I need to find heat loss from the greenhouse. I know how to find heat loss through the walls and roof , but what about soil ? how to find amount heat conductivity loss to the soil ? I would be gratefull for the any reference/method/ article or etc.
Easy answer: Net heat loss averaged over day/night is zero because the heat that's transmitted into the soil when the greenhouse is hot is stored in the soil and returned to the greenhouse when the greenhouse becomes colder than the soil.
Hard answer suggestion: This problem is called "One-dimensional transient heat conduction in semi-infinite body". Semi-infinite because the ground is infinitely deep in the downwards direction but not in the upwards direction. It's time dependent. The rate of heat loss will go down as it progresses because the ground becomes hotter. You can write and solve the DEs by hand if you wanted a hard exercise in calculus. Here's somebody's working: https://www.thermalfluidscentral.org/encyclopedia/index.php/One-dimensional_transient_heat_conduction_in_semi-infinite_body
Whiteboard lecture video: https://www.youtube.com/watch?v=X8ofOO-iHhk
Practical answer: I would use Finite Element Analysis (FEA) which does all that math for you. Download some FEA software and use time dependent or transient heat transfer analysis. This will also allow you to model 2D and 3D effects like heat flow around the edges. The two 1D suggestions above are not strictly correct because of lateral heat flow under the ground but the bigger the greenhouse, the more accurate the 1D approximation is.