More people are shopping on phones. Which retailers will reap the benefits?

When you look at the explosive growth in purchases made on smartphones, you can see why the retail industry sees so much opportunity. This year, for example, mobile sales are forecast to grow 43.2 percent to $115.92 billion, according to research firm eMarketer.

But a new report offers evidence that retailers who are aiming to ride the mobile wave will likely face some tough challenges in capturing shoppers' attention.

ComScore studied how U.S. consumers are spending their time on mobile gadgets, examining factors such as whether they're using websites or apps, how many apps they are willing to download, and which apps they while away minutes (or hours) in. Researchers found that consumers spent an average of 73.8 hours a month in smartphone apps, up from 68.2 hours last year. Monthly app time was up for every age group, and it was highest among the younger shoppers whom retailers are hungry to court. Consumers in the 18-to-24 age group spent 93.5 hours in smartphone apps, while those aged 25 to 34 spent 85.6 hours in smartphone apps.

These findings suggest that apps have become the foundation of our internet usage. That on its own doesn't spell trouble for retailers: It just means they needed to make their apps engaging and easy to use.

The problem for e-commerce players comes in how users are dividing up that time they spend within smartphone apps. Comscore found that, on average, consumers spent 45 percent of their app time on their single favorite app. About nine out of every 10 minutes people spend in mobile apps are spent in their top five favorite apps - and those aren't typically retail apps.

That means that most retailers are going to have to fight hard for a sliver of that one out of every 10 minutes that is not spent in apps such as Facebook and YouTube.

And that's if they can get people to download their app in the first place. Comscore found that people are getting choosy about how many apps they'll load on their phones. Some 49 percent of consumers don't download any new apps in an average month, and some 13 percent download just one every month. Retailers will face fierce competition from gaming, media and social offerings to be that rare app that you take the plunge and download. That won't necessarily be an easy battle to win.

Given how hard it is to get in users' app rotation, it is possible that some retailers will put more emphasis on building out chatbots - robots that mimic human conversation that consumers can find within other apps. Retailers and restaurants have been experimenting with chatbots that people can use to buy goods without leaving apps such as Facebook, Kik and Slack where they already spend a great deal of time.

Andrew Lipsman, vice president of marketing and insights at ComScore, said that the research suggests e-commerce sellers might need to think differently about getting their apps in front of shoppers.

"I think the retailers should be a lot more active in how they should incentivize people to download apps," Lipsman said.

One notable exception to the retail app usage patterns is Amazon.com, which has managed to become a sizable part of our smartphone diets. Comscore found that the Amazon app appears on almost 25 percent of smartphone users' home screens. This is noteworthy because people tend to give that kind of prime screen real estate to their most-used apps. Very few apps appear on so many people's home screens - Only YouTube, Google Maps and Facebook have more home-screen incidences. (Jeffrey P. Bezos, the chief executive of Amazon, owns The Washington Post.)

It's also striking to look at how engagement with Amazon stacks up against other online retailers. The Amazon app accounts for 34 percent of the time that users spend in the shopping sector. Every other retailer lags far behind, with eBay coming in second with 8 percent of time spent.