This is the exact opposite to efficiency and is a great way to suck margin out of farming. Living and breathing soils help plants to grow and soil to restructure, making tillage more sustainable in the long term. This is something that all tillage farmers need to address.

A few weeks ago, Monsanto and Deeside Agri organised a seminar which looked at the use of Roundup Flex, the Dekalb range of oilseed rape varieties and an interesting presentation on soils. The soils speaker was Philip Wright of Wright Resolutions based in Boston, Lincolnshire.

Soil texture

At the seminar, Philip spoke about ‘Soil structure — the current problems and how best to improve it’. He started by speaking about soil texture and the influence it can have on the things we do.

Soil texture is the relative amount of sand, silt and clay present in any soil, with sand being the biggest particles and clay the smallest. The more coarse particles that are present, the easier it is for water to move through the soil making it easier to dry out and easier to work but more prone to drought.

Texture is not altered by soil management, except through soil organic matter. Texture is fixed but increased organic matter can alter its characteristics by increasing moisture retention in a sandy soil or, possibly, by improving soil structure and increasing water percolation in heavier clay soils.

Increased organic matter can also help the friability of a soil, making it easier to cultivate and, thus, decrease diesel requirement.

Soil structure

Soil structure, on the other hand, is influenced by soil working machinery and by the process of cultivation. This effect on structure can be either good or bad, depending on the weight of the machinery, the design of the soil-working parts and the moisture content of the soil.

A standard soil contains approximately:

50% solid materials

23% water

23% air

4% organic matter.

These individual values can move independently of each other and plant roots will influence them by removing the water, by bursting up soil to allow more air and by adding to soil organic matter level.

Drainage

Philip stated that drainage is key to soil health. Successful drainage affects the air/water balance in the soil and this is critical to prevent anaerobic conditions. Plant roots must breathe in the soil but this cannot happen when it is waterlogged.

Soils that drain freely enable more timely cultivations and planting to help yield potential and there is less risk to soil structure from either heavy machines or soil working tines when the soil is dry enough. Fertilizer use efficiency is also increased in well drained soils.

Soil structure influences water movement. As soils are tightened or compressed, as distinct from compacted closer to the surface, it is the air spaces that are removed. This leaves less room for air to circulate as the roots breathe and also less capacity for water to percolate and escape down through the soil profile into the drainage channels.

The overall structure of a soil influences the severity of soils being tightened. Soils that have a natural crumb structure down to depth in both the top and sub-soil are least affected while soils that tighten easily to form slabs are most seriously affected.

Such soils are described as ‘massive’ if they naturally tighten to produce only occasional vertical cracks in the soil. They are described as platy when they only have horizontal cracks leaving little opportunity for water to escape through the soil.

Philip recommends that growers dig small inspection holes by hand in damaged soils and use spade resistance and plant rooting depth as the key indicators of what is actually happening.

effect of weight

One of the big challenges of modern farming is the ability of the soil to cope with the weight of modern high capacity machinery. Tyre manufacturers have done a good job of producing tyres with minimal compaction risk by spreading the weight of the machine across a big footprint area.

However, Philip stated that increasing axle loads increases the depth of tightening or compression of the soil. And this compression effect is likely to be even greater when the soil is wetter and on weak soils (see Figure 1).

Weak soils are generally lighter soils that are more easily damaged than clay type soils and have less capacity to self correct or repair themselves. But all soils are weaker when they are wetter.

To put this into perspective, Philip said that some large self-propelled potato harvesters can weigh up to 55 tonnes when fully loaded on the land. But even some of our larger combines are likely to exceed 33 tonnes loaded.

One of the most important rules to remember, according to Philip, is that the load on the axle/tyre influences the depth of its effect (its compressive force) (see Figure 2). In an example, Philip stated that a one-tonne loading on a tyre resulted in a 60cm depth of influence. But when this weight load increases several fold or when the soil is wetter, the depth of influence could be double. So, his advice is to keep axle weight down, especially when the soil is damp.

There is another factor that must be considered. The tyre alone exerts a straight vertical force when it rolls over the soil. But when this tyre is also providing traction you are adding a horizontal force to the vertical force. This combination of high axle weight and substantial wheel slip is the best way to destroy soil structure, Philip stated.

Controlling the passes

The damage caused by a machine tyre increases with the number of passes across a field. So, the temptation might be to keep moving the machine to different locations. However, Philip stated that 75% of the total damage is done in the first pass and it is this fact that lies behind the controlled traffic concept. This works on the premise of sacrificing the areas where the wheels will drive to maximise the area that is not driven upon at all.

Controlled traffic systems are increasingly used to help protect soils in different parts of the world and tend to be associated with minimum or reduced tillage establishment systems. They are also associated with the use of RTK satellite controlled auto steer systems to control the position of the vehicle wheels. Such systems require machine wheel-track modification, so that all vehicle wheels travel in the same tracks. This keeps the compressive forces in the same tracks year after year to help increase the productivity of the majority of the land.

organic matter

Soil organic matter (SOM) is a very important component of soils. Philip stated that levels in tillage soils around the world tend to vary from 1% to 6%. In general, the lower SOM levels occur in the warmer climates where disturbed soils undergo substantial organic matter breakdown and there is a major move towards min-till and direct drilling in such regions.

In cooler damper climates, many soils tend to level out around 2.5% to 4% and that is where the majority of Irish tillage soils are. Lighter soils often end up lower in SOM and these are the soil types that benefit more if levels can be increased.

In my opinion, it is not the absolute SOM level that affects a soil but the active and additional organic matter that is supplied to feed the soil biological system, which has earthworms at its core.

Active organic matter, which I would describe as added organic matter, provides an additional stimulus to biological activity. This brings many benefits in itself but some of the side effects are critical to soil health.

Keeping soils open

It seems highly unlikely to me that the size of the tools we use on the land will decrease, although there is some scope to do this. So, if we continue to use heavy machinery we must begin to use nature to help improve our soils on an ongoing basis.

If we are causing soil compression to over 1m depth by the wheels of heavy machines, there are only three ways that damage can be undone, excluding the option of digging it all up. Two of the options are climate related; the third depends on nature.

Frost could burst the soil to that depth but this is highly unlikely in Ireland. If we got that much frost, our water systems would freeze.

Drought is the second option and this will occasionally occur, 2013 being a good example. The cracking in dried out soils effectively opens the soil down to over a metre depth and this is nature doing a better job that we can do with machinery.

The third tool, and the only one that we have some control over, is the use of earthworms to burrow through the soil to open up pores and channels to great depth. These then help the movement of air, roots and water to keep the soil in better physical condition. But earthworm numbers depend on the amount of food available and, so, active organic matter addition is a most influential process for soil improvement.

Increasing the amount of organic matter in a soil helps improve soil structure, soil moisture retention and nutrient availability.

Increased organic matter helps to make the soil more spongy and increases its resistance to compressive forces. The increase in earthworm numbers associated with added organic matter brings many other benefits.

In The Field

Speaking in a soil pit later in the day, Philip showed in practice what he had been talking about earlier in the day.

The side wall of the pit showed a tightened layer at what was probably the base of the plough furrow but Philip said that the plant roots appeared to be getting through in adequate numbers.

Therefore this layer may not be an obstacle to plant root growth and was thought unlikely to benefit from subsoiling.

Indeed, the condition of most soils in mid-July reinforced the importance of good conditions if you were to consider subsoiling and the fact that when conditions are good enough nature is probably doing the job for free.

As one farmer put it, “when you need to you shouldn’t and when you can you needn’t.”

SOIL CULTIVATION

In general, the fineness of the soil influences the level and speed of plant establishment. This is the main reason we cultivate to plant seeds.

Planting depth also has an obvious effect on establishment, as does the subsequent firming of the cultivated soil. While you need a certain level of tightness, if a seedbed gets too tight it can hinder establishment.

So, the looseness and tightness of a seedbed must be balanced to suit the prevailing conditions of the soil. But these factors matter in the full rooting zone as well as in the seedbed. The continuous tightening of soils by some combination of heavy machinery and wet conditions is acting to tighten the soil. Reduced water percolation tends to be the most visible symptom.

Tighter soils also reduce root growth and root growth limits yield potential. The easier it is for roots to grow, the more access the plant has to nutrients and water and, so, the higher will be the yield potential. Nowadays, most of this tightness is occurring at depths that we cannot influence with basic cultivation or, indeed, by subsoiling because we are seeing compression or an increase in soil bulk density to below a metre depth.

Subsoiling may relieve a specific problem that is 12 to 14 inches deep but it may only move the visible problem off the surface leaving another problem between 15 and 40 inches deep.

For subsoiling to produce a desirable effect, the soil must be dry enough to shatter at depth. Philip reinforced the importance of that and stated that soils will not shatter when they are wet, whether that is for cultivation or subsoiling. Indeed, he said that soils will naturally compress when they are wet.

To put a subsoiler into wet ground (or at least wet where the subsoiler tip is working) is much more likely to compress the soil in the working zone leaving you with an even bigger problem for the future. While the water will drain off the surface down the slit, this action is only making for bigger problems in the future as water gets trapped at the subsoiling depth, making the soil more vulnerable to further damage.

Cultivation principles

Philip gave a brief summary of the principles of cultivation — dry soils shatter and wet soils compress. With tines, the effect is all about the angle of engagement with the soil, called the rake angle.

The bigger the angle, the more lifting you get but the soil might lift in lumps requiring further breakdown. The straighter the tine the more horizontal compressive force is applied and this could push soil particles together if the soil was not dry enough.

Discs tend to have a higher compressive force on the soil because of how they work but many claim that this is reduced by scalloped discs. However, this is the main reason why disc machines pose a bigger risk of soil damage if used when the soil is wetter than it should be.

These principles are also very important for subsoiling, Philip stated.

The first basic setting is depth and to know exactly where the pan is that one is attempting to break. Is this a uniform depth or is it variable? This is important because the working depth should be no more than 1.5 to two inches below the tight layer of soil. If the subsoiler is set much deeper, the soil below the target layer can act as a cushion to reduce the amount of fracture produced and lessen the benefit.

Asked about using wings on the subsoiler, Philip stated that wings will increase the amount of shatter and this can be used to widen the width between the individual legs.

As a general rule, Philip said that the leg spacing should be 1.5 times the working depth if no wings are fitted and that this can be twice the working depth where wings are fitted.

But wings need a bit of consideration, Philip stated. If you fit wings at an aggressive angle, they can burst more and generate a bigger pull on the tractor. But if the soil is damp at the working depth, this aggressive angle is more likely to result in compression of this soil leaving further damage. For this reason, Philip prefers to have wings with a gentle lift angle but to make them longer so that they still give a good lift with a much lower risk of compression damage.