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Increasing Soil Water Retention

Tony Callaghan 22/08/2019

Increasing Soil Water Retention

Improving soil water retention - what to add and why

Gardeners have traditionally relied on peat to improve water retention in soils. Peat is extracted from peat bogs, which take thousands of years to regenerate. Once extracted the peat decomposes quickly releasing the harmful CO2 that was trapped back into the atmosphere. There is now widespread acceptance to end all peat use asap.

There are many "water retention" improving products we can use, below we look at these and summarise their performance. 

There are two properties of soil-water we look at:

  • Water holding capacity - how much water (in g weight) can the soil/additive hold before it floods and water runs off.  This is easily understood as a multiple of the dry weight, e.g. sand does not hold water x1.0, colloidal humus holds lots x9). 
  • Water retention time - how long can soil/additive hold onto water under drying? This is usually measured in hours at a given temperature (e.g. 105 C). This can vary from a few minutes (e.g. sand) to 10 hours (e.g. colloidal humus. (10 hours at 105C equates to a soil that will not dry out under normal soil temperatures). ) This is one of the "laboratory" tests you can do at home if you wanted to check for yourself.  

We can broadly put the soil water retention enhancers into the following groups: 

  • Composts (cheap and easy to make - but not that good performances wise)
  • Colloidal humus (fantastic performance, natures wonder product, but hard to make)
  • Biochar (good, but currently still on the expensive side)
  • SAP gels (excellent, but artificially made)
  • Biochar composts (eg SF40, SF60) - the way forward!
  • Mulching (only a partial solution for certain areas)
  • Peat - no longer environmentally acceptable to use


Remember not all composts are made equal! There are huge differences between "manure and well-rotted manure", immature compost and a PAS100 certified mature compost, a compost containing a lot of woody material and one that does not, between a biochar-compost like SF40 and SF60 and PAS100 composts. (You can read a little more on composts at this blog)

It might surprise you to hear that your basic green waste compost (eg PAS100 certified and homemade compost) are not that good at water retention.  Typically they only have a water holding capacity of twice their dry weight (WHC x2) and make things worse they have really poor water retention times of typically 2 hours. This means they dry out very quickly when exposed on hot sunny days. To put this in perspective Caly is X2WHC and 16-30 hours of retention time. Sandy soils have 0 WHC and dry very quickly.  

This goes against the traditional advice of adding compost to improve soil structure and water holding. But when we look at our experience we can back up the science.  Most of us know that hanging baskets and containers using just compost (or peat) dry out really quickly - way faster than soil we end up watering them every day.

Let's look a little deeper at what compost is made of. Typically it is a tiny fraction of colloidal humus (wet and sticky) and a large fraction of small particles of degraded organic matter (POM) which is often dry and crumbly almost woody in nature. Composts vary hugely - try doing the splay test between your fingers - some are 'dry friable, almost woody' and others sticky dark brown. The POM part of compost is not good at holding water.

Try your observational test: add lots of compost to a patch of your soil (20%, 1 part compost to 4 parts soil). Over the summer check to see if the patch becomes very dry and loose (friable) versus the surrounding soil. Does it seem to dry out much quicker after watering?

We have tested many composts for their water-holding properties. Although they vary hugely,  green waste PAS100 tend to dry friable and lower WHC, whereas food waste and manure composts are around 10%. SF60 made using our patent-pending method plus humification agent is above 20% colloidal humus.  

Pie charts ration humus to compost

Colloidal humus

Colloidal humus is very good at holding water - X8 own weight and it holds onto it for days rather than hours.

Our best advice: added very mature sticky humus to improve water holding. If you want to know how to make compost with lots of colloidal humus follow our humification article.

SF60 made using our patent-pending method plus humification agent is above 20% colloidal humus. The next best is SF40 - a mixture of biochar and compost.  


Biochar is widely promoted for its beneficial water retention properties.  Yet, when you undertake very simple tests on biochar - its water-holding capacity is x2.5 and its retention time is 2-3 hours at 105 C - on par with compost and other soil components. 

Check yourself:  weigh a few grams of biochar out and drop them onto a paper kitchen towel. Flood with water. Leave excess to drain off. Re-weight the granules to give the WHC. Now leave the grains in the sun for an hour - and weigh again. You will notice they quickly dry out. 

However, the massive amount of scientific data proves soil with biochar has an improved WHC and resistance to drying. We have proposed the following: biochar has a huge affinity to associate with colloidal humus and microbes. When this has happens soil demonstrates improved water holding. It is the colloidal humus that increases WHC. It is the ability of biochar to foster colloidal humus production and survival in the soil that delivers water-retention improvement. 

Biochar-Composts (eg SF60 and SF40)

We are now seeing the whole biochar industry follow leaders such as SoilFixer and Sonnenerde in Austria who both add biochar into the composting process to make better compost. You get better performance and the best balance by using less of expensive biochar. 

Don't forget - Mulching

Adding layers of compost (or leaf litter) on top of the soil creates a thick cover that prevents the sun's heat and wind from drying out the soil below. They do not change the soil - they protect the water in the soil from drying.  

What about SAP gels

Water gel (sometimes referred to as superabsorbent polymer (SAP), is made from a water-absorbing polymer such as a polyacrylamide (frequently Poly(methyl acrylate) or Sodium polyacrylate).

They are powerful colloidal agents (x10 WHC). They work exceptionally well in "sterile" growing media (e.g. coir). But we cannot get away from it - they are artificial man-made polymeric (some would say plastic) chemicals being added to our soils. There is evidence they break down in hot composting. There is no evidence (at this early stage) that they will cause a long-term problem. Do we need them when we have natural organic colloidal humus?  Much better to educate gardeners on how to make organic biochar-colloidal humus ..... (aka SF60)

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