She said the two issues were completely unrelated.
Asked in a television interview by ARD public broadcaster about her investment plans and Schulz's criticism, Merkel said: "We now can not spend the money that we have".
Presenting a ten-point-plan for a modern Germany and a better Europe, Schulz said he would introduce an investment obligation for the state to boost public spending on infrastructure and education.
Germany has already earmarked billions of euros for schools, nurseries, hospitals and housing, but local authorities have so far spent only a fraction of that windfall due to planning bottlenecks.
"It's important to me that the citizens have a true choice", Merkel said.
On domestic matters Merkel defended her decision to host the recent G20 summit in Hamburg, known for being a hotbed of left-wing extremism.
"We say, for example, that we have to use at least one third of the additional tax revenues for investment".
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She said the violence the protesters carried out was "unacceptable". "We still have a lot to do in this regard", she added. Trading partners called on the government to invest more as a way of reducing its massive trade surplus, something US President Donald Trump has railed against.
Mr Schulz said he would push Germany into a period of increased investments and renewed eurozone integration.
The German leader made a similar argument three days ago in Paris after holding a joint cabinet meeting with French President Emmanuel Macron, who had joined criticism of Germany's current-account surplus.
Countries like France or the United States argue that while Germany is happy to rake in cash from selling its goods overseas, it fails to help other economies by spending at home to contribute to economic growth.
Merkel underlined her determination to run for a full four-year term in the September 24 election.
Right now it looks as if Merkel will remain chancellor after the election, but Martin Schulz should look to Britain if he wants to take the top spot.
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He urges the government to get a basic understanding of the impact and influence of Artificial Intelligence in the human society. Musk said the surging popularity of AI technologies is a "fundamental risk" the humankind is about to face in the coming years.