Water Absorbent Polymers

What are polymers and what are water-absorbent polymers?

A polymer is a string of repeating molecules that forms a long chain. DNA is a polymer, as are starches, proteins, etc. For example, casein – the protein that makes cow’s milk – is a polymer. About 30 years ago the plastics industry was looking to change the negative image people have of the word plastics, and pick up on a friendly sounding name to confound the public so they started calling a wide-range of their plastic products ‘polymers’.

Water-absorbent polymers (also known as hydro-gel, water crystals, super absorbent polymers, etc) – are simply a type of plastic that possesses some unique water absorbing qualities.

What makes this polymer water absorbent is the presence of sodium or potassium molecules that form bridges between the long hydrocarbon chains. These bridges – known as cross-linking – enable the polymer to form into a huge single super-molecule (desirable for a number of reasons), including its ability to degrade in the environment and break-down into simpler molecules, and hold significant amounts of water. The polymer crystals that you purchase, whether the size is small, medium, or large, will always be a single molecule (making it very difficult for you to chop up large crystals into smaller crystals – try it in a coffee grinder sometime.)

All water-absorbent polymers are cross-linked, and cannot work if they are not cross-linked. It is often written on labels to make it sound like a “feature” – don’t be tricked into paying more for polymers that are labeled as being “cross-linked”.

How are they used?

Water absorbent polymers (hereafter referred to as just polymers) can be used for two purposes: to store and hold water to add an extra few days between watering; or alternatively, to protect your plants from over-watering – especially if they are planted in an area that tends to pool water.

Are there different types of super-absorbent polymers?

There are about 800 to 1000 different recipes for these polymers – but they are divided into two big categories: polyarcylamide and polyacrylate.

Polyacrylate (called in the industry PAC) are used in disposable diapers, sanitary napkins, etc. and are capable of holding a huge amount of water – between 600 and 800 times its weight (purity of the water determines this range – the more dissolved solids in the water, the less liquid the polymers can hold).

Polyacrylates are usually made with sodium and are more environmentally friendly, breaking down first into ammonia salts and then nitrogen and CO2 in about 4 to 6 months. They are often sold with an environmentally friendly green label and retail for around $10 to $12 per pound.

In contrast, polyacrylamides (often known as PAM) absorb only about 300 – 400 times its own weight in water, use a variety of potassium molecules for cross-linking, and take between 5 and 7 years to completely breakdown. Because of the lower absorbency and longer time to breakdown, polyacrylamides usually sell for around $6 to $8 per pound.

Do polymers affect the taste?

Do water absorbing polymers affect the taste of finished marijuana? I experimented with the use of polymers on my canola farm over a period of years, and have grown cannabis in extreme drought conditions using these crystals.

Growers have reported that these polymers adversely affect the taste of the finished product. Many others have reported no impact on the taste of the quality of the smoke. This debate and the difference of opinion is due to the type of polymer used.

Polyacrylates will negatively impact the taste of cannabis. As noted, polyacrylates take approximately 4 to 6 months to completely break down – with intermediary chemicals including ammonia salts, nitrogen (nitrites), and CO2. This breakdown usually happens when most growers are flushing their plants – and it is the plant absorbing this residue that negatively impacts the taste, including a variety of sodium by-products.

In contrast users of polyacrylamides – which take 5 to 7 years to break down – will not be introducing intermediary residual chemicals to their plants, and will not notice any impact on taste or quality of the smoke.

Given a choice between the two – it is strongly recommended that growers use the less absorbent polyacrylamide.

How do I tell if the polymer is polyacrylate or polyacrylamide? (if there is no label)

If you cannot buy from a recommended online supplier, and are buying locally, look for these four clues to distinguish products: absorbency, price, “environmental friendly”, and a “safe for food crops” label.

Polyacrylates (the one you want to avoid), report absorbency of 600 to 800 times its weight, cost around $12 Cdn per pound, and almost always have on the label “Environmentally friendly”.

In contrast, polyacrylamides are labeled as absorbing 300 – 400 times its own weight, are priced at $6 – $8 Cdn per pound, and may have a note on its label that reads “Safe for Agriculture and Food Crops”. Only certain polyacrylamides (and very few polyacrylates) can be labeled as safe for food crops.

What size do I buy?

Avoid the powder, it is difficult to mix thoroughly. Likewise, the large size is also difficult to work with and to mix thoroughly with the soil. Both tend to cause the roots to clump up and bind around the polymers rather than grow outwards and search for water in the natural soil environment. Improper use of either of these sizes actually diminishes performance characteristics.

If you are growing indoor with a soil mix (perlite, etc), in fine sandy soil, or where water pooling is a problem, use the small grade as an amendment.

If you are growing in heavy soil or in drought conditions use the medium size. Why? In sandy light or fast draining soils you are most likely using the polymer to slow the rate of water drainage and you want to maximize the probability of water contacting the polymer and being absorbed.

In heavier soils and in drought conditions the medium grade is better for several reasons. One characteristic immediately noticeable when growing in drought affected areas is that the soil tends to harden and compact. There are several reasons for this – earthworms and other insects are not living in the top couple feet of soil and do not work and loosen the soil; plants can’t live naturally in drought, and the absence of a good root system causes the soil to compact; and finally, the heat itself causes the soil to expand slightly and fill the gaps that might have once existed.

Because the medium size polymers expand rapidly when they absorb water and contract when they release the water to the plant or environment – this expansion and contraction helps work and loosen the soil and promotes good root growth in what would otherwise be a very difficult environment.

How much do I use?

Consider that one cup of dried polymers (250 ml) will absorb 100 liters of water (about 24 US gallons). This would likely launch your plant out of its hole in a matter of minutes!

That said, an appropriate amount will be significantly less – between a teaspoon and a quarter cup of crystals. It all depends on your environmental conditions. In drought situations (no rain for more than 28-days), you want a lot of polymer crystals to absorb any water they come in contact with – and it is unlikely that any single polymer will absorb its maximum potential. A couple tablespoons to 1/4 cup of polymers is recommended for this sort of environment. In more “normal” meteorological conditions – a rainfall every seven to ten days – your objective is likely to minimize stress between rainfalls – and one to two teaspoons would be the more than enough.

How do I use them?

The optimum use involves some experimentation for your soil and growing conditions. As a general rule of thumb, this method worked well for the FAQ author in extreme drought conditions (no rain for 60-days). First – make a deep hole – at least 3 feet (80 cm). An 8-inch posthole auger is very good for this as it completely eliminates the back-breaking work of shoveling – and if you are lucky enough to use a power auger – it will be very fast too.

Back fill about 6-inches (15 cm) into the hole and sprinkle a tablespoon of medium-grade polymers over the soil. Continue backfilling another 6-12 inches and gently work the polymers into the soil. Make the soil wet with a couple liters of water and wait a few minutes. This will allow the polymers to absorb the water and expand.

In the next 12 inches (30 cm) add any amendments plus another tablespoon of polymers – pack the soil hard and when finished add another couple liters of water. With the final soil – add your soil mixes and at most a third application of polymers – about a half teaspoon maximum, moisten with a liter or two of water, and insert your plant. Do not fill your hole to ground level with the soil – you will need to leave at least a couple inches (5 cm) for expansion and heaving. The slight depression also acts as a natural pooling for water in the environment – which is important in drought environments.

As an added drought protection measure, use a brown grocery store paper bag as a liner for the hole (and mix the third half teaspoon of polymers in the soil in the paper bag). The paper bag slows the drainage rate significantly – and takes about a month or two to dissolve, in time to let the roots reach the deeper levels of polymers.

How many days will I be able to go between watering if I use polymers?

There is no real answer to this question because it depends on the environment and the plants themselves. In hot dry environments, the plants will transpire and loose moisture rapidly as compared to more humid dry environments. Dry wind also has an impact on the rate that plants transpire. Evening and night temperatures also impact the length of time that the polymers can hold water.

It is best to experiment and observe your plants, but as a general rule of thumb, if used correctly, the polymers should buy an extra three to five days between when you would normally water.

I once went 21 days between heavy watering in a severe drought situation. The plants with polymers, while definitely heat stressed, did not die; whereas plants without polymers did not survive this 21-day period of neglect.