What has not been made clear in the above descriptions is the process of water erosion.  Leaving aside stream bank erosion and tunnel erosion, the vast majority of water erosion is initiated when raindrops impact bare soil.  Rainfall of 1mm falling on one hectare of land has a volume of 10m3, and a mass of 10 tonnes.  So in a common rainfall event of 20mm the ground in one hectare is struck by 200 tonnes of raindrops hitting the ground at around 9.8m/s (35 kph), depending on the intensity of the rainfall .  The energy in such rainfall is far higher than the energy in the resulting run-off water, as demonstrated by Hudson (1971).  He compared the kinetic energy (energy imparted as a result of motion; KE = 1/2mass x velocity2) of rain and the runoff it produced.  Assuming that 25% of the rain became runoff, he calculated that the rain has 256 times more kinetic energy than the surface run-off it generates.  The result indicates clearly that rainfall has more energy to disrupt the soil surface than run-off flowing over the soil.  Geeves (1997) found that the rate and degree of surface sealing were influenced primarily by raindrop kinetic energy with highly energetic impacts leading to significant surface sealing.

The kinetic energy at raindrop impact has to be dissipated.  On bare soil this results in (1) soil compaction, (2) structural damage and detachment of soil particles from soil aggregates, breaking them apart, and (3) splashing soil particles into the air.  Particles splashed into the air have one of three fates.  If they land in runoff water they are entrained and carried downslope; if they land back on the soil surface they are small loose soil particles ready for water erosion by the next raindrop direct hit in this or the next rainstorm, or they are available to be eroded by wind in dry times.  On sloping sites soil particles are splashed differentially further downslope than upslope.  So, even without runoff, there is a movement of soil down slope. The third fate for splashed soil particles is to land back on the soil surface and be washed or sucked into a soil pore, thereby reducing its size and capacity to infiltrate water.  The vast majority of soil crusts that Runge and van Gool noted are developed in this way.  Any plant nutrients attached to these soil particles share their fate.  Soil erosion is also nutrient erosion.

Reduction in surface soil pores and development of soil crusts reduce water infiltration.  With nowhere else to go, the water runs off, and soil moisture storage is reduced.   Run-off, flooding, drought conditions and soil productivity are all made worse when raindrops hit bare soil.

Soil cover is imperative to reduce raindrop impact on the soil surface.  Soil cover is important for other reasons, amongst them minimising water evaporation, which causes loss of soil moisture and leads to capillary rise of salts, and reducing the harmful effects of the sun’s UV rays on soil microbes in the top 1-2cm of soil.

Advances in understanding – the problem with soil compaction and its link to Soil Organic Carbon (SOC)

Shaxson et al (2014) likened soil compaction to the demolition of a building.  In this analogue, all the solid, mineral parts are still present, but the spaces in which people lived and worked are dramatically reduced and all the services to the building (power, water, ventilation) are destroyed.  Soil compaction reduces pore spaces, particularly water-lined pore spaces in which the soil microbes exist and where the myriad chemical and biological activities of the soil occur.  A compacted soil has reduced ability to store and transmit water, and reduced capacity for gaseous exchange between the root zone and the atmosphere above the soil.  So, it is little wonder that compacted soils commonly produce poor crops or pastures.  Clearly a well structured soil with a good arrangement of pore spaces is essential for microbial activity, root elongation, water holding capacity and movement of water and minerals – and earthworms – within the soil.

Paradoxically, cultivation to aerate the soil commonly causes loss of soil pore space in the long term. Aeration causes increased oxygenation within the soil, which stimulates the respiration of soil microbes, encouraging increased utilisation of their energy source, which is soil organic carbon (SOC).  The increased microbial respiration also generates increased emissions of CO2 from the soil.  Reicosky (2001) reported on experiments that found CO2 emissions of 229 g/m2/24 hours from soil ploughed to a depth of 280mm compared to an emission rate of 10 g/m2/24 hours from un-cultivated soil.  The culmination of this is large scale reduction in SOC and the ‘soil services’ it supports.  Reporting on a longitudinal study of 100 years in Kenya, Marenya and Barrett (2009) showed that SOC declined from 12% to around 2% following the conversion of woodland to agriculture, and the decline in SOC was paralleled by declines in plant size and cob weights of the staple maize crops.   In Australia, Yin Chan (2008) suggests that there was a general halving of soil carbon in the thirty years from woodland clearing for agriculture, but a slow increase in soil carbon since reduced tillage techniques were introduced.

Soil microbes rely on SOC as their source of energy and nutrients.  Loss of SOC, therefore, adversely affects the ‘soil services’ fulfilled by soil microbes.  Amongst those ‘services’, it has been discovered that soil fungi exude a sticky material named ‘Glomalin’ (USDA, 2014).  It is thought to contain up to 30% of the soil’s carbon, and importantly glues soil mineral particles together to promote or re-build soil structure.  Similarly, mucilage and ‘glues’ from soil organic matter and other sources within the soil biota are likely to be contributing to the stabilising and reconstruction of soil structure and pore space.  However, these processes rely on an on-going supply of dead organic matter as their substrate.  Here is an underlying reason why minimum tillage or no-till farming methods with stubble retention are improving both soil structure and SOC, although the process is generally a slow one.  

Advances in understanding – the plant root zone

It has been established that there is a symbiotic relationship between plant roots and the microbes in the root zone (rhizosphere) (e.g. Stapper, 2006).  Plants have been found to exude carbohydrates, on which the microbes feed, and in return the suite of microbes attached to a particular plant help ward off pathogens, facilitate the flow of water and nutrients to and into the plant roots, and moderate the soil pH in the rhizosphere, allowing vigorous plant growth even in an acid soil (Stapper, op cit).  The diversity of microbes associated with plant roots is in the order of tens of thousands of species. This complex plant - microbial community is crucial for plant health.  Berendsen et al (2012) show that upon pathogen or insect attack, plants are able to recruit protective microorganisms, and enhance microbial activity to suppress pathogens in the rhizosphere.  The rhizosphere is a lot more complex than previously thought.  Yet to promote or maintain the beneficial activity of the soil microbes and allow the transmission of water and nutrients within the soil, the soil structure and, in particular the soil pore space has to be in good condition.  Now we can better understand Wilson’s (1986) observation that subsurface compaction increased the harmful effects of pathogens in the topsoil.  Compaction reduces the ability of the ‘good’ soil microbes to protect plants against pathogens.  Soil management to increase pore space will have a direct benefit on crop health.

Discussion and conclusions

Within a land husbandry context, this paper emphasises the importance of soil pore space in soil and water conservation and in promoting the beneficial activities of soil microbes, which in turn promote productivity of the soil and the plants that grow in it. Maintaining surface pore space is essential for rainwater infiltration, and minimising runoff and soil erosion.  Emphasis has been laid on the destructive effects of raindrops on a bare soil surface, and the need to keep the soil surface covered.  In cropping systems, stubble retention is a primary method for achieving this, and ‘keeping the soil covered’ is enshrined as one of the three principles of Conservation Agriculture. 

Mechanical structures such as contour banks and grade banks have been viewed for too long as the first line of defence against soil erosion.  They are an important back-up and second line of defence for those very intense storms when rainfall intensity outstrips soil infiltration rate.  However, to rely on them solely misses the point that erosion starts at the soil surface when it is allowed to be bombarded by high-energy raindrops.

‘Keep the soil covered’ should be equally applied to livestock farming.  Overgrazing, particularly in summer and autumn can leave the soil exposed to wind erosion in dry weather and exposed to raindrop impacts, soil and nutrient erosion, runoff and reduced infiltration during summer and autumn rainfall (which tends to be intense), and exposed to the opening rains of the next autumn/winter.  Three grazing management techniques are likely to reduce the incidence of bare areas in paddocks.  The first is rotational grazing.  By resting paddocks from grazing, the plants have increased opportunity to recover from grazing, re-cover the soil surface, and grow larger, thereby increasing annual pasture production compared to systems of set stocking.  The second technique is reducing livestock numbers going into summer.  However the possibility of doing this is largely controlled by marketing opportunities and arranging the whole livestock production system to suit.  The third technique is known as deferred grazing, under which the livestock are removed from paddocks over summer and autumn, held in a well shaded holding paddock and maintained on conserved fodder and supplements.  Such holding paddocks should be on level, stable sites, and should be rested from livestock for the remainder of the year. 

Improving soil pore space starts with management of the soil surface; regaining and retaining pore space throughout the soil relies on the soil biota, and keeping it fed.

Soil cultivation increases microbial respiration, depletes SOC and leads to increased CO2 emissions from the soil and reduced soil structure and porosity.  As shown above, soil microbes, their exudates and the mucilage from organic matter produce the ‘glues’ that can repair, maintain and improve soil physical conditions.  No wonder the second principle of Conservation Agriculture is ‘minimise soil disturbance’.   An exception would be deep ripping massive subsoils and plough pans to expand soil moisture holding capacity, promote increased root depth and root and microbial foraging for mineral nutrients, and alleviate seasonal waterlogging.   However, the implement chosen should minimise soil aeration for the reasons provided earlier.  (For completeness, the third principle of Conservation Agriculture is ‘use crop rotations’, however, this third principle is not discussed in this paper.)

Minimum or no-till farming remove the opportunity to use cultivation as a means of controlling weeds; other options are needed.  Weed control is important for optimum crop growth and yield, purity and price.  Herbicides are widely used to control weeds, particularly around the time of seedling emergence.  Results from both field and laboratory research have shown symptoms of microbial stress (reduced new microbial growth) even at recommended application rates for up to six to eight weeks following the application of some herbicides (Gupta, n.d.).  These effects were found to be soil type dependent. For example, in sandy soils with low organic matter and low microbial activity the negative effects of these herbicides were found to be greater.  However, this six to eight week period is when the new crop seedlings should be developing symbiotic relationships with soil microbes and starting to build the rhizosphere to help ward off pathogens and improve availability of nutrients and water to the establishing crop.  If used, herbicides need to be carefully chosen on the basis of their effect on soil biota, or alternative methods to cope with weeds need to be considered. 

In this time of climate change and continued growth of human population we face an uncertain future, with some dire predictions if comprehensive mitigation of greenhouse gas emissions is not undertaken soon. Much of the land used for agriculture here and around the world is degraded and less productive than previously.  Inputs to agriculture that are derived from fossil fuel are becoming more expensive and cause emissions of greenhouse gases.  It is timely that more is being discovered about the remarkable properties and abilities of the ‘soil organism’, and ways are being discovered to naturally rejuvenate the soil and return it to higher productivity.  In the quest to feed the world’s growing population the biological and chemical concepts and practices relating to soil health are sure to become more widely adopted.  Within such land husbandry it will be equally important to adopt concepts of physical soil management for improved surface conditions and soil pore space so that maximum benefit can be gained from soil hydrology.