"Listen and hear my voice; pay attention and hear what I say. When a farmer plows for planting, does he plow continually? Does he keep on breaking up and working the soil?" NIV translation
Isaiah begins to share a sorta mini parable with the leaders of Jerusalem. Using the example of farming, he explains that a farmer that desires a harvest does not go on plowing and preparing the ground for the entire season. Most of the people of this day did not run out to a grocery store or a restaurant to get the food that they needed. In order to have food on the table they had to plan and prepare in advance.
For a farmer, that means the ground must be broken up so that the seed can be covered and so its roots can dig in to provide nutrients for the plant. When the soil is broken into smaller and smaller pieces, it allows the roots to penetrate easier and go deeper to ensure proper water and nutrients throughout the growing season.
There is a point in the process where the effort in breaking up the soil does not provide a significant benefit anymore and this is what Isaiah is describing here.
"When he has leveled the surface, does he not sow caraway and scatter cumin? Does he not plant wheat in its place, barley in its plot and spelt in its field? His God instructs him and teaches him the right way." NIV translation
At some point, the seed has got to be put into the ground if a farmer is expecting a harvest of food for his family. Isaiah goes on to remind them that there are different seeds for different crops and they must be put into their proper place. Caraway and cumin are from the parsley family of plants and are used for flavoring while wheat, barley, and spelt were grains that were used to make bread. The flavoring was nice but it would have been more important to have the grains for making bread.
Isaiah reminds them (and us) that the wisdom to know the proper time to plant the seeds as well as where to plant them comes from God. This would have been a sharp contrast to the way that the people of Israel were casting aside the wisdom of God to get ahead in business and to live it up.
So, what does this have to do with the people of Jerusalem during Isaiah's day and what does it mean for us? Isaiah was using the example of farming to show them that God had prepared (plowed) them to be His people when they were in Egypt. The seeds were planted when the words of God were given to them at Mount Sinai and so God (as with any farmer) was expecting a harvest. The same thing is true for us today, as the Holy Spirit prepares (plows) our hearts (the soil) to hear the words of God so that we might be saved (the harvest).