GGU-FIELDCAPACITY: Calculation according to KA5¶
GGU-FIELDCAPACITY follows the Bodenkundliche Kartieranleitung, 5th edition (Ad-hoc-AG Boden, 2005). The parameters are determined per sample in the following steps. Each step appears as a row of its own on the evaluation sheet, so the calculation stays traceable.
1. Fine soil fractions¶
The KA5 tables refer to the fine soil. The program therefore rescales clay, silt and sand so that they add up to 100 % without gravel:
Tfine soil = T / (T + U + S) · 100, likewise for U and S.
These values appear on the sheet in the rows clay content T, silt content U and sand content S. If clay, silt, sand and gravel add up to more than 100 %, the program does not continue for this sample.
2. Soil type (KA5, Tab. 30)¶
From the fine soil fractions the program determines the soil type according to the KA5 texture triangle, for example "Sl3" (mittel lehmiger Sand, medium loamy sand). You can look up the limits of each soil type under KA5 soil types.
3. Dry bulk density class (KA5, Tab. 68)¶
The dry bulk density is assigned to one of the classes pt1 to pt5; the limits are listed under Entering samples.
4. Humus content and humus class (KA5, Tab. 15)¶
If the TOC content is given, humus = TOC · 1.72. The humus content is assigned to one of the classes h0 to h7; the limits are listed under Entering samples.
5. Fine soil parameters (KA5, Tab. 70)¶
For the soil type and the density class the program reads from Tab. 70:
- the field capacity FK,
- the air capacity LK,
- the wilting point PWP (dead water),
each in vol. %.
6. Surcharge for organic matter (KA5, Tab. 72)¶
For the soil type and the humus class the program reads the surcharges for field capacity and air capacity from Tab. 72 and adds them to the values from step 5. Tab. 72 gives surcharges for the humus classes h2 to h5. For h0, h1, h6 and h7, and without humus data, the surcharge is zero.
7. Reduction for the gravel content¶
Gravel does not contribute to water storage. Field capacity, air capacity and wilting point are therefore converted to the whole soil:
valuesoil = valuefine soil · (100 − gravel) / 100
This gives the rows soil field capacity, soil air capacity and soil wilting point. This step needs the gravel content, so for gravel-free soils enter 0.
8. Available field capacity¶
nFK = FKsoil − PWPsoil
The available field capacity in vol. % is the share of water that the soil can store in a form available to plants.
Example¶
Sample P1 from the picture in the Preface: clay 8 %, silt 32 %, sand 52 %, gravel 8 %, dry bulk density 1.45 g/cm³, TOC 1.5 %.
| Step | Result |
|---|---|
| Fine soil | T 8.7 %, U 34.8 %, S 56.5 % |
| Soil type | Sl3 |
| Density class | pt3 |
| Humus | 1.5 · 1.72 = 2.58 % → h3 |
| Tab. 70 | FK 27.0, LK 15.0, PWP 9.0 vol. % |
| Tab. 72 | surcharge FK + 5.0, LK + 2.0 vol. % |
| Fine soil with surcharge | FK 32.0, LK 17.0 vol. % |
| Gravel reduction 8 % | FK 29.4, LK 15.6, PWP 8.3 vol. % |
| Available field capacity | 29.4 − 8.3 = 21.2 vol. % (calculated unrounded) |