~に足る This is not the same as above 足る.　 The above 足る/足りる means "enough quantity of something". 水は、足りていますか。はい、足りています。Do you have enough water? Yes I do. 水は、足りる？　うん、足りる。　　うん、足る→few occasion. ~に足る is usually used in academic writing. 信用に足る人物。--->信用できる人物。A man you can trust.

足る is hardly used. It is an old fashion like Samurai time's language. So you find 足るmore in proverb. or you hear some old men are using it or maybe used in some regions like a dialect. I do not recommend you to use 足る because it is not easy to use it at the right moment. 足るを知る = knowing enough. In modern time, even though we supposed to have enough food, clothes, shelter,,,,. but we still complain that I need that, need this, this is not goo enough, so on. When you know that you have enough, you will have some satisfaction. However, I believe that it is also important to complain sometimes. :-)

足るを知る 足るを知る者は富む（たるをしるものはとむ） 努力が足りない 水が足りない I think it has the same meaning but do not use 足る a lot on a daily basis....