Topsoil and growth layer — thickness, quality and tolerances

Topsoil (the growth layer) is the top, fertile soil layer that plants grow in. Topsoil that is too thin or too heavily compacted gives poor growth, so thickness, structure and bulk density all matter.

• 01Topsoil layer thickness: 25–40 cm (NfA 1992, section 2).
• 02Tolerance: area < 1000 m² → at most ±3 cm; area > 1000 m² → at most ±5 cm from the required.
• 03Reusable topsoil: the top 15–35 cm is regarded as reusable topsoil for plant growth.
• 04Tilling is carried out within the thickness of the topsoil layer, but no deeper than 25 cm.
• 05Topsoil stockpile: must not be heaped higher than 1.5 m without being lifted into the stockpile.

Max. dry bulk density ensures the topsoil isn't packed too hard (reference-measured values, NfA 1992):

Optimal topsoil for grass (weight-%, NfA 1992): 35–45 % coarse sand (2–0.2 mm), 35–45 % fine sand (0.2–0.02 mm), 12–18 % clay+silt and 3–5 % humus. Optimal pH is 6.5–6.8.

Danish soil is classified into JB classes (soil type classification) according to the content of clay, silt, sand and humus — the higher the JB number, the more clay (up to JB9; JB10 is silt soil, JB11 humus soil). See the full JB table and explanation in »Soil Science and Soil Types«. Combined with the texture triangle, the JB class gives a quick picture of whether the soil needs to be improved, drained or replaced.

A soil analysis from a laboratory (e.g. according to SEGES/Ministry of Agriculture guidelines) tells you what the soil is lacking. The key parameters:

• 01Reaktionstal (Rt): jordens surhedsgrad (svarer til pH). Lav Rt = sur jord, der typisk skal kalkes.
• 02Phosphorus number (Pt): plant-available phosphorus. Low Pt may require phosphate fertiliser.
• 03Potassium value (K): plant-available potassium. Low K may require potassium fertiliser.
• 04Conductivity value (EC): the soil's total salt content. High EC can damage roots and germination.
• 05Humus/organic matter (%): affects structure, water retention and nutrient pool.
• 06The laboratory indicates whether each value is low, adequate or high — use that interpretation rather than guessing optimal figures.

The analysis is translated into action. Liming raises Rt/pH; the lime amount per Rt unit depends on soil type — the more clay and humus, the more lime is needed to shift Rt. Fertilising (N-P-K) is dosed according to the crop's needs and the analysis's Pt/Kt, and starter fertiliser is often given at new establishment. Always follow the analysis's recommendation and the product's dosing guide.

The fact that clay and humus-rich soil requires more lime is due to buffering capacity: the many clay and humus colloids bind hydrogen and prevent pH from shifting, so more lime is needed to raise Rt by one unit on clay soil than on sandy soil. The relative relationship looks like this (agricultural lime per Rt unit):

The relative factors are orders of magnitude that show the direction — the binding kg quantities per Rt unit per soil type are read from SEGES' liming guidance or follow the laboratory's recommendation on the analysis result. Never guess an absolute lime amount; look it up.