how efficient is conversion of nitrate or phosphate fertilizer to plant biomass?

when we put 1kg of the fertilizer into the soil, should we expect to get additional amount of biomass containing approximately that amount of the substance? Or will a great deal of it get pointlessly destroyed by denitrifying bacteria, carried off by the water and otherwise lost? If fertilizer were to become more expensive, could this conversion efficiency be significantly increased? E.g. maybe could the denitrifying bacteria be suppressed?

Runoff certainly results in loss of the fertilizer. This is a reason for avoiding quick release (water soluble) fertilizer, and can result in less of the fertilizer you put down being available to your plants.

I don’t believe a significant amount of the fertilizer is destroyed by bacteria, though. Remember that fertilizer is mostly providing Nitrogen, Phosphorus, and Potassium, which are elements, so they can’t be literally destroyed biologically. In the case of Nitrogen, plants can’t use Nitrogen in the form of N2, like what’s present in the atmosphere. If the bacteria was converting it back into N2, then it would be essentially “destroyed” as far as plant use, by why would bacteria have a process to do that? They need fixed Nitrogen too, not N2, and any Nitrogen they consume will still be in the soil, usable when they die. And if they consumed the water soluble fertilizer from the first paragraph, they’ve preventing that from being lost in runoff in the meanwhile.

Organic fertilizers will often state how much nitrogen is immediately available as soluble nitrogen, and how much non-soluble nitrogen is available over time through the action of soil microbes, so in that case, the bacteria or other microbes are actually making the nitrogen available to plants, rather than destroying it. The non-soluble Nitrogen is much less susceptible to loss through runoff.

Some plants, notably clover, have nodules in their roots with bacteria that are capable of fixing atmospheric nitrogen, and converting it to a form usable by the clover.

with root fertilization in soil you have to oversupply it. the minerals are distributed in the soil, only that which gets to the root is used.


the denitrifying bacteria are discussed here Denitrifying bacteria - Wikipedia . The article does not give any numbers about their effect.

By way of rant, this illustrates a pretty common situation with “popular science” (or, for that matter, “popular politics”) where they talk about some interesting process but give no meaningful numbers to help the reader put its significance in perspective.

I think you have a basic misconception, or two, going on here. The mass of a growing plant is not taken up from the ground, it comes out of the air. Water and soluble nutrients are taken up from the ground but the contribution to the dry mass of the growing plant is very small.

If you have a large tree growing in your yard that weighs several tons, it did not suck several tons of matter out of the ground. The tree has used a small amount of water borne nutrients and converted carbon in the air to plant mass.

Soluble chemical fertilizers can be taken up directly, although a lot can be lost leaching through the soil and the plant will only use a certain amount at a time so much of it is lost and has to be reapplied.

Organic fertilizers cannot be used directly by the plant, the fertilizer feeds the bacteria in the soil which grows and dies and the waste products of these bacteria become the soluble nutrients that are utilized by the plant. If you start with sterile soil and sterile organic fertilizer you aren’t going to grow much of anything.

A most useful clarification. But, this bit is actually wrong, isn’t it? That tree would have sucked many tons of matter out of the ground, evaporated all but a tiny bit of it, and built most of itself out of other things it got from the air.

The tree will have used a lot of water, but the contribution of the nutrients, minerals, etc. from the soil, to the mass of the plant is quite small. You don’t see trees sinking into a hole in the ground created by the matter being sucked up into the tree. With proper addition of water and a small amount of nutrients you can grow a tree on a rock.

In basic photosynthesis a CO2 molecule is drawn in from the atmosphere, then split and the O2 oxygen is expelled and the plant captures the carbon and this carbon is what makes up the bulk of the plant. The nutrients, water and sunlight are the engine that fuels this process of constructing the plant out of carbon from the air, but add very little to the overall mass/dry weight.

The mass/weight/dry matter of a plant comes almost totally out of the air.


there is a useful saying which, frankly, I myself often don’t follow: “don’t deem people around you dumber than yourself”. Or at least try not to do so too much.

If you read my OP, you will see:

the THE is my edit in this here post for the clarification. By substance I mean the nitrate/phosphate/whatever substance, not the generic biomass. As you may have noticed, other people responding in this thread were quite able to decipher that. And yes, I am aware of the fact that biomass is mostly stuff synthesized from CO2 and H2O, as are most other people on this forum. Thanks for the update.