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I just bought some Scotts (4M Snap) Lawn Fertilizer. On the container it says:

Water soluble: No

How does fertilizer that is not soluble in water get absorbed by the root system of a plant?

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3 Answers

up vote 6 down vote accepted

Actually it's a simple question and I'll give you a simple answer too ;-)

With "water soluble" they mean, "able to dilute with water before applying".

In the soil, every fertilizer is probably soluble with water, but fertilizer is not depending on water to be taken up by the roots. Water is for distribution of nutrients like fertilizer and aeration of the soil. But water is not the only part responsible for distribution to the roots.

It's not that the plant just only 'drinks' the water with the fertilizer by it's roots.

The soil is a very complex area of all kinds of mechanisms and organisms. The roots (rhiza) communicate with several organisms to get their nutrients in exchange for the sugars they release. Some of the organisms are fungi and one of them is the mycorrhizae family which is responsible for finding the nutrients the plant wants.

To make it easier for the mycorrhizae to find the nutrients, the water distributes the fertilizer more easily all over the soil and deeper into the soil.

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Aschwin is right with the comment about 'water soluble' meaning it can be diluted in water for application. Some Lawn treatments are water soluble, and should be mixed with water at the correct dilution rate, usually in a can, before application.

The product you have purchased though, works differently. I haven't seen it physically, but from the description, this will be either granular or pelleted, because it feeds the grass over a period of 6 weeks. The way this is achieved is a formulation which breaks down very slowly over that time, releasing amounts of feed as it does so. Take up by the lawn will be through the drinking route, if you like, that is, the solution will be present in any water in the soil in small amounts as the product breaks down, and will be taken up by the roots and thus into the plant.

Obviously, such a slow release product cannot be formulated to dissolve instantly in water, the point of it is to slowly degrade, releasing small amounts of nutrients as it does so.

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Yes there are different rates of solubility. The label refers to dilution speeds (i.e. seconds), but many minerals can take days, weeks, or even millions of years if you're a geologist! –  winwaed Oct 22 '12 at 14:34
    
For sure, Winwaed, but a granular formulation for lawn feed is, nonetheless, not designed to be soluble in seconds or it wouldn't do the job... specifically, the NPK contents will be released over weeks for the plantlets to take up. –  Bamboo Oct 24 '12 at 17:40
    
I think I explained myself poorly, we're in general agreement... –  winwaed Oct 26 '12 at 13:47
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This question has not been answered fully:

How does fertilizer that is not soluble in water get absorbed by the root system of a plant?

A company would have to work pretty hard to find a way to make the N component insoluble in water (apart from sulfur coatings, which is done to make a slow release N, yet the NO3 or NH4 components are still soluble in water after the S is gone). Also, the cheapest potassium and most popular K compound in fertilzers is KCl (potasium chloride), which is salt that is fairly soluble in water like NaCl (table salt). So the only component of fertilizer that isn't soluble is phosphorus, which is because in the cheapest form, its a rock. Rocks don't dissolve in water. Even other compounds of phosphate dissolve only minimally in water. So how do roots take up something that won't dissolve into the soil solution?

Acids in the soil break them down. Rain and decaying organics contribute to the acidity of the soil.

Lime and sea shells are calcium carbonate. Obviously, sea shells don't dissolve in water, yet we use them to add calcium to our soils. Acids in the soil solution or in the rain break them down over time and allow the Ca to be taken up by roots.

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