendation.
The central lesson is not that biosolids belong in every nutrient management plan. It is that they deserve a field-by-field evaluation that accounts for their full value and their nutrient-management constraints. When the analysis, site, crop rotation and application method align, biosolids can support productive agriculture while protecting water quality and building healthier soils over time.
About the Authors
This article is based on material developed by Ron Alexander and Jeremy Bertier.
Ron Alexander is president of R. Alexander Associates, a company specializing in market development and research for organic recycled products. A horticulturist with more than 35 years of experience, he has worked extensively with biosolids-based products and other recycled organics in North America and internationally. He is the author of The Field Guide to Compost Use and hundreds of papers on recycled-organic production, use and market development.
Jeremy Bertier is a Maryland farmer, agronomist and commercial nutrient management planner with experience in biosolids and residuals management. Raised on his family’s 165-acre farm in Union Bridge, Maryland, he has worked in production agriculture, soil sampling, plan writing and land application, including roles with Synagro, Old Line Environmental and The LAND Group. He earned a degree in agricultural business management from the University of Maryland’s Institute of Applied Agriculture.
Nutrient-management requirements and approved calculation methods vary by jurisdiction and may change. Planners should follow current state guidance and product-specific analyses when preparing a regulated plan.
