Soil is the most basic requirement for the majority of food production to feed the world's population. That latter challenge continues, and is increasing because of the growing population and dietary requirements.

But one of the major challenges for farming is to withstand the huge volatility in agricultural markets, coupled with the increasing challenges of climate. The quality and health of our soils are key to helping us achieve these objectives.

High levels of organic matter in soil help to increase the supply of nutrients like P to plant roots. Nitrogen is obviously another key nutrient for crop growth and high soil organic matter (OM) fields tend to have more N to release and this can often result in higher yield potential.

Where soils have the capacity to release more N, it is likely that some N will be available to fuel growth on every occasion over winter and spring when the plants are able to grow. This is generally visible in the crop as increased leaf and tiller numbers and apparent higher yield potential.

Higher OM levels in the soil also stimulate higher biological activity, which can have many other benefits in the soil, many of which originate from additional biological activity. One hugely important benefit is the impact of organic matter and soil biology in improving soil structure which can help to improve soils by taking the fine soil particles and regrouping them into bigger crumb units.

This is a big challenge in continuously tilling soils and is more easily done during a grass break, but it should be possible to bring about some structural improvement through the addition of OM.

Soil nitrogen

One of the benefits of increased soil OM is an improvement in soil fertility and an increased capacity to supply more nitrogen to the growing crop through mineralisation. This is the process of converting soil OM to its various components, one of which is nitrogen, but carbon can also be lost in this process.

It is important to realise that as farmers we only supply a portion of the nutrients required for crop growth. The balance of N, P and K in particular is supplied from the soil and most or all of this is returned to the soil as the crop matures. This will be available again to the following crop. This is particularly the case for P and K where the returned nutrients are held for future use by the soil particles or on the soil humus.

Nitrogen is continuously on the move in soils. As well as the crop roots, nitrogen is also used by soil organisms of all sorts and this is partly the reason why the nitrogen we apply is only used at 50% to 60% efficiency, i.e. of what we apply only this amount is actually taken up by the crop. The balance of the N requirement is supplied from the soil and Richie Hackett of Teagasc described the challenges of attempting to improve the accuracy of estimating this supply.

Uptake is the only exact measure

The only true way to estimate what a soil supplied in any year is to grow a crop on top, without any applied N, and measure the off-take. But off-take is not the only issue as demand is not uniform across the year and tends to peak at certain times in spring/summer.

A crop like winter wheat has a peak demand for N in the April-July period and our ability to meet this demand helps secure the delivery of high yield potential. However, the supply of nitrogen from the soil is variable and cannot be relied upon to meet crop demand.

Richie explained that the supply of nitrogen from the soil in spring is partly a result of N left over from the previous season, which is still in the soil in springtime (some of this may be leached out by winter rainfall).

Our second supply line comes from the mineralisation of organic matter in the soil. This is even less predictable, as it is influenced by the amount of OM in the soil, the soil temperature and moisture availability.

Because of the unpredictability of these supply sources, Teagasc use the Index system, based on previous land use, to predict a soil's contribution to soil N supply.

Richie stated that there are up to 15 tonnes of pure nitrogen in the top 60cm of a fertile hectare of tillage soils. But very little of this becomes available annually because its release depends on mineralisation, which is the conversion of organic nitrogen into mineral nitrogen that the plant can take up. The amount of mineralisation is influenced by the conditions for microbial growth.

The amount of nitrogen that is available from the soil is influenced by the use of rotations, the application of organic matter, the management of the soil over winter and previous fertilizer applications.

I was in a field of winter wheat in Britain earlier this summer which carried a nitrogen rate trial last year. Each of last year's treatments was clearly visible this year because drought inhibited the uptake of much of the N applied this spring. So excess applied N can be used in the future but only if it is not lost from the soil by leaching.

Richie pointed out that increased soil OM levels give increased yield, both short and long term. He stated that the presence of higher soil OM levels helps the use of the applied nitrogen. However, this effect or benefit will plateau off in time and he suggested that the benefit appears to become saturated following continuous FYM/slurry applications.

The use of cover crops over winter will prevent the loss of much of the nitrogen present post harvest ö as much as 10kg to 15kgN/ha. However, the re-release of this nitrogen for the following crop is unpredictable and so does not really present a solution for N loss. But Richie said that protein levels were sometimes increased in the following crop, possibly an indication that the nitrogen in the decaying cover crop was being released late in the season.

Nitrogen is always coming and going in the soil but long-term experiments would suggest that it never runs out. The soil supply can be influenced up or down by management but there will always be nitrogen present.

Improved N prediction

The ability to more accurately predict the supply of nitrogen that will come from a soil would benefit husbandry. The Index system is relatively blunt and takes no account of soil type.

David Wall of Teagasc told us that he is assessing a number of new testing systems for soil N supply and that one in particular, the Amino Sugar N Test (ASNT), appears to be quite suitable and takes into account the organic matter level in the soil.

Test results appear quite representative of the plant growth but there is still some inaccuracy. However, it would appear that this test is potentially a step forward in nitrogen application systems and the potential of a specific soil to release N in the future.

The test has been more widely used in the US where results appear to accurately reflect soil N availability across the full season at different depths in the soil profile. So we may hear much more about this test in the future.

Compaction

Dermot Forristal, Teagasc, has previously reported on the concerns for soil damage resulting from bigger machines with bigger axle weights.

He said that the risk of soil damage from machinery is directly related to soil moisture content (wetter soils damage much more easily), the tyres fitted to the machines (bigger tyres spread the weight and reduce ground pressure) and the amount of traffic on the land.

In addition, the soil type also has an impact, as heavier soils (with a higher clay content) can better resist compaction but these soils can also repair themselves better through the normal swelling and shrinking processes, which are a characteristic of clay.

Organic matter also plays a role here as higher soil organic matter levels help a soil, to some degree, to resist the compressive forces of weight to help minimise compaction.

While bigger tyres with higher air volume capacity tend to reduce physical ground pressure, there is growing concern that this bigger footprint is sending compressive forces much further down into the soil to over a metre in depth. If this is happening there is nothing much we can do to loosen the soil to this depth except by promoting soil biological activity through the addition of OM. And the great fear for farmers on this island is that the level and depth of any damage is likely to be even greater when the soil is wetter.

Structure

In the outdoor session in the afternoon, Rachel Creamer from Teagasc, Johnstown Castle, showed us an example of a very difficult soil which was naturally limited in use by its structure and composition. She also explained how to assess a soil for its natural structure.

Mark out a lump of soil on five sides ö five widths of the spade to complete the cut. However, only make four cuts, as the shearing by the space will cause some effect to the immediate structure. After the four sides are cut, lift out the lump of soil which will have to break off on the uncut side.

After this is done, insert a cut behind the uncut side and lift out a 1.5 - 2 inch slice of soil. This is likely to be much less damaged as less force is needed to extract it from the soil. Take this slice to a solid surface and drop it from waist height (knee height if it is a sandy soil). The soil will break into its natural components.

The ideal structure is a nice crumb-type size which is largely round in shape to allow easy water and root penetration. Rachel explained the importance of having a well structured soil for ease of use but stated that some soils, like the one we were looking at, was naturally poorly structured and could not be helped by deeper loosening because water would just sit further down, but would still have nowhere to go.

She emphasised that sub-soiling is very difficult to do properly in this country because it is very seldom that the soil is dry enough to fracture adequately. And sub-soiling when conditions are not good enough can result in even more soil damage, but further down in the profile.

*This article was previously published in the Irish Farmers Journal on 9 July 2011.