Calculating:
SIP (Raised Bed)
Potting Mix

The type of soil that you use to fill a sub-irrigated planter (SIP) will have a huge impact on the overall performance of your garden plants. SIPs, aka wicking beds, work much better when they are filled with a water absorbent substrate. Without good wicking (capillary) action, how can you expect the water to make its way upward from that bottom reservoir? At the same time, the growing media also needs to have good pore spacing. Otherwise, the plant roots will not receive adequate aeration.

For this reason, I only ever recommend "potting mix" type components for the growing substrate. This does present some challenges though. In particular, potting mix has become increasingly expensive in recent years. That's why it's absolutely critical that you estimate your potting mix requirements BEFORE starting a large SIP project! People may not realize it, but the potting mix may very well be the biggest individual expense of your entire SIP raised bed. And imagine planning to build half a dozen, large 4' x 8' beds. You need to know what you're getting into right from the beginning!

Calculating the Soil Capacity of an SIP Raised Bed:
• How to Estimate Potting Mix Requirements for SIPs?
• SIP Mix Volume Calculators
• How to Use SIP Calculators
• What to Remember About Mix Estimations

How to Estimate Potting Mix Requirements for SIPs?

Calculating the internal volume of a standard raised bed is extremely easy. You simply need to multiply the length x width x height. Then convert from cubic inches to the volume unit of your choice. But SIPs are a little more complicated. First of all, you need to account for those drain pipes. Additionally, I always make a separate mix for the lower portion of the bed (to ensure maximum wicking performance).

SIP Mix Volume Calculators:

To streamline the process, I've developed some easy to use volume estimators. These two potting mix calculators, will help you to anticipate the internal capacity of your SIP wicking bed. These estimates are based on the general Albopepper technique, in which the water reservoir is constructed using 4" corrugated, perforated drain pipes.

How to Use SIP Calculators:

INNER MEASUREMENTS: The top section of the mix calculators is pretty straightforward. You simply need to plug in the INTERNAL length, width and height measurements. I typically suggest building SIPs with an internal depth of 16.5 inches. This depth is then broken down into two discreet subsections. The bottom (reservoir) section would be 4.5 inches devoted to wicking mix. This leaves you with exactly 12 inches for the upper portion of standard SIP mix.

DRAIN PIPES: What should you plug in for the "Drain Pipes" section? For an accurate calculation, you need to be using 4" (internal) corrugated drain pipes. If each pipe segment is identical in length, you can simply input the length of one of the pieces. The enter the quantity of pipes. If you have various lengths of pipes, you can enter the TOTAL length (combined) of all pieces. Then put in a quantity of 1.

All measurements are based on US measurements (inches, feet & gallons). If you are using the metric system, you can input the number of centimeters and the "cu in" will really mean "cu cm". However, you need to do your own conversions if you want to know the liters, etc. For the bottom layer, which is filled with "wicking mix," It is assumed that you are using standard 4" pipes. These actually have an outer diameter of about 4.5 inches. But they are corrugated, so I use an average diameter of 4.25 inches to estimate how much volume gets displaced by pipes.

What to Remember About Mix Estimations:

Firstly, my calculations on the wicking mix are just rough estimates. I used an average outer diameter for the drain pipes, and then used the basic formula for the volume of a cylinder. This is not 100% exact.

Another important thing to keep in mind is that potting mix settles over time. Even when you first install your SIP, you can fill the potting mix with various levels of compaction. This will obviously impact the final volume occupied by that mix. For reference, I don't aggressively pack in my mix. I throw it into the planter and level it out with a bow rake. I'll lightly tamp it in a little. Then I'll top-irrigate the mix using the shower setting on my garden hose.

Plants need good pore spaces for their roots to get oxygen. So it's not helpful to over-compact things. At the same time, if the mix is too loose, it may not have the capillary properties needed for wicking.