These agricultural activities disrupt the delicate soil profile which was sustained by the rich rainforest, because crops only use up certain chemicals and leave the others.  When left to natural means, the rainforest uses these soil minerals cyclically thus keeping a balance, which is not observed by the soil use of crops, especially in the case of cash crops where crops using only a few chemicals from the soil are grown year after year, giving the soil not even the slightest chance to cycle back to normal.  This is obviously very inefficient because:

1.  After depleting the soil of its necessary components, more crops cannot be grown on the same land, so farmers have to move on.  

2.  Once the soil balance is ruptured, new trees cannot grow again because the soil now lacks the necessary components to sustain it.

3.  Pesticides and fertilisers used on land favor certain chemicals of the soil while destroying others.

4.  Cattle ranching and use of heavy machinery compacts the soil further disrupting the balance and making it more susceptible to erosion.

If this agricultural method is used extensively (which it is), the end result is the diminishing of a finite rainforest inch by inch (or hectare by hectare, in this case) up ahead, and an infertile wasteland left behind which cannot be used to regrow what is being lost.


References:
Whitmore, T.C. (1998). An Introduction To Tropical Rainforests
Pictures:
1. Picture from: http://www.ddbstock.com/mexfarm1.html