Water relations in the soil

This blog relates to the old syllabus: R2102 Plant Nutrition and the Root Environment – Pore space and Water. This is also relevant to Unit 1 Topic 3 Element 1 which covers drainage although some of the more technical terms such as field capacity and permanent wilting point are not required for the exam.

Learning outcomes from the old syllabus:

1.4 Relation between pore space, air and water. Saturation point, field capacity, available water content, permanent wilting point, soil moisture deficit.

Importance of an appropriate balance between air and water for the healthy growth of plants.

1.5 Managing soil water content: identify poor drainage. (Surface symptoms, soil colour, soil smell, indicator plants, surface run off)

Identify causes of excess water (compaction, run off e.g. patios, high water table)


Air and Water in the Soil

At the time of writing it is pouring down and has been for many days so I bet the last thing you want to hear about is rain, water, drainage and floods; otherwise known as water relations in the soil. I started this topic before the torrential downpours flooded homes and gardens across the UK over the last week. Water is essential for life and it is likely that us humans, yet again, are causing the dramatic change in weather patterns.

Here’s my front garden – This picture was taken in Summer 2019 when we had none- stop rain for days and days and lots of local flooding where I live. It looked the same last week (Jan 2021)and then froze into a mini ice skating rink. A good example of surface ponding.

Why are the air spaces (pores) in the soil important?

Pore spaces (gaps) containing air or water are interlinked. More water = less air and vice versa. For healthy plant growth, there should be enough available water in the soil so that the plant does not wilt or suffer water stress, and there should also be some pores filled with air so that there is some gaseous oxygen for respiration of the roots. If all the spaces are filled with water for long periods of time, there is no oxygen available and roots ‘drown’ and die. The term used for the ideal soil; ‘Free draining but moisture retentive’ sums up this utopian state when both oxygen and water are available to roots.

The following diagram shows the proportions of air and water occupying pore spaces in a fertile top soil when the same sample of soil is very wet, very dry and ideal (i.e. in between the two extremes)

Pores in the soil

Porosity = the size and quantity of pores in the soil which influences water and air availability.

A balance between air and water is required for plant roots to function.

A simple way to look at pores is to classify them as 3 sizes:

  • Large pores are called macropores and these can drain easily within hours after being saturated (Filled with water). Gravity pulls the water downwards.
  • Small pores are called micropores. These continue to contain water after saturation and drainage has occurred. The water ‘sticks’ to the sides of the tiny pore spaces due to surface tension. This water is unavailable to plants as roots do not have enough force to draw up the water from these small pores spaces.
  • Medium pores are called mesopores. These pores hold water or can drain freely. They hold water which is available to the plant. Mesopores hold water which can be drawn up by the roots.

The optimum for plant roots is a combination of pore sizes where macropores contain air and mesopores and micropores contain water. This is often a Loam texture soil type. Incorporating organic matter can also help to increase the range of pore sizes.

What makes up the solid part of a soil?

Answer: Mineral particles (Sand, silt and clay) and organic matter

The following diagram shows the rock particles (minerals), sand, silt and clay and the different sized pore spaces. There will also be organic matter in the soil which will affect the size and number of pore spaces. The more sand particles for example, the more macropores, which is why sandy soils are free draining.

The Role of Water in the Soil

Water is taken up by plant roots and taken to all plant parts. It is involved in many biological processes such as photosynthesis. When water is plentiful, cells are turgid (plump) allowing the plant to expand its leaves holding them erect so they are exposed to maximum light (as opposed to wilting)

The soil water will also contain dissolved substances such as plant nutrients and pollutants.

Water is essential for all the microorganisms and fauna in the soil such as worms which contribute to making a healthy soil.

It binds the soil particles and organic matter particles together.

Factors which cause soils to be too wet (saturated) and too dry.

Factors which cause a saturated soil:

  • Contours of the land – lower areas collect water.
  • A high water table.
  • Prolonged periods of rain.
  • Too much irrigation.
  • Compacted clay soil at surface causing surface run off.
  • Drainage is impeded due to a cultivation pan and the soil above becomes saturated.
  • Drainage is impeded due to concrete footings and the soil above becomes saturated.
  • A burst drain floods the soil.
  • Soil texture: Clay soils have more micropores which hold on to water and so are more prone to becoming saturated as they do not drain well.

Summary diagram to show causes of saturation (excess water):

Factors which cause a well-drained (dry soil)

  • Drought
  • Over cultivation during hot weather bringing moist soil to the surface which dries.
  • High temperatures which cause plants to lose water faster by transpiration and also evaporation from the soil surface.
  • Drainage systems incorrectly installed – take water away too fast
  • Contours of the land. Higher areas lose water.
  • Soil texture: Sandy soils have more macropores so drain more freely so are prone to becoming dry.

Water Definitions

To make the discussion of water in soils a universal language, there are some terms which are used to describe the amount of water in the soil. This means that a particular soil type can be assessed objectively for its water holding capacity in relation to growing a particular crop. This may determine which agricultural areas suit different crops and whether supplementary irrigation or installation of drainage systems are required.

  • Infiltration: Water soaking downwards into the soil with gravitational flow.
  • Surface run off: Water running off the surface of the soil as it cannot go downwards. This may be due to surface capping of the soil or compaction. Surface run off can also be due to impermeable surfaces which have been sealed, such as concrete driveways or patios.
  • Soil erosion: Soil is moved from its original source to somewhere else. It is washed away.
  • Ponding: Water is seen on the soil surface as rainfall is faster than the rate of infiltration. Often due to a compacted layer below.
  • Waterlogged soil: Is saturated and has little to no oxygen and therefore anaerobic respiration will occur which causes toxins to build up and roots to die. All the pore spaces are filled with water.
  • Saturation Point: The point at which water occupies all the pore spaces in the soil. When there is a lot of heavy rain soil can get to saturation point which is a point at which it can no longer absorb anymore water. All the pores (macro, meso and micro) are filled with water. This is indicated by standing water on the surface of the soil (ponding) or flowing water off the surface (surface run off). Soil which is below the level of a water table is always saturated. Most plants will not grow well in soil which is permanently saturated. Saturated soils can also be due to impeded drainage such as a soil cultivation pan.
  • Field Capacity: This is the maximum amount of water held by a soil after it has been saturated and then allowed to drain freely. The macropores are filled with air as the water has drained away, the micropores hold hygroscopic water which is not available to the plant roots and the mesopores hold water which is available to the plant roots. A soil at field capacity has the maximum water content for the optimal plant growth.Field capacity is measured in mm of water. Aiming to achieve and maintain a soil at field capacity is good for many crops as there is plenty of available water AND oxygen in the pore spaces. Some call it the goldilocks of drainage as it is ‘just right’.
  • Water holding capacity: Is the total amount of water a soil can hold at field capacity.
  • Permanent wilting point: Is the amount of water held in the soil when a specified test plant does not recover after wilting. The macropores and mesopores are filled with air and the hygroscopic water in the micropores is unavailable. The plant has been starved of water and will not recover when watered. At permanent wilting point there is water in the soil but it is not available. It is so tightly bound within the small pores spaces that it cannot be drawn up by the roots.
  • Available water content: This is the amount of water held in the soil between field capacity and permanent wilting point. This is the water that is available for plant uptake and is held in the mesopores. It is the soil water volume measured at field capacity minus the soil water volume at permanent wilting point which gives us the value for available water. It is the maximum amount of water that the soil can supply to the plant. Here are some different soil types with available water content values:
  • Soil Moisture deficit: The amount of water needed to bring the soil back to field capacity. Field capacity is optimum state for the soil as there is water available but also air spaces. This is important to know in relation to irrigation schedules for crops.
    The diagrams below illustrate saturation, field capacity and permanent wilting point:

Symptoms of Saturated soils:

Surface ponding and pooling.
This can be a problem on new build gardens where heavy machinery has compacted a clay soil. Surface ponding can also be seen on lawns where soil is compacted due to high levels of foot traffic. The water sits on the surface as it can not infiltrate downwards because there is an inpermeable layer below it.

Lack of plants, dead plants.
Prolonged periods of anaerobic respiration cause build of toxic ethanol. Roots die and secondary rots invade the roots.

Grey areas of colour.
Grey patches of colour may be noted in the top soil or sub soil. This is called gleying. This is a deposit of a grey iron compound which forms when there is no oxygen present.

Poor root system.
The root system is superficial as it has not had to seek water. It will also be in poor condition due to anaerobic respiration when growth is much slower.

Devoid of soils dwelling macro organisms such as worms.
All living things require oxygen

Bad odours.
Microorganisms which thrive in anaerobic conditions produce a range of smelly odours such as hydrogen sulphide or bad eggs smell.

Disease of roots / Root rots
Many pathogenic bacteria and fungi thrive in saturated soils.

Yellow leaves and stunted growth
This may be one of the first plant symptoms you see as roots cannot absorb nutrients. Wilting may also be a symptom of compromised root growth.

Symptoms of very dry soils

  • Wilting plants, dead plants.
  • Yellowing of leaves and Leaf drop
  • Symptoms of nutrient deficiency as nutrients are not dissolved in soil solution for uptake.
  • Succulent plants thrive while other plants die as they require less water.
  • Surface capping – cracked soil surface with a hard crust on top, often on silty sandy soils. Water cannot infiltrate downwards so soil below is dry and the excess water runs to other areas of the garden. Surface capping causes localised dry and wet areas.

Soil Agony Aunt – Let Dr. Peat answer your Soil Questions:

Question 1

Dear Dr. Peat
I bought a new-build house and I have surface water (ponding) on my new lawn which was only laid 6 months ago. What has caused this and what do I do?

This is probably due to compaction of your soil which is more common on a clay soil. It happens when heavy machinery drives across wet clay soil. Here is a picture of the area on the new estate where you live. This area by the fence is now where your garden is situated.

I suggest you dig up your lawn and start again. Double dig and add organic matter to the top soil layer and then re-turf. If the problem is due to water running off a large patio, (so ponding occurs where the lawn meets the patio) then you may have to install a drainage system to take away this surface run off.

After you have returfed you will still have to aerate the lawn regularly as wet clay soils are always prone to compaction due to foot traffic so as part of maintenance you may have to aerate it regularly to relieve the surface compaction and hollow tine aeration would be the best way to do this.

Question 2

Dear Dr. Peat
After 2 days of rain my whole garden has flooded – this has never happened before when we had around the same amount of rain.

If you have a drainage system beneath the garden, perhaps it is blocked?

Have you or your neighbours extended the hard landscaping in your garden recently so there will be more surface run-off from these impermeable areas to other areas of the garden which can cause flooding?

Is the soil compacted which can cause it to collect in other areas?

Have the neighbours installed a drainage system and channeled the outflow towards your garden?

Question 3

Dear Dr. Peat
I live on Marsh drive and there is a natural pond quite close by. The local ducks have found a new home in my garden because the lower areas have remained under water for 7 days. What can I do to get rid of the water problem.

Unfortunately, there is not a lot you can do if you want to grow plants in this area – a raised bed is your best bet. If you can see a pond, this indicates that the natural level of the water table is high. Water will always drain to the lower areas of the garden where it will accumulate until it can filter downwards. The name of your road also suggests that it is a boggy site.

Question 4

Dear Dr. Peat
I have moved into a new house and my plants just keep wilting – I keep watering with a watering can but I can’t keep up with the watering. What can I do – I feel like I am a slave to the garden!

Plants can wilt due to dry or constantly wet soil so I would have to know what soil type you have. I presume however that it is sandy soil which is very free draining. There are many things you can do to retain moisture in the soil (improve the water holding capacity) so that your plants do not reach permanent wilting point. You can:

Select plants that prefer dry conditions such as succulents

Mulch the surface of the soil with organic (garden compost, leaf mould) or inorganic mulches (gravel, slate) to stop water evaporating from the surface of the soil.

If you can dig into the soil without damaging roots you can incorporate organic matter which will help moisture retention.

You can install an irrigation system such as seepy hose or a dripper system which can be automated with a timer. Seepy hose can be buried just below the soil surface so water does not evaporate from the soil surface.

Remember that you will also have to add fertilisers to sandy soils as many nutrients are washed away when water filters downwards.

Finally I have put together a little video to explain the 3 main water terms; Saturation, field capacity and permanent wilting point.