Crop growth is influenced by three factors: Air quality, Hydration, and Nutrients.
And Industrialcraft currently has no way of measuring the value of any of these numbers. Hence my suggestion: The Soilnalyzer, an upgrade to the cropnalyzer. Right-clicking on a crop block displays the air quality, nutrient, and hydration values for that block as a text message (similar to right-clicking on a bed during the day or with monsters around). As a possibility, right-clicking on air switches between modes: One for measuring nutrient/hydration storage (the value that can be directly changed), and one for measuring the actual value (which apparently is what affects plant growth).
My suggested recipe: 
(where is a cropnalyzer and is a placeholder)