Coping with wet ground conditions

Published 19 October 12

How wet are your soils….and can you do anything about it?

In a normal year in south west Wales we'd experience around 1000mm of rain - and around 600mm of that would leave the soil in the form of evapotranspiration, meaning that around 400mm has to pass through (or over) the soil. This year rainfall has already exceed 1100mm in many parts of Pembrokeshire and Carmarthenshire and with a lack of sunshine and warmth, evapotranspiration rates have been around 20% lower, meaning that soils have had to process about 300mm of extra water than normal.

The amount of water in our soils can be measured in terms of "soil moisture deficit" (SMD) - the amount (in mm) that a soil can absorb before all of its pore space becomes saturated and it starts to drain freely. The graph below shows how this year's soil moisture deficit in Wales (black) differs from the average (green) and the extreme of 1976 (red). Once SMD falls below 30mm it can start to reduce grass growth.

Check out your regional figures at
 
  www.environment-agency.gov.uk/research/library/publications/100459.aspx

The net result is that soils in many parts of the country have a SMD of 0 and are termed to be at or above 'field capacity'. At field capacity almost all of the small pores in the soil contain water, and only the larger pores (earthworm and root channels) have air in them.  When rain falls on a soil in this state the water will move through these large pores as fast as it can - draining through to the subsoil or to man-made drains. The rate at which soils that are already at field capacity can drain varies, depending on the following:-

1) Soil texture - a light sandy soil can easily drain 150mm a day, whereas a heavy clay soil may only drain 2 or 3 mm a day.

2) Soil structure - if there are any barriers to water movement such as compaction layers, surface capping and plough pans then movement down through the profile will slow dramatically.

3) Macropores- if the soil has lots of worm channels and root channels then water can pass through the soil profile more easily.

4) Drainage- low water movement rates can be increased by introducing drainage networks, but the limiting factor to the efficiency of the drains will still be the texture and condition of the soil above the drains. Without adequate drains the water table will rise up through the soil. It has been estimated that for every 1cm of rain that falls on a soil where drainage is impeded, the water table can rise by up to 15cm.

The upshot of all this is just because you have standing water in your fields does not necessarily mean you have soil compaction issues - it may simply be that your soil does not have the capacity (even in a perfect state) to move the volume of rain through the soil profile fast enough.  So subsoiling or slitting may be a complete waste of time.

However there is a strong relationship between soil wetness and how easy it is to compact and damage - so a drainage problem can very quickly turn into a compaction problem as well.

Plan of action

Identify your problem areas - those fields that particularly struggled to grow grass, the ones where you know cows made a mess, the ones where surface water has stood for a few days at a time.  It unlikely that you can sort everything out; so prioritise.

You are looking to establish if soil structure is at fault, and if so, at what depth is the problem and how severe is it. You also need to establish if soil drainage is the underlying issue.

Step 1- the simple stuff. Are all your ditches and drains running as they should be? It is amazing how many problems can be solved by regular maintenance of open ditches and existing drains.

Step 2- take a look at compaction issues. Dig your hole to around 25cm and look for signs of any barrier to both water and air movement through the soil; restricted rooting depth, large blocky structures (called soil peds) that are hard to break up, horizontal cracking. Go down the profile 5cm at a time and take a handful of soil and squeeze it to assess where the water is holding. Very wet conditions are not the best time to look at soil structure as it is hard to assess friability and blocky structures when everything is like porridge, but it is a good opportunity to look at exactly where all this excess water is moving, or pooling.

Compaction layers can only be corrected when the soil is dry enough to lift and crack. When conditions are right will depend on your soil type and for many it is unlikely to be dry enough for many weeks to come.

Compaction in the top 10-12cm can be alleviated with slit aeration and deeper compaction can be broken with grassland "subsoiler" machines  although success does vary with different soil types.

Where you identify compaction damage make sure you avoid further damage next spring - target early grazings in dry conditions and think about grazing strategies. Soil conditions should take priority over a paddock's place in the grazing round, worry about soil more than grass cover.

Quite often where water has held on the surface then the soil may become capped; with small particles blocking up the pores and forming a thin impervious layer. Some soils will correct themselves with wetting/drying and frost action - others won't and may need light harrowing to help break it up. Where you have surface capping avoid applying slurry and dirty water as it will be highly prone to run-off.

Step3 - if you have found wet areas towards the bottom of your hole and signs of waterlogging at depth (rust deposits and grey coloured soil) then it is time to go a little deeper and check out what's happening further down the profile.

If you ignore problems of deeper water movement then any aerating or shallow 'subsoiling' will be ineffective as the water table will continue to influence soil moisture levels as water backs up towards the surface. As you dig deeper keep assessing the structure of the soil blocks, pore spaces and signs of waterlogging. Look for where water starts to seep into the sides of the pit. The minimum depth of water table that you would want to see in grassland is 30cm - so any drainage system needs to be well below that level. 

A recent publication from Teagasc in Ireland, explores some of the methods and costs of drainage schemes for heavier land where high water tables restrict grass growth and grazing efficiency. http://www.teagasc.ie/publications/2012/1334/Greenfield_2012.pdf

They recommend digging inspection pits up to 2.5m deep to assess drainage problems - so you may need a bigger spade.

Summary

It has been an unusually wet year that has put all systems and soils to the test.

  • Soils may have been compacted by grazing and machinery and may need to be corrected - although many may recover over the winter. Dig and monitor over the next few months. Only try and correct when soils are dry enough to lift and crack without smearing.
  • Try and protect any compacted ground so it gets a chance to recover - on off grazing, back fences etc.
  • Many instances of standing water may be more associated with drainage than compaction - make sure you have a clear understanding of what is happening underground before you spend money and time correcting surface and shallow compaction.

soil moisture