Even if you don’t give the plants so much fertilizer, as long their roots are strong, they'll be able to absorb them, so it's fine.
But if their roots aren't so strong, then you need to give them a lot of fertilizer, because they would otherwise be unable to absorb them.
ご質問ありがとうございます。
"Even if you don’t give the plants so much fertilizer,"=「肥料をあまり植物にあげなくても」
"as long their roots are strong,"=「根っこがしっかいしていれば」
"they'll be able to absorb them, so it's fine."=「ちゃんと吸収できるので、大丈夫。」
"But if their roots aren't so strong,"=「でも、もし根っこがあまり強くなければ」
"then you need to give them a lot of fertilizer,"=「肥料は沢山与える必要がある」
"because they would otherwise be unable to absorb them."=「そうしなければ、(肥料を)吸収できないから。」
ご参考までに。