An engineer might analyse the soil and the plant water needs. Inject water to right levels, or set a timer. These settings need to be readjusted constantly without a hydrometer.
I found an ancient solution that worked incredibly efficient and required no electricity and far cheaper than spending hundreds on electronics. Ollas. The science is called capillary action.
I went a step further as an engineer, and did DIY terracotta pots + silicone. Added a testing stage, of course. Filled pots and sealed it over night. If leaking from seal, more silicone. For filling them up, I used irrigation fittings hooked up to a tank. If I wanted to, I could use rain water into tank (with overflow). That way watering is a fully automated pipeline.
I don't have a picture on hand but others have done it like this: https://www.abc.net.au/gardening/how-to/olla-watering-pots/1...
Other fun topics I looked into:
- Companion plants (avoid spending on pesticides)
- Crop rotation (avoid spending on fertilisers)
- Composting (for compost)
Those are not easy to automate but other topics to make it far cheaper to grow crops without trips to nurseries.