Dealing with Water-Retentive Soils

I posting this message on a few forums in the hope that those who read it will feel it merits sending growers struggling with water retention issues in containers over for a read as you come across them in your forum adventures.

Al

Karen

anyhow great effort to inform others.

len

tj

Most of the replies are pretty funny though, apparently you are going to have to make more appearances on this forum (you have been gone too long) lots of people don't know about your vast (and may I say...awesome) knowledge ,when it concerns Containers... you may have to do another primer on the basics of container gardening.

Jolj - did you miss that I was referring to container soils?

Al

Mums and hydrangeas can grow side by side in the same substrate and flourish. You may need to adjust the pH of the substrate (soil) solution for the hydrangeas by acidifying the fertigation solution if you want them blue, but other than how you approach nutrition, both will respond exceptionally well to a porous, free-draining substrate in any size container ..... with the proviso added that container size does not cause root constriction to become a limiting factor.

One of the things that we take into consideration when we choose a substrate is that it must provide support for the plant. This actually has more to do with the age (state of lignification) and volume of the root mass than it does substrate choice. Potting up into too large a container produces toppling not because of any substrate shortcoming, but almost always because the roots haven't sufficiently colonized the soil mass and/or the roots are still unlignified/succulent.

"Commercial greenhouse ranges are unique in that they are watering and fertigating far more freely than a homeowner would, they make their mixes good for their growing conditions but not necessarily good for a homeowner."

Commercial ops water as infrequently as possible w/o inducing drought stress, so - on an 'as-needed' basis. 'As-needed' is dictated by substrate, temperature, humidity levels, and growth rate of the plant material. What you said may be true if taken in a 'real time' sense, because greenhouse conditions are usually (close to) perfect; plants actually grow and USE water/fertilizer faster than in the natural environment where they will soon reside. Hopefully the idea and measure of real time fertigation can be discarded and replaced by the end-grower with the same approach the greenhouse ops use, which is the aforementioned 'as needed'.

Al

Size range of media particles AND uniformity are major players in determining aeration and drainage characteristics. For example, mixing pea stone into a commercially prepared peat-based substrates does nothing to promote drainage or aeration unless it becomes the primary fraction of the soil. Where the peat, or in some cases coir or compost fraction of the soil has a tendency to compact, sand, depending on its size, may or may not improve drainage, but because most tend to use play sand or builder's sand in this application, it is highly likely that any improvement in flow-through rates will be accompanied by reduced air porosity, increased water retention, and an increase in the ht of the perched water table. None are favorable contributions to the properties of a substrate that is already struggling with aeration and drainage - or there would have been no need to amend the soil in the first place.

Al

From another of my posts:

Obviously, I think the grower's soil choice when establishing a planting for the long term is the most important decision he/she will make. There is no question that the roots are the heart of the plant, and plant vitality is inextricably linked in a hard lock-up with root vitality. In order to get the best from your plants, you absolutely must have happy roots.
If you start with a water-retentive medium, you cannot improve it's aeration or drainage characteristics by adding larger particulates. Sand, perlite, Turface, calcined DE ...... none of them will work. To visualize why sand and perlite can't change drainage/aeration, think of how well a pot full of BBs would drain (perlite), then think of how poorly a pot full of pudding would drain (bagged soil). Even mixing the pudding and perlite/BBs together 1:1 in a third pot yields a mix that retains the drainage characteristics and PWT height of the pudding. It's only after the perlite become the largest fraction of the mix (60-75%) that drainage & PWT height begins to improve. At that point, you're growing in perlite amended with a little potting soil.

You cannot add coarse material to fine material and improve drainage or the ht of the PWT. Use the same example as above & replace the pudding with play sand or peat moss - same results. The benefit in adding perlite to heavy soils doesn't come from the fact that they drain better. The fine peat or pudding particles simply 'fill in' around the perlite, so drainage & the ht of the PWT remains the same. All perlite does in heavy soils is occupy space that would otherwise be full of water. Perlite simply reduces the amount of water a soil is capable of holding because it is not internally porous. IOW - all it does is take up space.

If you want to profit from a soil that offers superior drainage and aeration, you need to build it into the soil from the start, by ensuring that the soil is primarily comprised of particles much larger than those in peat/compost/coir, which is why the recipes I suggest as starting points all direct readers to START with the foremost fraction of the soil being large particles, to ensure excellent aeration. From there, if you choose, you can add an appropriate volume of finer particles to increase water retention. You do not have that option with a soil that is already extremely water-retentive right out of the bag.

I fully understand that many are happy with the results they get when using commercially prepared soils, and I'm not trying to get anyone to change anything. My intent is to make sure that those who are having trouble with issues related to soil, understand why the issues occur, that there are options, and what they are.

Al

Steve

Al