Even the north-east, which has received plentiful winter rains, is expecting widespread crop failure.
The United Nations says the drought in Afghanistan is more widespread than previously thought.
Northern and central areas have also been gravely affected, with many farmers expecting total crop failure.
It says three to four million people are severely affected, about a fifth of the population.
The devastating effects of the drought in southern Afghanistan have been clear for some weeks.
But it is only now that surveys are revealing how many farmers in the rest of the country are also facing ruin.
Crop failure
The UN says those farmers in the north who have no access to irrigation are the most seriously affected.
But many people in the central highlands, already one of the poorest areas in Afghanistan, are facing their third poor or non-existent harvest in a row.
Even the north-east, which has received plentiful winter rains, is expecting widespread crop failure.
Its problems started in April, when low rainfall and soaring temperatures meant crops burned in the fields.
Many farmers are simply letting their animals graze on the failing crops.
People with access to irrigation are faring better, but even their yields are down.
The north usually provides most of Afghanistan's home-grown wheat, crop failure there will have a knock-on effect on the rest of the country's capacity to feed itself.
Imported wheat
The UN estimates Afghanistan could be short of two million tonnes of wheat in the year starting in July.
That is far higher than Afghanistan has ever experienced before.
News on Tuesday that neighbouring Pakistan has produced a bumper wheat crop, and intends to export 600,000 to Afghanistan, will come as some relief.
But incomes are greatly reduced by the drought and it is not clear how many Afghans will be able to afford this imported flour.