There is a configuration parameter in nutch named "solr.commit.size" which according to the description in nutch-default.xml is:
Defines the number of documents to send to Solr in a single update batch. Decrease when handling very large documents to prevent Nutch from running out of memory. NOTE: It does not explicitly trigger a server side commit.
As it said, it does not explicitly commit, because it is more optimized to left the decision of commit times to solr. So you should also tune your solr configuration parameters: autoCommit and autoSoftCommit. You can find their descriptions in solrconfig.xml file.