A recent workshop on the function of soils provided a very interesting day for farmers at Teagasc’s Johnstown Castle research centre. The workshop, which was organised by the Irish Tillage and Land Use Society (ITLUS), concentrated on the functions of soil rather than just fertility.

Opening the morning seminar, head of Teagasc’s crops, environment and land use programme Paddy Browne told us that the 400ha estate is only about half agricultural.

Land use is primarily grassland and the major research areas include soils, nutrient supply, environment, water quality and gaseous emissions.

All of these areas are linked to soil and soil health, so they are of direct relevance to agriculture.

Johnstown Castle was formerly the soils research centre for An Foras Taluntais. While it is still a major Teagasc research facility, the complex is now also home to the Environmental Protection Agency and Department of Agriculture offices.

Five main functions

Rachel Creamer of Teagasc (who is about to leave Teagasc to take up a senior position in Wageningen University in the Netherlands) commented that soil is influenced by its environment, which affects both its use and functional characteristics.

Our soils are classified into 11 major groups and nine of these actually occur at Johnstown Castle.

Rachel emphasised that soil has five major functions:

Understanding soil should be an integral part of good farming. A good soil should be about 50% solid and 50% water plus air (about equal).

The relative proportions of sand, silt and clay govern the texture of a soil and this remains fixed over time. What will or can change is the amount of non-solid and when this decreases due to compaction or compression, etc, the characteristics of a soil can be altered.

Understanding structure

Jer Emmet Booth, a soils researcher with Teagasc, described soil structure as the organisation of the aggregates in the soil in combination with the macropores and micropores present.

The macropores enable water and air movement in the soil, while the micropores within the soil particles help to retain moisture to support plant growth when surplus water has been drained away.

Good soil structure is important for plant growth and it represents the physical health of a soil. Structure controls the entire rooting environment in the soil, as well as water movement and storage.

It also affects air exchange, which is important, because roots must be able to breathe, and air movement influences the temperature of the soil.

And both temperature and air influence nutrient cycling by providing air to the soil organisms responsible for nutrient recycling.

Soil and structure also influence the ability of roots to grow and explore the soil volume to source essential nutrients. And this in turn is important for growth and also for root and plant anchorage.

Structure, air, organic matter and temperature are also important for the soil biological complex, which affects nutrient recycling and also possibly disease interactions.

Jer said that it is important to realise that structure is always changing because aggregate degradation is always occurring, while soil biology can help to restructure the fine soil particles. When soils become compressed, it is the pore space which is altered and this affects the various functions of the soil.

Pore space is described by bulk density (BD) measurement, which is a measure of the dry weight per volume.

Soil measurements show BD values ranging from 0.5g to 2g/cm3, but Jer said that root growth tends to be adversely affected at values greater than 1.5g to 1.6g/cm3.

Organic matter

Carbon sequestration is one of the important functions of soil. A soil’s carbon level is proportionate to its organic matter level and this is a store of organic material or carbon, Gemma Torres-Sallan, a Teagasc Walsh Fellow, told the meeting.

The challenge is that cultivation exposes soil organic matter to more air and oxygen and this fuels biological activity, which mineralises it to release nutrients.

The problem is that this process also releases carbon dioxide to the environment. So tillage increases organic matter degradation, which is negative for soil structure and it releases carbon.

Gemma explained that organic matter is approximately 58% carbon and so the higher the organic matter level in the soil, the higher its store of carbon.

And because soil organic matter is negatively charged, it attracts positively charged nutrients, which protects them from leaching and makes them more available to plant roots. She also explained that soil particles and organic matter make up the structural aggregates.

Organic matter is important to make strong aggregates, which help resist degradation and maintain good pore space.

Nutrient cycling

Phosphate (P) is a very important nutrient for plant growth. Teagasc researcher Jessica Graçe explained that P is an important building block for cells and it is also important for energy transport within the plant.

Most soils are naturally rich in P, but less than 1% of total soil P is available to plant roots. The rest is tied up in some way in the soil.

Jessica said that P availability is heavily influenced by soil pH and that this is an important key to unlock availability. P is also stored in soils as organic matter (could be 35% to 60%), but this must be mineralised to an inorganic state to make it available to plant roots.

P is also consumed by microbes, which are part of the soil biology and recycling process, and this too can be made plant-available in time.

Moving on from this, Teagasc’s David Wall explained that tillage systems front-load nutrients, but inefficiencies in the soil system rob a proportion of these nutrients. While this is undesirable, it is normal and the soil supplies 35% to 45% of the crop’s needs. So soil type and field (how it was managed previously) provide essential base nutrients to support plant growth.

The soil type effect on grain yields is not just related to soil organic matter (OM), David stated. He presented results of recent trials where site had a big impact on yield without fertiliser where both sites responded similarly to applied nitrogen. In these results, urea looked to be as good as CAN.

David explained that the type of soil OM present is also important. Where OM is readily available for degradation and nutrient mineralisation, yield level tends to be naturally higher.

This is what happens in fresh versus worn land. Worn tillage soils have lower nutrient release due to lower OM, but this will increase if organic matter levels are increased in the soil.

The ability of the roots to explore the soil volume is also important here. David reported that root length density (centimetres of root per cubic centimetre of soil) appears to have decreased in the past 30 years. Many factors could influence this, but the fact that bulk density has increased, even at depth, is a fact that should not be ignored.

Importance of earthworms

“Soil is a living thing which is mediated by organisms.” While we do not yet know what many of these are, UCD’s Olaf Schmidt stated that they all fulfil some function, which is probably directly or indirectly influenced by earthworm activity.

Olaf said that there should be around five million earthworms per hectare or about 500/m2. But to have this number of worms, it is essential that they be fed with some form of organic matter.

He also said that minimum disturbance of tillage soils helps numbers, as there is less disturbance of their burrows, etc. But this still takes time to happen, unless there is active supply of feed.

Lack of feeding is at least one contributory factor to the falling numbers in recent decades. However, numbers have been increased again where organic matter has been added back to the soil.

Potato seedbed preparation is very severe on earthworm numbers, mainly as a result of the aggressive cultivation and destoning processes. This also decreases the number of species present, Olaf said.

While undisturbed field margins are good for earthworm numbers, research has shown that relatively few spill over from the margins into the field proper. They do not tend to be very mobile horizontally.

Slurry, especially cattle slurry, is often maligned as killing earthworms. However, while one will see an occasional dead worm on the surface post-application, Olaf said that research with cattle and pig slurry over 30 years showed a slight, but not significant, drop in earthworm numbers following application of 50, 100 and 200 m3/ha/year.

Indeed, Olaf suggested that pig slurry could be slightly more negative on earthworms than cattle slurry.

Assessing structure

Structure is arguably the most important characteristic of soil. While you can disagree with aspects of this comment, in general a soil will not retain good structure where something is wrong.

So good structure can be taken as a sign of good health, but it can be damaged by management such as tractor tyres and cultivation equipment.

With this in mind, researcher Julia Bondi, with PhD student Jer Emmet Booth, explained one aspect of the soil quality assessment research (SQUARE) project which aims to develop simple soil structure assessment techniques based on visual assessment to indicate its general condition.

Research is currently working with two systems – the widely used visual evaluation of soil structure (VESS) method and what the researchers term the double spade method. The VESS method takes a 20cm x 20cm x 20cm block of soil and then examines and scores it manually to show how it behaves relative to a range of different criteria.

It was originally developed for tillage and grass soils. As the grass root mat on pastures influences the results, a specific GrassVESS is being developed to take this into account.

A further modification, the double dig method, is being developed and examined for tillage soils.

While the top 20cm of soil is examined normally, a second access point is dug out from 20cm to 40cm to look for the presence of compaction, plough pan, etc, between 20cm and 40cm.