First, activate the virtual environment (with Python 3) with virtualenv env/bin/activate. Then run the following command within the soil-carbon directory:
streamlit run app.py
A browser window should open at localhost:8501.
With the proper Heroku permissions, you'll be able to deploy the application with a simple command:
git push heroku master
Note, however, that if you are developing in virtualenv and you install a new package, then you'll have to add the packages to requirements.txt. The deploy on Heroku runs on Docker, which rebuilds the environment from requirements.txt on each deploy.