Add a small handful of crushed coral to your filter if it has baskets. I did this with my 75G, and it took almost 3 weeks before I started to see it register. It's the safest method to increase your kh, as it increases very slowly over a long period of time. Replace the CC about every 3-5 months by removing what may be left, as it does disolve. Or just add new to what's in the filter. Either method is fine.
If using baking soda, it must be added with each PWC, and if you don't get the dosages correct, or accidentally do too much, the kh swings could result in deaths. I don';t like that method myself, but others use it pretty effectively. I personally prefer the more natural approach with crushed coral.
As for benefiting plant growth, it won't do alot for plant growth. What does affect growth is CO2, nitrates, phosphates, potassium, and all the micros.