Screening, Finishing & Storage

Why finishing matters

An active pile that has cooled to ambient is not yet compost — it is stabilized raw material. The finishing stretch (curing, screening, blending, storing) is where a farm decides whether the year's work ships as a premium product, a commodity bulk amendment, or a liability that comes back with a complaint. Three truths organize the chapter:

• Curing is a biological process, not a parking period. Fungal and mesophilic communities re-colonize the cooled material, phytotoxic compounds degrade, and nitrogen re-immobilizes into stable organic forms. This chemistry cannot be rushed by machinery — only by time, moisture, and a little oxygen.
• Screening is the product's haircut. It removes the woody skeleton (bulking-agent overs), homogenizes texture, and can drop moisture several points by exposing fresh surfaces. It is also the last chance to see — really see — what the process missed: plastics, glass, gravel, uncomposted clumps.
• Storage is where quality is quietly lost. Uncovered finished compost re-wets, leaches nutrients, re-heats, and grows weeds on its own surface. Rain falling on a finished stockpile recreates, in miniature, the whole leachate problem (see Water & Leachate).

Curing: the quiet phase

Curing is the 4–8 weeks (often longer) after the active phase, when material rests at ambient temperature with modest oxygen demand. What's happening inside:

Practical management: pile cured material 1.5–2.5 m high (high enough to shelter from rain, low enough to stay aerobic without turning), on the same drained hardstand as everything else. A curing pile that smells of anything other than forest floor needs turning or more time. End-of-curing targets: ambient temperature for 2+ weeks, stability index of Solvita ≥6–7, germination ≥90%, C:N ≤20:1.

Screening: equipment, sizes, overs

Screening separates finished compost from the persistent wood skeleton it was built on. Three things decide the outcome: moisture, screen size, and feed rate — in that order. Wet material blinds screens (holes smear shut) no matter what machine you own; dry, cured material screens beautifully even through modest equipment.

Equipment at farm scale

• Static inclined screen (grid/deck): loader-fed gravity deck; $5k–20k; 10–40 m³/h with good material. The default first screen for a farm.
• Rotary (trommel) screen, towable or PTO: $10k–80k; 20–100+ m³/h; handles slightly wetter material; self-cleaning; the standard for consistent product at volume.
• Star/deck screen: gentle on fibrous material, good for fine screening of compost; cost sits between static decks and trommels.
• Custom hire: mobile contract screeners charge per tonne or per day — often the cheapest route below ~1,000 t/yr (see equipment economics).

The overs are not waste

Overs — the chips and woody fraction that didn't pass — are the most valuable "byproduct" on the yard. They are partially decomposed, microbially colonized bulking agent: recycle them directly into the next active batch at 10–30% by volume. This re-inoculates fresh feedstock, reuses paid-for carbon, and improves structure. After 3–5 cycles the fragments are small enough to pass the screen — that's when they become compost, which is exactly how the mass balance was supposed to work.

Blending & product grades

Most farm composts land between grades — too good for the field, not consistent enough for the nursery. Blending closes that gap. Common, honest product lines:

• Agronomic bulk (unscreened or coarse-screened): sold or spread as-is; the volume product.
• Landscape/screened grade: 12–16 mm product, consistent moisture, STA-style testing; the margin product.
• Media blend (10–30% compost): compost blended with peat/bark/perlite for potting — the compost component must be mature and low-EC (see container media); blends let you sell small tonnes at large prices.
• Composted mulch: coarse fraction with >25–30% finished compost by volume — weed-suppressive and biologically active, distinct from raw bark mulch.

Blending rules: blend dry materials (moisture differences segregate a mix in minutes), calculate the blend's salt and nutrient load rather than assuming the compost's lab sheet applies (dilution changes EC per volume), and label by content ("30% compost by volume") rather than by promise. Blends inherit the claims and the liabilities of every component.

Storage & stockpiles

Finished compost stores well — that's part of its pitch — but "well" means managed. The storage decision tree:

• Moisture drifts toward the weather. Sale-spec moisture (typically 35–45%) is a product property — weigh it into every price, because water is what you're shipping (see the moisture calculator for the arithmetic).
• Weed seed rain is real: an open stockpile grows its own weeds, seeds its own surface, and ships the evidence. A compost skin or cover prevents it.
• Nutrient holding: rain through finished compost leaches K and mineral N first. Covered product keeps its lab sheet honest.
• Frozen product: in cold climates, frozen clumps shatter screens and annoy spreaders — screen in fall before winter stockpiling, not in spring.

Pre-sale quality checks

Before any batch leaves the yard with your name on it — a 30-minute gate check, plus the lab cadence:

1. Look: coarse wood OK for mulch grades; plastics, glass, and gravel are never OK. Grab five handfuls from five spots.
2. Smell: forest floor or nothing. Any ammonia (immature) or sourness (anaerobic pocket) holds the batch back.
3. Squeeze: damp, holds shape, no drip (roughly 35–50%).
4. Temperature: ambient at core. Warm means re-heat — restabilize before shipping.
5. Solvita / quick stability: ≥6 for bulk grades, ≥7 for media use (see field stability tests).
6. Lab cadence: full panel (nutrients, metals, maturity, pathogens where required) per feedstock change and at least annually for sold product — keep certificates with batch records (see getting a lab test).

The economics of finishing

Finishing is where compost stops being a waste-management outcome and starts being a product — and where its costs concentrate:

• Curing area is pad area. Curing occupies 1–2× the active-phase footprint for months. On a constrained pad, curing competes with throughput — roofed curing bays often unlock more capacity than a second windrow line.
• Screening costs are real but small per tonne (equipment amortization or custom hire, typically a few $/t) — while the price difference between unscreened and screened product is routinely 2–4×. Screening pays wherever a landscape/nursery market is within reach (markets & pricing).
• Yield discipline: every 10% of product lost to overs, spillage, and rework is 10% of the year's margin left on the pad. Log yields (see screening above).
• Covers vs roof: tarps and compost skins are cheap moisture control; roofed storage is the premium answer where wet winters coincide with spring sales. Price both against your climate (see the water balance).
• Storage is inventory finance: product made in fall and sold in spring is capital sitting in a pile. The return on finishing well is a product that sells itself; the cost of finishing late is discounting it.

Where a farm draws the line — sell everything unscreened to neighbors, or build a screened grade — is the classic make-vs-buy of composting: screen when a market pays the increment; otherwise spend the hours on the field. The worked dairy ledger in Economics shows exactly how sale revenue enters the math.

Finishing troubleshooting