Aerobic Compost Making for Biological Inoculation

Detailed method for producing high-quality aerobic compost as a biological inoculant (not fertiliser), based on Simon Cowell’s 15 years of practice on heavy Essex clay. From his extended Farmer Focus in Direct Driller Issue 4.

Core Principle

Compost is a biological inoculant, not a fertiliser. At 4 tonnes/acre application rate, the nutrient value is negligible — the benefit comes from introducing diverse, thriving microbial communities in their own stable environment.

“If your soil is not in a good enough condition to live and multiply, inoculants will soon die out. What is unique to compost is that it is a way of applying the good biology in its own, perfect environment.”

[Source: dd-issue-04, 2026-04-11]

Ingredients

Target carbon:nitrogen ratio of approximately 30:1 (matching the C:N of the microbes that will consume it).

Ingredient Source Purpose
Horse manure Local riding schools/livery yards Primary N source; contractor delivers
Woodchip Local tree surgeons Carbon source; fungal substrate
Grass cuttings Landscape maintenance company N source; diversity
Leaves Autumn collection Carbon source; diversity
Gypsum Purchased Calcium + sulphur in biological form; counters high Mg (see The Albrecht Method — Base Cation Saturation Ratios)
Topsoil or previous compost On-farm Inoculation with existing soil biology

Diversity of ingredients is key — the wider the range of input materials, the wider the range of bacteria, fungi, and other soil life in the final product.

[Source: dd-issue-04, 2026-04-11]

Method

Windrow Setup

  • Tip manure in a straight line in the field where compost will eventually be spread
  • Tidy with loader to 4m width (optimum for turner)
  • Wider = anaerobic centre; narrower = insufficient volume

Turning Schedule

  • Use tractor-driven windrow turner (aggressive mixing, shreds all ingredients)
  • Monitor temperature daily with probe
  • Turn whenever temperature reaches 65°C
  • Typical schedule:
  • Days 1–4: turn daily (bacteria go “crazy” after first mix; can reach 70°C in 24 hours)
  • Days 5–6: one day gap
  • Days 7–10: turn every 3 days
  • Then: turn once per week until temperature stabilises
  • Critical: never exceed 70°C — beneficial soil life cannot survive above this temperature
  • Total active phase: 4–5 weeks from first turn to temperature stabilisation

Maturation

  • After turning stops and temperature stabilises, leave to mature as long as possible
  • Ideal: up to one year
  • During maturation: larger organisms (fungi, arthropods, protozoa, nematodes) colonise and multiply
  • Turner is too aggressive for fungi — the maturation phase is essential for fungal development

Two Quality Determinants

  1. Diversity — the biggest range of different species at the start
  2. Maturity — how long the compost is left for larger organisms to develop

[Source: dd-issue-04, 2026-04-11]

Application

  • Rate: 4 tonnes/acre (approximately 10 t/ha) — surface application
  • Not enough to build organic matter directly — this is inoculation
  • Applied to fields that need biological improvement
  • 2018 plan: lower rate across wider acreage to maximise coverage

[Source: dd-issue-04, 2026-04-11]

Results

Simon Cowell’s original “accidental” compost application (~2013):

  • Old muck heap (6–7 years accumulation) spread at ~6 tonnes/acre on one third of a heavy clay field
  • Following wheat: at least 1 tonne/acre more on compost area
  • Following OSR: twice the yield on compost area
  • Soil visibly darker, aggregated, open structure; water passing freely to drains
  • Walking the field: clear boundary between treated and untreated by wetness/stickiness
  • The difference still shows up years later despite subsequent whole-field applications

[Source: dd-issue-04, 2026-04-11]

Related Pages