I think for your application Lite would be perfect.
You can draw lines indicating the field and lines for the irrigation system very easily with dimension information for any feature. (like length of irrigation line, diameter of pipe, type of pipe, fitting type etc.) You can also store any drawing or part of any drawing as a block. When this is done, you just add that block to your current drawing and you don't have to draw it again.
There are also predrawn blocks of common stuff available (some with Lite, some separately) Like shrubs, trees, and even pipe fittings.
CAD packages are very different to use than other drawing packages. I highly recommend a course or tutorial to get started. It takes some getting used to in order to take advantage of all the convenient features.