Nothing notable in these chapters. Chapter 8: Poor quality land requires more labour time than socially necessary (which is to say the average labour time required to produce a given output). Chapter 9: Socially necessary labour time is permanently established in nature, in industry it is constantly disappearing. Chapter 16: The value of labour is established by the customary means of subsistence of the labourer (this appears to be a notation on Ricardo’s view).
In chapter 17, Marx discussed the crisis. (Brad DeLong called it Marx’s Half-Baked Crisis Theory) The first appearance of socially necessary labour time is in an assumption that the time for producing the commodity is socially necessary. The crisis arises out of the separation between the act of selling the produced commodity and using the money from that sale to buy another commodity. Marx acknowledged the possibility of partial crises resulting from disproportionate production — where one commodity contains more than the socially necessary labour time — but that is not the crisis Marx was concerned with in his analysis. To repeat, Marx was concerned with the crisis that resulted from the separation of the acts of selling and buying.