Do nutrients always flow downward in soil?

Question

I've been reading and watching planting techniques. Many people place fertilizer at the bottom of the planting hole before putting in the plant. The first example is from Osmocote's fertilizer instructions:

When transplanting annuals from flats or when re-potting, mix into the soil and growing media at the bottom of the hole before planting.

The second example is from this guy's guava planting video. He chucks a frozen cut of fish into the hole.

I always thought that nutrients would flow downward. Water can dissolve nutrients, and since water flows downward, the nutrients go along with it. So why then would one place fertilizer at the bottom of the planting hole? There's a chance that the roots can grow so fast that it can chase after the nutrients that are flowing downward, but why take that chance? Why not place nutrients at the top of the soil so it drains down into the roots? Unless, of course, nutrients can flow upward. Is that possible?

Answer 1

It's called capillary action or wicking. The passages between soil particles are usually narrow enough to support this (if they weren't, you really couldn't call it soil). The water in the soil does flow downward during rainstorms or watering, but a lot is pulled back up during dry spells.

Generally you do want to fertilize from the top, because nutrients do tend to spread out more evenly in the soil that way. I usually mix a weak slow release fertilizer with the fill dirt when planting things, so I don't have to worry about nutrient leaching so much.

Also, from the Osmocote's fertilizer instructions, they were talking about planting annuals. These have such small root balls at planting time that the fertilizer at the bottom of the hole is usually quite near the surface, and the plants will quickly put out lots of new roots into the fertilized soil, utilizing it quickly.

From: "The second example is from this guy's guava planting video. He chucks in a frozen cut of fish into the hole." This has to do mostly with convenience, and sanitary reasons. You do not want rotting fish on top of your soil or very near the surface for several reasons: 1) It would smell strongly of rotted fish. 2) This would attract flies and other insects that can carry harmful bacteria. 3) Wild and domestic animals will tent to eat this type of fertilizer. 4) as the proteins break down, more of the soluble nitrates that form will evaporate into the atmosphere. There are possibly more.

Fertilizing at the bottom of the planting hole on larger trees/shrubs is used by many people to encourage deeper root growth. I personally don't think it works as well as top-dressing methods, but I can't find any evidence to prove this.

Answer 2

You are right, fertilisers are taken down into the soil by water, but there is a reason why you put fertiliser in the planting hole. Whenever I plant permanent items such as shrubs and herbaceous perennials, I always put the fertiliser I'm using in the planting hole, usually mixed with the soil at the bottom, and add some to the back fill soil I'll be replacing in a minute. I may also be mixing in humus rich organic material with the back fill soil too, though more usually this has been added when prior digging has taken place.

There's a good reason why this is done; nutrients in the fertiliser are taken up by roots, once they're wet - a granular fertiliser such as Growmore takes 6 weeks to break down completely, which means the plant's roots can access the nutrients more readily whilst they're spreading out and down into the soil, without waiting for fertiliser to percolate down from above. Placing fertiliser in the planting hole is the quickest way to allow your new planting to access nutrients whilst they get established, and it's the only time you'll be able to do that. Future applications of fertiliser, naturally, will be forked or raked into the topsoil around plants, and plants are then reliant on its percolating into the ground to reach the roots.

The exception to this is planting in autumn/fall, when you don't really want to encourage plants to grow too rapidly by adding chemical fertilisers, to avoid the risk of sappy new growth which won't have time to harden off before winter sets in.

As for planting annuals, that's the one time I don't worry too much about adding fertiliser in the planting hole - most annuals are rabid growers anyway, a lot prefer poorer soil, and summer bedding type planting will be receiving a higher potassium feed than shrubs and other planting, often in soluble form.

UPDATE: I watched the guava planting video with interest, I'd heard of this practice but never seen it being done. He's put a fish in the bottom, presumably some that's been in the freezer too long or is out of date, because it will rot down quickly, and in the process provide a fairly quick shot of organic fertiliser to the roots, in the same way that chemical fertilisers would. Better than inorganic fertiliser, because it is humus material, which increases biodiversity in the soil, which in turn makes your plants better able to grow.