This is a matter of the heart, and the prophet moves immediately into graphic language. Of course, he is not interested in them ploughing their fields; he is using these illustrations concerning the hearts of the people, as he goes on to say in the passage.
All worship of God that does not come from a renewed heart is unacceptable to him. It is mere formality and not in spirit. We cannot approach God with our spirits, while they are still at enmity with him and all that flows out of them is hostility and rebellion, or reluctant conformity. Those in this unrenewed state can do little more than worship in a formal way with external expression. That is the highest they can achieve. But God is a spirit and he must be worshipped in spirit, as well as in truth. To worship him only with external rites and ceremonies is not a real approach to him. God is a spirit, and man has a spirit and a body, and to approach him only with the body and not the spirit is to give him the least important part of us, and to withhold the more important part. We must worship him with our minds, with our deepest feelings, with our wills – all these are part of our spirit, and without these being engaged in worship, we are ignoring the true nature of God. We are like those with an electric car, who seem to want to push it everywhere, instead of turning on the ignition.