Soil drainage is important for many plants, especially those that like an open soil with lots of organic matter. For other plants it isn’t as important, but can still make them more content if they are suffering from too much water infiltration.
There are several ways to improve the natural drainage of your substrate (soil or compost). The best way to do this is to mix some coarse sand into the top layer (2-3 cm) and then dig it in lightly by mixing it together with some potting soil again. The sand will act as small pieces of gravel in the drain holes of your pots, making sure that all excess water has somewhere to go before it causes problems. You could also use finer sand or sifted compost instead if you don’t have any coarse sand available.
If you prefer a more natural option you could add some leaf mould or small bark to your top layer. This will help create a nice habitat for the tiny animals in your substrate and will also provide some protection from the elements while simultaneously improving drainage. In fact, lots of tiny holes in the soil are very important for air to get inside, which is vital for healthy plant growth.
Steps to improve your soil drainage
There are three steps you can improve porosity:
Clay soils are heavy because they have microscopic particles which are very smooth and therefore do not allow water to drain very well. Colloidal clay, or “biosoil” is a type of clay which has been processed so that surfaces are roughened up by microbial action– this makes them much better at drawing in water because the surfaces have less points of contact between themselves, but still retain enough surface area to form strong hydrogen bonds with water molecules. In order to achieve optimal porosity it’s best for biosoil to be used undiluted, mixed into soil as a slurry first before being added. Other alternatives exist but all require thorough mixing with existing soil constituents instead of just dumping the stuff on top and calling it good like you see sometimes in garden centres (they’re only interested in selling so they don’t care whether it actually works).
Adding coarse sand or gravel
Gravel and coarse sand both have very large surface areas compared to their volume, so water molecules clump together on the surfaces and flow slowly downhill like a glacier instead of trickling more quickly through smaller pore spaces. This is why putting rocks (big ones!) in your pot increases drainage: there’s always room for another stone at bottom of pot because of increased surface area of soil particles. Coarse sand can be made from brick fragments ground up with mortar mixer. Make sure it doesn’t get stuck in carpet fibres though by moistening first before putting under vacuum cleaner nozzle– you’ll know what I mean when do this! Use sparingly though.
Reducing depth of pot
This is the easiest method, but also the most work because you’ll have to repot your plant more frequently for a few weeks until it gets used to the new conditions. The main idea here is that container depth must be reduced relative to its diameter at top– this makes soil arrangement similar to a flag pole standing upright with no flags attached. Water will pool at bottom near rootball and still evaporate slowly instead of running off like water down a slope: this increases porosity while retaining necessary pore size gradients which plants need (e.g. xylem tissue in roots and phloem in leaves). Planting deeper than normal therefore causes drowning issues while not planting deep enough results in low porosity. Note however that soil depth should never be so shallow that roots are exposed to air at surface! The benefit of this method is the intensive pore space created where water accumulates, which promotes root growth and branching. One downside is that plants must be watered more frequently during times of rapid transpiration because there’s a greater direct path for water loss from container, but overall it’s a better method than other choices if your plant tolerates frequent re-potting while still growing well.
Organic matter doesn’t improve drainage rate by itself, but forms strong hydrogen bonds with water molecules when fully decomposed, helping them to stay in place instead of breaking free and running downhill through soil pores (known as “hydraulic conductivity”). This is why we add organic matter (manure, compost, peat moss) to soil when transplanting: it increases long-term retention of water for plants by binding excess in place.
Perlite is a type of volcanic glass with very large pores and therefore great ability to trap air when mixed into soil– this traps space in the pot which would otherwise be filled with water. However, perlite has a low cation exchange rate so doesn’t improve nutrient uptake at all.
What is drainage
First, you’ll need to understand why technically-speaking, “drainage” is bad for roots. It’s technically known as the “relative air humidity surrounding plant root systems”, or simply put; too much air around roots causes them damage. There are two reasons for this: 1) oxygen kills cells, which plants can’t afford to die (except when they do it on purpose like in autumn), and 2) lack of water causes wilting (and other dangerous problems). When plants’ roots are surrounded by too much air, they dry out and suffocate. Sometimes, roots actually need to be a little bit in the air because certain plants have evolved to exploit that by growing aerial roots which absorb oxygen from above ground where there’s plenty of it. Examples include strangler figs, asparagus ferns, banyan trees and many others.
In order to improve drainage (i.e. reduce relative humidity), you need to “improve porosity”– allow more water through the soil so it doesn’t just sit there, but can flow on through into lower layers of soil until it hits an impermeable layer like rock or clay at the bottom of your pot where excess water accumulates instead of draining away with gravity.
Know your soil
When you water your plants, the most important thing to know is that there are two kinds of soil: airy and heavy. Airy soil has good pore space for water to flow through easily. Heavy soil doesn’t, so when excess water accumulates in it, it stays there until you irrigate again. All heavy soils have a certain amount of pore space available to them which can be filled up with water before they become saturated (aka “wet”)– but only temporarily! If you do nothing about it, eventually enough will evaporate out of the soil due to high humidity around roots or lack of sunlight hitting ground that all that is left will be clay or rock at bottom of pot with no open air at all. Airy soil, on the other hand, will either be dry after a rainfall or irrigation event because it drains quickly and never becomes wet to begin with– this is called “voids” in water retention terms because there’s nothing but open air where pore space could’ve been.
Now that you understand differences between “airy” and “heavy” soils, you probably know what kind of soil yours is already. If not, the following guidelines should be able to help:
1) Heavy soil needs improved porosity. If your pot always feels heavy when you pick it up even if there’s no visible water pooled underneath then it has poor drainage and should be improved before damage occurs to roots (see below).
2) Airy soil will either be dry or wet, so take care when watering. You can recognise it because plants in airy soil tend to be less stressed out by irregular watering– lack of water for a few days won’t hurt them that much.