Re: How can I measure the water retention in soil?

Dear Jenna,

Thank you for your message.  Soils are very important for plants and 
other organisms as they provide them with nutrients and water.  The 
amount of nutrients and water can vary greatly between different soil 
types and this can lead to big differences in the plants and organisms 
that like to live in these different soils.  In my lab, I have a machine 
called a pressure plate extractor in which I put wet soils and then the 
machine sucks the water out of these at different strengths.  If I put it 
on a low setting more water is retained in the soil than if I put it on a 
high setting.  I can also see if water is retained better in different 
soil types by using the same setting for both soils.

An important term in soil science is ‘field capacity’.  This is the 
amount of water that is retained in soil when gravity has removed the 
rest (you can think that gravity acts in a similar way to my machine 
by ‘sucking’ water down out of the soil).  You can measure the field 
capacity of a soil by using a simple method called the flower-pot method 
which I will describe:  

Take the two soils in which you want to measure the water retention (one 
with recyclable materials and one without) making sure they are dry - you 
can leave them sitting out for about a week to make sure - and do not have 
any large stones or other items in them.  If you have a sieve, it would 
be good to sieve the soils to remove these.  Take two small plastic 
flowerpots (about 5 cm diameter will be fine) and cut a small circle of 
blotting paper (or filter paper if your school has it) to cover the holes 
at the bottom.  Weigh both of the flowerpots with the paper in.  Now fill 
one flowerpot with soil and recyclable material and the other with soil 
only.  Reweigh both of the flowerpots with soil in.  Put the two 
flowerpots in a bowl of water until the soils are wet through (maybe half 
an hour).  Now stand the flowerpots in a beaker so that the water can 
drain out of them freely (be ‘sucked’ out by gravity).  Now reweigh the 
flowerpots a third time.

By doing some simple mathematics, you can calculate how many grams of 
soil contain how many grams of water and the one with the greater value 
has the better water retention (e.g. if 5 g of soil contains 1 g of water 
then it retains 20% of its weight in water).

I hope you have fun doing this and that you get some interesting answers 
to your question.

Francis Brearley
Dublin, Ireland