Unable to post new forum topics in Drupal site When I try to post a new forum topic getting this error 

Fatal error: Allowed memory size of 157286400 bytes exhausted (tried to allocate 91 bytes) in /sai/vij/modules/field/field.attach.inc on line 696 

i have a free space of 4389 MB in server. and I given a memory_size as -1 in php.info file.

     Any ideas for implementing this in drupal is appreciated.
Yes i think @No Sssweat is right. You need to increase PHP memory limit.

ini_set('memory_limit', '256M'); in your sites/default/settings.php file.

Increase PHP memory limit

Increase PHP's memory limit, e.g. to increase it to 256M you could try adding:

memory_limit = 256M to your server's main php.ini file (recommended, if you have access)
memory_limit = 256M to a php.ini file in the Drupal root
ini_set('memory_limit', '256M'); in your sites/default/settings.php file
php_value memory_limit 256M in your .htaccess file in the Drupal root