modules/salinity/ — salt building up in a bed (layer 2)#

Under a tunnel no soaking rain washes salts below the roots, while fertilizer, compost and hard water keep adding them, so salt builds up season after season (University of Maryland Extension). A soil test sees it a few times a year. A probe that reports electrical conductivity, as the TEROS 12, the HydraProbe and the LoRaWAN probes do, sees it climb between them. This sets a bed's salt beside the tolerance of the crop growing in it:

Salt in tunnel bed 2 is climbing: about 1.1 dS/m, as a saturated paste's extract would read, 86% of the 1.3 at which the yield of lettuce starts to fall.

Salt in tunnel bed 2 has reached about 2.3 dS/m, as a saturated paste's extract would read, past the 1.3 at which the yield of lettuce starts to fall: about 13% of it is lost at this level. Water deeply to wash it below the roots, or uncover the bed to the winter's rain, and have the soil tested.

(The test's bed, its salt raised a week at a time.)

From the probe's reading to the crop's scale#

Three steps, as METER's TEROS 11/12 manual (section 3.3) gives them:

1. The pore water's conductivity from the bulk soil's. The probe measures the bulk soil's, which rises with the water and the temperature as well as with the salt. Hilhorst (2000): σp = εp·σb / (εb − 4.1), with the pore water's permittivity εp = 80.3 − 0.37 (T − 20). Not below 0.10 water content, where the denominator is small; about ±20% in moist soil.
2. The bulk soil's permittivity εb is the probe's own measurement, but it sends only the water content made from it. It is recovered by Topp, Davis and Annan's (1980) curve, the one most probes' water content is calibrated by. That is an approximation on top of Hilhorst's.
3. The saturated paste's extract is the pore water diluted to saturation: ECe = σp · θ / φ, the porosity φ = 1 − ρb / 2.65 from the bed's bulk density (a loam's 1.4 Mg/m3 unless measured).

The bed's salt is the median of the last week's hours whose three readings (water, conductivity, temperature, from one probe's body) the health checks passed. A wetter soil reads a higher bulk conductivity for the same salt; the estimate does not move (tested at 0.22 and 0.34).

The tolerances are Maas and Grattan's (1999), as FAO Irrigation and Drainage Paper 61 (Tanji and Kielen, 2002, Annex 1) prints them: for 25 vegetables, the ECe up to which the yield does not fall, and the share lost per dS/m beyond it (data/salt-tolerance-maas-grattan.csv, data/MANIFEST.csv). Beans, carrots and strawberries start losing at 1.0 dS/m; asparagus at 4.1.

Read-only and advisory. Each alert is a CEL rule judged through layer 1 (modules/rules); a decision is written when an alert is raised or cleared, and a monitor that restarts reads which stand from the log. The crop is named in the reader's language (Rule.key_params).

pip install -r modules/salinity/requirements.txt         # cel-python, via modules/rules
cd modules/salinity && python3 -m unittest discover -p 'test_*.py'
python3 salinity.py --socket … --config beds.json        # once a day, as a module process

with beds.json as {"beds": [{"feature": "…", "crop": "tomato", "bulk_density": 1.3}]}.

Not tested#

Nothing here has met a real bed (UNTESTED):

• the chain is tested against the manual's worked example and Hilhorst by hand, on readings made to fit it; a real probe's bulk conductivity has its own errors;
• Hilhorst's offset of 4.1 is generic; he gives a simple way to measure a soil's own, which the manual says improves the estimate in most cases;
• the bulk density is assumed unless given;
• no bed's estimate has been set beside a laboratory's saturated paste.