Field capacity theory
I originally prepared this post last year when I was new to blogging. As someone innocent and pure in heart with a soil science message I was quite pleased with my effort…
You can’t publish that I was told in no uncertain terms. Mention was made of ‘Fifty Shades of Grey’; what on earth did they mean? “Juvenile humour” I was told. How appropriate from one who has entered his second childhood from his first without any intervening period! Publish and be damned! My original piece was un-illustrated. I have learnt in my year of blogging that pictures help to make one’s real meaning explicit (oh dear, there I go again). There will be no pictures of me bathing!
In the bath with my toy ducks
Lying in the bath my thoughts inevitably wander… to soil water holding capacity. My hand idly goes to... a small rectangular bath sponge. I gently lift it out of the water and hold it horizontally. Water immediately pours out. If I continue to hold it steady, no more water drains. Is water in the soil like that? Yes! Growers would say my sponge -representing the soil - was at field capacity. The water that spills from the sponge as I picked it up equates to drainage. No more water falls from the sponge as I continue to hold it steady, even though it is still very wet. Is a soil like that? Yes, no water drains from a soil that is only at field capacity.
I take my empty coffee cup and gently pour a little water over the still horizontal sponge (Damn, I need an extra hand). An equivalent amount of water drains from the sponge. Soil water behaves like that. Soil normally needs the winter rains to restore field capacity before it loses water to drainage. Not this year, (this was 2012) our soil, unusually for summer, is still at field capacity and when it rains, it drains! How topical! How tropical?
I suddenly sit up really excited now. What if the sponge was compost in a seed tray or a plant pot? I experience a moment of acute revelation as I gently tip the sponge from the horizontal to the vertical and water cascades out. The depth of the sponge has affected its ability to hold water. Eureka, compost in thin layers holds a lot of water at field capacity, more than the same compost in a deep container. If the compost holds less water it is better aerated.
I examine the still vertical sponge closely. It is much wetter at the bottom than the top. Compost in pots is the same. Maybe old fashioned crocking of pots wasn’t so daft after all!
|Sponge held steady on a horizontal axis. It’s holding a lot of water but none drains away|
|By tipping the sponge the vertical profile becomes deeper and lots of water rapidly drains away|
|Water ceases to drain when the quantity of water settles to the lower field capacity of a deeper profile. But look the base of the profile is wetter than the top! Does water in a pot behave like that?|
Excited now, I squeeze the sponge and water runs out. Perhaps my squeezing represents the plants as their leaves transpire, sucking water from the soil. Perhaps it also represents the water that is lost by evaporation from wet soil.
Squeezing the sponge a little harder, I only get a little dribble. I squeeze really hard, no more water. But the sponge is still wet! Soil is like that, the plants cannot exert enough root pressure. Clay soil hangs on to as much as half the original water it was able to hold when at field capacity. Plant roots cannot squeeze hard enough to extract it.
I relax and sink down in the bathwater, still pondering that when I water a plant in a container I will raise the compost to field capacity. If I generously water, as I usually do, the surplus rapidly drains away.
What if my plant pot was in a saucer and the water had nowhere to go? The compost will be water-logged. It will be above field capacity and will be poorly aerated. I sag deep into the bathwater with a quiet shudder.
By this time I have got goose pimples and resolve next time to take a shower.
I used to use this analogy to my students. One year they presented me with a set of red toy ducks at the end-of-course alcoholic celebration!
Now what does the water that leaks from the wet sponge represent when my hand trembles ….? Maybe a soil scientist will post a suggestion.
I get out of the bath and go to play in the garden
|To carry out my experiment I need to make a slit in the bottom of my plastic tray|
I fill a seed tray with compost and thoroughly water it.. and again… and again, to demonstrate that it really is at field capacity. At each watering surplus water drains. I leave the seed tray for about a quarter of an hour. Because it is only at field capacity no more water drains over this time.
|I convert the shallow tray to a deeper profile by tipping it. Oodles of water rapidly drains away|
|Just look at all that water lost from the seed tray|
|I have made a slit in this pot and filled it with sand and thoroughly watered it to examine the profile after drainage. It is wetter at the base than the top.|
Practical significance of field capacity theory.
I used to refer to field capacity in a pot or a seed tray (or a sponge for that matter) as ‘container capacity’. A growing medium in a shallow container will have a higher field capacity than the same medium when it is part of a deep profile such as in the ground.
- In a normal season in the UK, soil in the ground wets up overwinter and is restored to field capacity. Only then will further rainfall run away to drainage. February is not called ‘February fill-dyke’ for nothing.
- Because a great deal of water per unit volume is held at container capacity, garden soil will often hold too much water if used in containers and will be poorly aerated. Seed and potting composts are formulated with ingredients that are well aerated when at field capacity.
- Shallow seed trays hold a lot of water after watering. A good thing for a tray of vigorous water hungry bedding plants but not so good for small seedlings if you water too often.
- Some gardeners cause themselves endless problems by economizing in the amount of compost they use and sowing or pricking out into only half filled trays. The compost lies too wet.
- The real reason that some plants are found to do better in deep containers (e.g sweet peas are often sown and grown-on in ‘long pots’) is that the growing medium is better aerated after watering.
- The reason that outdoor plants in shallow display containers frequently die in wet weather is that they are badly aerated when they remain for a prolonged period at a high container capacity.
- Thin layers of ‘amended’ soil in the ground, especially when overlying a gravel drainage layer(!) may hold excessive amounts of water and be poorly aerated. Such a gravel layer provides an unfortunate discontinuity to capillary water movement down into the soil.
Definition of field capacity
A soil is at field capacity when it holds all the water it can against gravity assuming free drainage.
Container capacity is a term used by some growers to refer to the field capacity of a growing medium in a container - or a block of compost - or even a sponge!
I failed with this experiment to demonstrate clearly that water levels at the base of a soil profile is greater than at the surface. By now some of you will have decided I am somewhat mad. I present you with evidence - I make mud pies. Guilty m’lud as charged.