You are the salt of the land; but if the salt has become foolish, how shall it be salted? It is now good for nothing except, having been thrown out, to be trampled by people.
In the context of Jesus’ address to the community of renewed Israel it makes more sense to translate gē as ‘land’ than ‘earth’. The issue here is the role of the disciples in the eschatological transition from second temple Judaism under judgment to a restored people of the Spirit.
The syntax is difficult, but the verse could be read as follows: the disciples are the salt of the land; but if they become foolish (as Israel is sometimes said to be foolish: eg. Deut. 32:6; Ps. 93:8; Is. 32:5-6; Jer. 5:21), how will the land be salted?
Mark 9:49-50 suggests that ‘salt’ is a metaphor for judgment. The statement that Israel will be ‘salted with fire’ sums up the preceding warnings about the judgment of gehenna. Jesus tells the disciples to ‘have salt in yourselves’, which appears to mean: make sure that you will not be subject to the judgment of gehenna, that is, to the judgment of AD 70.
The context of the saying is different again in Luke: the disciples are effectively ‘salt’ insofar as they are willing to take up the cross and follow Jesus (Luke 14:26-35).
If we may legitimately attempt to amalgamate these arguments, it would appear that the metaphor of ‘salt’ has to do with the effect that the presence of a community of disciples, whose righteousness exceeds that of the Pharisees (cf. Matt. 5:20) and who are willing to pay the price of suffering, would have within Israel under judgment. The emphasis, however, is less on the actual impact of the community than on the need for the disciples to maintain their integrity and commitment. The actual effect of the ‘salt’ should probably be interpreted in terms of the core ministry of disciples, which was to proclaim the imminence of the reign of the God - that is the imminence of judgment and restoration. They will fail in that mission if they do not first judge themselves and if they do not count the cost of following Jesus.