The list below names each data source and what part of the calculator relies on it. A more detailed citation page (with method numbers and page references) is linked at the bottom.
Soil data
- USDA NRCS Soil Data Access (SSURGO)
- Provides the live soil profile at any US coordinate — map unit, component, horizon-by-horizon pH, organic matter, CEC, bulk density, sand/silt/clay percentages, drainage class. Queried on demand for every Calculate request; the calculator does not store soil data.
Compost characterization methods
- USCC TMECC — Test Methods for the Examination of Composting and Compost
- The standardized analytical methods that every reputable compost lab (A&L Western, Control Labs, ANR Analytical, and similar TMECC-compliant labs) follows. Defines how moisture (TMECC 03.09), bulk density (03.01), pH (04.11), total elementals via acid digest (04.05), sulfate-S (04.06), organic matter (05.07A), calcium carbonate equivalent (04.08), stability (05.08-B), and electrical conductivity (04.10) are measured. The calculator expects values exactly as TMECC reports them; the input form labels each field with the underlying method.
90-day plant-available nutrient delivery
- Stanford & Smith (1972) — Nitrogen mineralization potentials of soils (Soil Sci. Soc. Am. J. 36:465–472)
- Establishes the first-order kinetic model describing how organically-bound nitrogen becomes plant-available over a growing season.
- Gale, Sullivan, Cogger, Bary, Hemphill & Myhre (2006) — Estimating plant-available nitrogen release from manures, composts, and specialty products (J. Environ. Qual. 35:2321–2332)
- Incubation/field basis for how much compost nitrogen mineralizes in the first season, and how that shifts with compost maturity and C:N ratio.
- Sullivan, Bary, Nartea, Myhre, Cogger & Fransen (2003) — Nitrogen availability seven years after a high-rate food-waste compost application (Compost Sci. Util. 11:265–275)
- Long-term record of residual nitrogen release in the years following application, informing multi-year carryover.
- Cabrera, Kissel & Vigil (2005) — Nitrogen mineralization from organic residues (J. Environ. Qual. 34:75–79)
- Temperature dependence of soil nitrogen mineralization, used to relate laboratory rates to California growing-season soil temperatures.
- Eghball (2002); Eghball, Ginting & Gilley (2004) — residual effects of manure and compost (Agron. J. 94:128–135; 96:442–447)
- Multi-year residual nutrient and soil-property effects of repeated compost and manure applications.
- Sharpley (1996) — Availability of residual phosphorus in manured soils (Soil Sci. Soc. Am. J. 60:1459–1466)
- Basis for how compost phosphorus carries over and becomes available relative to soluble fertilizer phosphorus.
- Whalen (2014) — Managing soil biota-mediated decomposition and nutrient mineralization in sustainable agroecosystems (Adv. Agric. 2014:384604)
- Potassium release from compost is largely dissolution-driven rather than mineralization-limited.
- Western Fertilizer Handbook, 9th ed. (California Plant Health Association)
- Reference for secondary- and micronutrient behavior and the standard oxide reporting conventions (P₂O₅, K₂O, CaO, MgO).
Crop nutrient demand (90-day window)
- OSU Extension — Pacific Northwest Crop Fertilizer Guides
- Per-crop nitrogen, phosphorus, potassium, and sulfur demand at typical commercial yields. Specific guides cited: PNW 611 (field corn), EM 8903 (processing tomato), EM 8918 (sweet corn), FG 30 (lettuce), FG 63 (broccoli), FG 79 (carrot), FG 80/82/84 (cole crops), EM 8963 (winter wheat), EM 9086 (alfalfa), EM 9099 (almond), EM 9304 (residual N decay).
- PNW Crop Fertilizer Guide — consolidated reference set (Aug 2017)
- Pacific Northwest Extension's tab-separated reference dataset compiling pH-min/max, N rates, and soil-test-stratified P/K/S rates for 39 crops. Provides the new preferred-pH window field for each crop and expands the demand database to 36 crops total (was 12). The published guide is a joint OSU / WSU / UI Extension product distributed via the Western Plant Diagnostic Network.
- UC ANR — University of California Agriculture & Natural Resources
- California-specific crop production manuals for strawberry, avocado, almond, and table grape. Calibrates the demand database to CA Central Valley and Central Coast yields.
- NRCS Practice Standard 590 (Nutrient Management)
- Federal nutrient-management framework. The calculator's nutrient-balance presentation follows the 590 convention of comparing what's delivered against documented crop demand.
Agronomic convention
- Western Fertilizer Handbook (CPHA, 9th ed.)
- Source for the elemental-to-oxide conversion factors used when the calculator reports P, K, Ca, Mg in agronomic-convention oxide form (P₂O₅, K₂O, CaO, MgO).
- CDFA FREP — Fertilizer Research & Education Program
- California Department of Food & Agriculture peer-reviewed research underpinning sulfate-S handling and CA-specific nutrient-budget conventions.
Soil-test (optional) — methods & lab examples
SSURGO does not report plant-available nutrient pools. When the user supplies a recent soil test, the calculator overrides the SSURGO pH with the lab-measured value and adds a soil baseline + compost delivery row to the crop-demand card. Any TMECC- or NAPT-participating soil-fertility lab works; the calculator is vendor-neutral.
- Soil-pH (1:1 water or saturated paste)
- Standard methods for the Examination of Soils, Sparks (Ed.), SSSA Book Series No. 5. The 1:1 water and saturated-paste readings are the two most common lab pH conventions; either is accepted as a baseline override.
- Olsen-P (NaHCO₃ extract) · CA / arid-soil convention
- Olsen et al. 1954, USDA Circular 939. Reported by UC ANR Vegetable Production Series and the CDFA FREP soil-test guidelines as the appropriate P extract for alkaline / arid California soils.
- Bray-P (acid extract) · PNW / acid-soil convention
- Bray & Kurtz 1945, Soil Science 59:39–45. OSU Extension EM 9099 and PNW 678 use Bray-P for acid soils west of the Cascades.
- Exchangeable K (ammonium-acetate extract)
- Brady & Weil, The Nature and Properties of Soils, 15th ed., Ch. 14. NH₄OAc-extractable K is the field-standard first-season K pool across both CA and PNW labs.
- Sulfate-S (mono-calcium-phosphate extract)
- Recommended Chemical Soil Test Procedures for the North Central Region (NCERA-13). Reports the plant-available SO₄²⁻ pool directly.
- DTPA-extractable micros (Zn, Cu, Mn, Fe)
- Lindsay & Norvell 1978, SSSA Journal 42:421–428. The standard Western-US micronutrient extract; correlates with first-season uptake.
- Hot-water-soluble Boron
- Berger & Truog 1939, Industrial & Engineering Chemistry, Analytical Edition 11(10):540–545. The field-standard B test across both CA and PNW labs.
- Conversion convention (ppm → lb/ac)
- Western Fertilizer Handbook (CPHA, 9th ed.), Ch. 4: soil mass (lb/ac) ≈ bulk_density (g/cm³) × depth (in) × 217,778, so 1 ppm at 6-in depth and BD ≈ 1.4 g/cm³ ≈ 1.83 lb/ac. The familiar field heuristic "1 ppm ≈ 2 lb/ac at 6 in" is the upper bound of that range. P-elemental and K-elemental are then converted to P₂O₅ / K₂O so the rows line up with the crop-demand values.
- Lab examples (representative, not endorsements)
- Any TMECC- or NAPT-participating soil-fertility lab works. Western-US labs include A&L Western Laboratories (Modesto, CA), Wallace Laboratories (El Segundo, CA), Ward Laboratories (Kearney, NE), ANR Analytical (UC Davis), Spectrum Analytic (Washington Court House, OH), Servi-Tech (Dodge City, KS), and Brookside Laboratories (New Bremen, OH). Pick whichever your operation already uses — the calculator does not weight one lab over another.