In Breaking Bad Walter wanted Jesse to believe that Gus was behind the poisoning of Brock. Then why would he use Lily Of The Valey, at the risk of it being traced back to him? Won't using Ricin ensure that Jesse believes Gus did it?
In my opinion Walter White is not truly evil. He just want to earn money for his family. His aim is not to kill Brock but to use Jesse's emotion towards Brock in his favor. Ricin is riskier than Lily of the valley. So he used Lily of the valley and removed the Ricin just to put suspicion on Gus.
Note- I haven't completed season 5 yet, so don't know if it is addressed later.
When Walter White was contemplating how he was going to topple drug kingpin Gustavo Fring, he noticed the potted lilies in his backyard. ("End Times")
Brock Cantillo was poisoned by Walt in an unknown way with the Lily of the Valley berry. The doctors initially thought he was poisoned by ricin as Jesse Pinkman had suggested but later discovered the true toxin. The doctors and Jesse believed that Brock came across the flower growing naturally and ate some poisonous berries. ("Face Off")
After returning home from successfully killing Gus and getting Jesse back on his side, Walt disposed of the Lily plant from his backyard. ("Live Free or Die")
According to the doctors taking care of Brock, Lily of the Valley is not as poisonous as ricin, and they came to the conclusion that Brock might of thought of them as fruits (as they claimed that it would be easy for kids to mistake the venomous plant as consumable), and slipped one in his mouth, causing him to poison himself.
Now the episode makes you believe that Walter White poisoned him, without actual evidence, because he had the plant in his backyard, and disposed of it after killing Gus. Source: http://breakingbad.wikia.com/wiki/Lily_of_the_Valley
So Walter might of wanted to temporarily have Jesse's attention by making him believe Gus poisoned Brock, thus sort of brain-washing Pinkman to get him onboard to kill Gus, without causing real harm to the child.
Ricin was always Walt's go-to poison. This is becuase it's not at all common and very hard to detect.
As knowlegable as Gus was, it's very unlikely that he'd know about a poison such as ricin.Gus is a business man nad owner of a chicken franchise, not an expert in chemistry.
If it was actually found to be ricin in brock's system, Jesse would have immediatley blamed Walt despite it coming from his own pack of cigarettes. By using Lily of the Valley (stated in the show as a commonly found poison) it's much easier to believe that Gus could be aware of it.
Walter used Lily of the Valley instead of ricin because it has similar attributes to ricin without actually BEING ricin. If it was actually ricin in Brock's system chances are Jesse would have been taken by the FBI.
Also Lily of the Valley has less of a chance of killing the kid so Walt could get Jesse on his side and against Gus without killing the kid (possibly. There's always a chance it could have killed the kid as well. I guess Walt took the gamble deciding that Brock's life wasn't as important as his family's)
Ricin would have definitely killed Broch and this was not what Walt wanted.
So he used a milder stuff just to get the kid ill and then plant suspicion in Jessie's mind. It was not necessary to use Ricin or anything else in particular.
I remember the conversation between Jessie and Walt, where Walt says that
"think Jessie ! Who do we know, who is known to use children for his own benefit" Walt's whole purpose was to relate Broch's condition to the killing of Tomas, Andrea's 11 year old brother who was killed by Gus's men. This was highly effective in making Jessie believe that Gus was involved.
It would have been a lot easier to get Brock to ingest the pleasant looking berries as apposed to somehow getting the ricin into his food. This coupled with the fact that the Ricin would take a little time to kick in and therefore may not have had the desired affects in time to spur Jesse into his reactions means that this was the better option for Walt in this case.