Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If your pond is the enclosed rubber type of liner or the hard plastic 'preformed' pond then the fertilizers on the outside may find their way in when it rains so hard the run off ends up in the pond, flowing across the surface of the soil, but ferts cannot leak through the rubber or plastic liner.
Some water-borne materials can seep though porous concrete and other masonry products, but you would notice water leaking the other way: The pond would leak. Even if there are no cracks, just an unsealed masonry surface can allow water (with dissolved materials) to move either direction.
A natural pond, open to the soil can certainly be polluted by below grade water movement carrying fertilizers usually down, but sideways, too. The worst offender I hear more about than any other is Nitrate. In agricultural districts where KNO3 and other nitrates are used to grow pretty much everything the wells test 10 ppm NO3 or more.
If you are worried about the dry ferts we use in aquariums (Agricultural products) finding their way into your pond, why not use organic products like bloodmeal, bonemeal, greensand and similar products. They become part of the soil micro-ecology and get bound up via the actions of microorganisms in the soil.
Keep the soil loose, and mulch heavily with bark, coarse compost or similar material.
Minimize foot traffic on the soil. Put flagstone where you know you will need to walk, and stay on the path. This will reduce soil compaction so when it does rain the rail will soak into the soil, not run off. The improved air movement through the soil will benefit soil microorganisms as well as the plants, and this will contribute to the better use of the natural fertilizers.