If it was me id put 3 bags of 46-0-0 on it....obviously clover fixes nitrogen but you say that your clover is not very good. Well its said in this thread that good clover can fix upwards of 100 units of nitrogen, so yours only fixes 30....well that means with 3 bags and the clover youd be very close to 100 units, which should be plenty to grow good brassicas....with that being said im putting 4 bags on my clover ground...and my clover was very good...im hoping ill be in that 150+ units range...we shall see! if we dont get some rain soon we wont have to worry about it much!!