Martin Marietta (MLM) could be a solid addition to your portfolio given its recent upgrade to a Zacks Rank #1 (Strong Buy). This upgrade primarily reflects an upward trend in earnings estimates, which is one of the most powerful forces impacting stock prices.The sole determinant of the Zacks rating is a company's changing earnings picture. The Zacks Consensus Estimate -- the consensus of EPS estimates from the sell-side analysts covering the stock -- for the current and following years is tracked by the sys ...