How can I merge a branch into master but continue working on the branch?

Here is the process you are looking for:

1. git checkout master
2. git merge Farmcrops
3. git push origin master
4. git branch -d Farmcrops
5. git checkout master
6. git checkout -b Farmcrops
7. continue your commits on the branch Farmcrops...

Branches are just pointers, it's very easy to create/delete a branch and if your branch Farmcrops isn't pushed on remote repository, there is absolutely no dependency with it. You can delete it after the merge and recreate it from the master.

Hope this will help you.

If I understand correctly, you're starting from

-- o -- o -- o [master]
    \
     o -- o [Farmcrops]

You shouldn't merge Farmcrops directly into master, because you run the risk of breaking the code in master, which, by convention, is supposed to be more stable. Instead, check out Farmcrops and merge master into it.

git checkout Farmcrops
git merge master

Then you'll get

-- o -- o -- o [master]
    \         \
     o -- o -- o [HEAD -> Farmcrops]

Run some tests; make sure everything works as expected. Then check out master and merge Farmcrops into it:

git checkout master
git merge Farmcrops

Your repo will then look like this:

-- o -- o -- o
    \         \
     o -- o -- o [HEAD -> master,Farmcrops]

(Note that this merge is a fast forward: it doesn't create a merge commit because Farmcrops is a direct descendant of master.)

Now check out Farmcrops again and continue your experiment, make more commits on it, etc...

-- o -- o -- o
    \         \
     o -- o -- o [master]
                \
                 o -- o -- o [HEAD -> Farmcrops]

You can always fall back on master (which now contains "the first round of changes", as you put it) if your new experiment on Farmcrops doesn't pan out so well.