Hello!
I am one of the people struggling with this.

It is because, if your plants get too dry, you can "burn" the roots with chemical fertilizers diluted in water.

At least, that is my understanding. Whereas, if you don't let your plants get that dry, then you don't have to worry and you can "fertigate," which is watering and fertilizing all at once.

But I hope Al or someone will answer to confirm.

What I do makes very little sense, since I use fish emulsion, which I think cannot burn ... but I keep watering first anyhow. (I am careful because at some point I will probably start using something synthetic, so it is a better habit. And I don't always water as often as I should.)

Short version: elevated levels of fertilizer salts in the outside soil solution suck water through osmosis out of the roots or in the least prevent water from penetrating into the roots. Roots shrivel and die, which affects the health of the plant above the soil line.

Fish emulsion can still burn, but the risk is lower compared to quick-release chem ferts.

Fert burn is more of a concern with full strength quick-release dry ferts or liquids. That's why lower qty/higher frequency generally works better and minimizes burn risk.

Gritty pots are served better with CRFs than Foliage Pro in my opinion. I still FP my Gritty pots with light dosings, but realize you may literally be throwing expensive liquid fert right down the drain given the porous nature of Gritty. When I fert Gritty with FP, I often collect the drainage water and reuse....double dip saves money.

They call it the algal bloom mix in Ohio.

Thanks, Oxboy!!! I didn't know that. Good to know.

I'm sure if Al wants to chime in, he'll give an awesomely thorough explanation about fertilizer burn.

Never miss a chance to get in a jab, eh, Drew?

The reason you're admonished not to fertilize dry plants is because as the moisture level of the soil goes down, the concentration of salts in the soil solution goes up. Adding more salts to what's already retained in the soil can cause plasmolysis (fertilizer burn) or make it exceedingly difficult for water to move into plant cells. I don't worry about those conditions when using the 5:1:1 or gritty mix because when you water you're regularly flushing from the soil any accumulation of salts that might cause problems were they added to the concentration of the fertigation solution. Where you DO need to be careful about dry conditions is when fertilizing soils you're forced to water in sips because they retain excess water if you don't. These are the soils that inspired the admonishment I mentioned above.

Al

I like to pre-water lightly to moisten the top inches of mix, then I fertigate, and then I hit the plants once more with a light post-watering to rinse the lower leaves and to drive the fertilizer a bit deeper into the mix.

Josh

Thank you Al.

For plants that are in a dense soil, is this still a concern if you flush the soil regularly?

When you talk about watering in sips, do you mean until the soil is saturated but before a significant amount comes out the drain holes, or a lesser amount than that (just enough to wet the soil, for example)?

Also, are only certain components of fertilizer retained as salts in the soil? In other words, is it mainly nitrates or one of the other components that stick around, or do all the components stick around in proportions similar to what they were in the original fertilizer solution?

Watering in sips means that the soil retains too much water and so one must water sparingly in order to avoid rotting the roots by oversaturating the root-zone. Watering in sips is also a technique used to moisten those dense soils that have become hydrophobic.

The excess salts in a soil are those that the plant doesn't use....since we know that most plants use nutrients in approximately a 3:1:2 ratio, if one were to fertilize with a 1:1:1 fertilizer, the excess would predictably be P and K.

Josh

Salt buildup is definitely more of a concern in dense soils. If you water with hard tap and fertilize regularly, flush once a month at least from spring to fall. Flushing with rainwater, RO or distilled is preferred but not necessary if you're thorough.

Re: sips -- the latter. Best practice is water until 10-20% of the irrigation water drains out the bottom. I find about 80% comes out the bottom of my Gritty pots, which is fine and what I want. That said, there can be a fine line between watering deeply and leaching too much. The balance is keeping the right amount of nutrients in the soil solution near the roots -- not too little and not too much. This is how it works in nature.

In outside garden soils, some nutrients leach out readily (ex: nitrogen) and some tend to bind up and hang around (ex: phosphorous). In Gritty and 5-1-1, it's safe to assume with a final pH around 6-6.5 that pretty much all of the nutrients are either taken up readily or washed out pretty quickly. Hence the point of a) using a fert like Foliage Pro or Dynamite/Osmocote Plus that contains ALL macros and micros and b) using them on a light, but consistent basis. I've seen some newly installed plants in 5-1-1 start suffering from N deficiency inside of 2-3 days post install.

All my life I have been told " never fertilize a dry plant" , I believed it without the explanation. Calistoga Al