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Soil Husbandry and Tillage

Published as a three part article in 'Organic NZ' the magazine of the Soil & Health Association of New Zealand. Part one'The big dig debate' May/June 2008, Vol 67, No 3, pp 30-33. Issues two and three are yet to be published.

Introduction

Healthy soil is the foundation of organic agriculture and while much is made of the role of compost in creating healthy soil, the role of tillage (cultivation) seems like the elephant in the room.  This is most odd as the effect of tillage on soil health can easily dwarf the contribution of compost and/or other forms of organic matter.  Its also odd because soil husbandry is not exactly a new topic of discussion, after all, it’s not unreasonable to think that after 10,000 odd years of agriculture, 150 odd years of soil science and 80 odd years of the organic movement, that such a fundamental issue as how best to till soils would be done and dusted.  However, this is far from the case.  While organic arguments over the importance of soil biology have been mostly won, with the soil chemists pretty much throwing in the towel and agreeing that soil is much, much more than a chemical barrel, arguments over tillage still reverberate around the globe. 

While much of the founding organic research and argument focused on soil husbandry, for example the Haughley Experiment, it has not had such prominence in organics for some time.  I suspect that this may be due to the environmental revolution in the organic movement in the 1960s, which saw a dramatic swing in focus from soil health to issues such as the environmental and health impacts of biocides.  However, since the pioneering soil husbandry work was completed, there have been huge advances in scientific understanding of soil.  Despite this great library of new knowledge, I am increasingly concerned that this is not being taken up at the practical level in organic agriculture, home gardening nor is it reflected in certification standards.  For example, while many audits/inspections have a strong biocide focus, questions about tillage practices are almost non-existent.  This has resulted, in my view, in the increasing fossilisation of organic soil husbandry, as exemplified by the attitude, “this is how it has always been, and always will be, done” regardless of new evidence.  For example, the enthusiasm for composting is in a large part based on the work of Albert Howard.  However, despite the large increase in understanding of the production of compost since his time, organic devotees are still following Howard’s Indore method, even where it has been shown to be outdated. 

In such a milieu of global debate and organic soil husbandry history, it would be somewhat foolhardy to claim to have the truth and nothing but the truth about tillage management and soil husbandry.  However, I hope to shed some light on what is often a misunderstood area of organic and non-organic agriculture.  I have tried to take a ‘principles of baking’ approach, i.e., giving a person a loaf feeds them for a day, giving them a recipe for bread allows them to feed themselves with bread for the rest of their lives, but if you teach someone the principles of baking, they can work out how produce lots of different breads and even discover new areas of baking such as cakes.  I hope that by explaining some of the underlying processes of soil husbandry and tillage, i.e., a ‘principles of baking’ approach, it will allow a deeper understanding which will allow farmers and gardeners to devise new and better means of optimising, soil husbandry. 

Terroir and soil variation

First up is a couple of big caveats: Discussing soil in detail without discussing the climate that the soil exists under is often pointless because climate has such a huge effect on soil behaviour.  Further climate also affects what types of plants will grow, and vegetation has a big influence on soil behaviour.  The French have a lovely word for the combined effect of soil and climate on a crop (mostly wine!), ‘terroir’.  Soils are also stunningly varied, this is so obvious that it is a truism, but it is rare for detailed articles on soils, especially those espousing compost, to define the type(s) of terroirs that the recommendations apply to.  For example, discussions of soil structure simply don't apply to sandy soils or peats which can not form the structure found on soils with a significant clay content.  However, as this is a high level look at soil husbandry with limited space, I have committed the sin of not being ‘terroir specific’, while imploring you to be wary of such advice.  Put simply, ‘your mileage will vary depending on your terroir’ and that ‘for every rule there are exceptions’ are about the only certain things you can say about soil husbandry.  So please use this advice as a basis for understanding soil husbandry, not as a recipe, or worse a loaf.

The ‘un-scientific’ nature of tillage

To make it clear right from the start very little of current tillage practice has a ‘scientific’ basis.  By this I mean a thorough bottom up understanding of the optimal soil husbandry required to establish and grow a crop, and designing tillage and planting machines based on this knowledge.  The principles and designs of much of our tillage equipment originated hundreds, if not thousands, of years ago, long before science was invented.  Most modern machines are refinements, or rather more often, enlargements, of the original ancient ideas.  This means there has been a lot of top down use of engineering science based on widely held, but false, assumptions of what tillage is required to establish and grow crops.  To put it more bluntly, nearly all tillage and planting machines have been designed by engineers who have little or no biological understanding.  You don't believe me?  Go to any agricultural machinery manufacturer and ask them to explain the effect of their machines on soil physics, chemistry, biology or ecology.  I bet none of them have the foggiest idea. 

The same goes for home gardening.  Recommendations for soil management are rarely evidence based, most are hand-me-down ideas, that often originated centuries ago, e.g., double digging, which were often driven by other concerns than soil health, e.g., an ultra tidy Victorian garden to keep the ‘master’ happy. 

Its not just tillage that lacks foundations.  Many basic agronomic ideas are being found to be misplaced.  For example, on the fundamental issue of how seeds absorb water from soil, we have all been wrong.  They absorb most of the water as vapour from the soil atmosphere, not liquid water through contact with soil particles, see http://extension.oregonstate.edu/catalog/html/sr/sr1047/69.htm .  So, considering that some of our most basic assumptions about soil husbandry are wrong, most of the people designing agricultural machines have practically no understanding of soil processes, especially eco-biological ones, and gardening is infused by old lores that are often just plain wrong, it is not really surprising that the whole topic is a bit of a mess.  This is also why I prefer the original (and American english) term ‘tillage’ than the newer (British english) ‘cultivation’ as there is nothing ‘cultured’ about the damage done to soil by ‘cultivation’. 

No-till and its soil husbandry lessons

The one exception to the top down engineering approach to tillage is that of no-till which I believe has some very valuable soil husbandry lessons for organics.  The drive for no-till started with the American dust-bowls, which were caused by transferring European tillage practices to the terroirs of the mid-west USA, a soil-climate combination they were wholly unsuited for.  No-till unfortunately means many things to many people, so to be clear, it is a production system where there is absolutely no soil disturbance at all, with the exception of drilling / planting.  In addition, crop residue must cover the soil at all times and the aim of drilling is to place the seed in the soil with the absolute minimum disturbance of soil and particularly crop residue.  No-till and no-dig are effectively the same thing, just at different scales.  From the bottom up scientific perspective, no-till started with the question, ‘is it possible to produce crops without any form of tillage at all, and if so, how can this be done?’.  For such a simple question it took decades of research and farmer experience to make no-till work.  Much of it is dependent on drill designs that can successfully plant seeds though inches of thick crop residues in un-tilled soils of all moisture states and get them to reliably germinate, establish and produce good yields.  No-till is now a unequivocally proven success on hundreds of thousands of farms, from small subsistence to massive corporate holdings, on millions of hectares, across continents and many terroirs [1].

So what can we learn from no-till, no-dig and associated soil science?  Rather obviously, for most terroirs, it is totally unnecessarily to till or dig soil to grow good crops.  However, more importantly, the long term experience is that under continual no-till soil health is better than under tillage.  The key to this is that once the soil is no longer tilled and residues are left on the soil surface, earthworms can flourish, and with large earthworm populations comes excellent soil structure.  Further, without regular tillage introducing air (oxygen) into the soil, organic matter levels build up, sometimes to higher levels than under permanent pasture on the exact same terroir. This is the opposite of agricultural experience to date where cropping always depletes organic matter.  This shows that it is tillage, not cropping per se, that destroys soil organic matter.  With hindsight this makes sense, as soil organic matter is determined to a large extent on the amount of organic matter being introduced to it.  Crops which leave large amount of plant residue behind turn over more organic matter than pasture, so organic matter should be higher under cropping, if the soil is undisturbed, the same as soil is undisturbed when under pasture.  The importance of maintaining a layer of crop residue on the soil surface in no-till is impossible to understate.  Even the word ‘trash’, that has traditionally been used to describe crop residues, has been deliberately exorcised from no-till discourse due to its inference that the material is waste and has no value, while the opposite is true.  Another major difference in no-till is the change from a bacteria to a fungi dominated soil, especially the highly beneficial filamentous mycorrhizal fungi that are symbiotic with plants, which are ripped to bits by tillage. 

Having sung the praises of no-till, from the organic perspective it has a huge Achilles heel: it is utterly dependent on a handful, and principally one, systemic herbicide.  With the alarming spread of herbicide resistant weeds (from the non-organic perspective), big problems are on the horizon for such herbicide dependent systems.  Despite this I believe it is inherent that the organic movement takes on board the soil health lessons of no-till as well as the new knowledge coming from related agricultural systems and soil science. 

The starting point for optimal soil husbandry

My starting point for optimal soil husbandry is therefore:  “That for nearly all terroirs, if they are not subjected to compaction, the soil will be healthier if it is not tilled, a layer of recently dead organic matter is always maintained on the surface and/or there are growing plants covering the soil”.  With this as an ideal, farmers and gardeners need to work out how best to achieve it while making the inevitable compromises that the rest of the farm system places on soil husbandry.  However, just because it is not possible to reach the ideal, it is no reason to chuck the baby out with the bath water and give up on the idea entirely. 

For organic systems the way to achieve these compromises is a minimum-tillage (min-till) approach.  This is where only the soil surface is tilled, with the aim of ‘minimising’ the tillage to the absolute necessary (depth and amount) required to establish a good yielding crop.  Min-till, in non-organic farming, is attracting an increasing global following for farmers that don't want to, or can’t, achieve full no-till, and it is now an internationally recognised agricultural system with many benefits over traditional inversion tillage. 

Is no- and min-till possible on all terroirs?

The first caveat in the above aim is ‘nearly all terroirs’.  As soil has been around for hundreds of millions of years longer than man, it’s a pretty fair assumption that it’s pretty much able to look after itself if left alone.  This is part of the reasoning that led to no-till and dig in the first place.  However, there are claims, that no-till/dig (NTD) and min-till are not possible on some terroirs.  I suspect that much of this is down to mind-set, insufficient knowledge, incorrect machinery (especially seed drills), and/or the system has not had long enough to bed-in.  However, it is not impossible that there are soils (terroirs), that even with optimal management, can become compact to the point of limiting crop growth even in the absence of any compacting force being applied to them and therefore needing compaction relieving subsoiling or digging.  However, I suspect that such soils are really very few and far between and that it is lack of understanding and management failure that are the real problem. 

However, the problem may not be with the soil itself, but some other aspect of the system.  Often the first words that farmers and gardeners say when told these ideas is “yes, but what about the slugs under all that residue?”  Well, much like organics, where it takes time for the soil to ‘convert’ from ‘chemical’ to biological fertilisers, soil takes time to convert from tillage to no- or min-till.  NTG practitioners often report slug problems in the first couple of years, then they all disappear forever because the slugs’ natural enemies, such as carabid beetles, can build up their populations once the devastation effects of tillage are stopped and then keep slug numbers below problem levels.  It is therefore strongly recommended that you don't bet the whole farm or garden on any new approach, including min-till.  Min- and NTD has a higher knowledge and management overhead than traditional inversion tillage and digging, the same as organic has a higher knowledge requirement than non-organic, so it essential to do the research and trial it on smaller areas first.   

Compaction

The second caveat is compaction.  Compaction, is the bane of agricultural soil scientists.  They have huge amounts of research showing how horribly bad it is for the soil and the terrible effects it has on crop yields and profit. Despite this, most of their protestations fall on deaf farmer and gardener ears.  It would be easy to write a whole article on compaction management, but very briefly.  Minimise the weight of all equipment, the heavier the equipment, the deeper the compaction it will cause, almost regardless of tyre pressure.  Always use the lowest tyre pressure possible for the job in hand, and where possible use duel wheels, flotation tyres or other means of increasing the ‘foot print’ of a machine, as this will minimise the density of compaction.  Where compaction is inevitable, try to confine it to the minimum area of the field, e.g., minimise the number of passes across the field, use controlled traffic farming, tramlines and the vegetable bed system [2].  Livestock, especially large modern cattle, can cause considerable compaction in even moist soils, definitely remove them if poaching / pugging is occurring.  For the home garden, the advice is very simple, made permanent paths used for all traffic, foot, wheel barrow, etc. and permanent beds which are never, ever, walked on or subject to any form of compaction at all (even the kids) with all work done from the paths. 

Moving on from prevention, if compaction does occur then it is almost always best relieved by deep loosening techniques such as chisel ploughs, subsoilers or forking.  Subsoiling requires a chapter to itself, which it gets in “Resource Management: Soil. [2]” (the 6th edition of ‘Soil Management’) which every farmer, organic or otherwise, should have a well thumbed copy.  Briefly, the tools of choice of for remedying compaction are either the traditional (top down design) subsoiler with a straight leg and wings, or the parabolic subsoiler or Howard paraplough (bottom up designs).  It is also utterly essential to only subsoil at the correct soil moisture, which is when all of the soil lifted by the subsoiler shatters into crumbs.  If soil deforms, or it breaks into blocks put the subsoiler straight back in the shed as more damage will be done than remedied.  The only way to truly determine if good shattering has been achieved is to do a short test run and then dig a hole to the full depth of the subsoiler and observe the effects (several places in the field are best).  However, subsoiling should only be used where compaction is a definite problem, as research has shown that it can decrease yields where no compaction exists, i.e., regular subsoiling, e.g., less than three years, is likely to be highly detrimental to soil and a waste of labour, machinery and money.  Ploughing, the traditional means of relieving compaction, nearly always makes things worse (see below).  For the garden use a large strong digging fork, or if the compaction is at depth one of the special ‘deep forks’ designed exactly for such work.  The aim is to very slightly lift and fracture the soil into a reasonable crumb structure.  The aim is definitely not to lift whole blocks of soil and turn them over and shatter them or create a fine crumb structure. Only enough loosening and fracturing is required to allow soil processes such as drainage to re-establish themselves. 

Why then is compaction such an issue for min- and no-till?  If you are trying to avoid tillage, you must avoid any practices that increase the need for tillage.  Compaction is the major reason tillage is required, so no compaction, no tillage, end of story. 

The financial aspect of tillage

An often overlooked factor in tillage debates centred on soil health, is that every agricultural tillage operation costs time and money, independent of its effect on soil.  From the crude financial point of view, all expenditure has to generate a return, so if a tillage operation costs more money than it generates (i.e., in increased yield), or worse, reduces income (reduces yield), then it makes no financial sense to do it, so leave the tractor in the shed and do something more profitable.  Digging, especially techniques such as double digging are dam hard work and very time consuming.  Why bother if they are not needed, or worse damages soil?

Soil organic matter: much more than humus

It is difficult to underestimate the benefit of organic matter to soils, and there are virtually no terroirs, even peat soils, that cannot benefit from the addition of organic matter, particularly if they are deficient or run down.  However, there is considerable confusion over the terms humus and organic matter, which is not helped by the lack of a scientifically agreed definition of humus.  Generally, humus refers to soil organic matter that has reached a stage of decomposition where it is recalcitrant (resistant) to further decomposition.  It has a lifespan of years to decades with very tiny amounts surviving for centuries.  Compost mostly consists of humus as the composting process has destroyed (decomposed) all the valuable non-recalcitrant material.  Humus is however, only one part of the organic soil fraction.  In a healthy, biologically active soil, most organic matter is in the form of living organisms, from microscopic single-celled species, up though invertebrates such as earthworms and insects, plants, both microscopic and higher plant roots, and all the way to vertebrates such as moles.  Finally there is the freshly dead organic fraction which is the halfway house between the living and humus fractions. 

The living fraction is in constant turnover, particularly the microbes, with individuals constantly being ‘born’ dying, decomposing and being used as food for the next generation.  However, this constant cycling needs a constant source of energy to drive it.  The living fraction is what powers most soil processes and is mainly responsible for maintaining soil structure and soil health, i.e., a soil with high levels of humus but without active biology is a dead soil.  Freshly dead organic matter acts as a short-term store house of nutrients.  A key role of humus is as a cation exchange site; cations are the only form of nutrients that plants can take up in any significant amount, and cation exchange sites are those places in the soil that can hold onto cations strongly enough to prevent their loss to water or air, but weakly enough that plants can ‘absorb’ them.  Clay minerals are the only other significant soil cation exchange site. 

As the living organic fraction is the most important for a healthy soil, where does it get its energy from to keep itself going?  The same as humans and all other non-photosynthesising life forms, the chemical energy in food.  However, as I pointed out in my article on the pros and cons of composting, compost is a very poor source of energy for soil life, because much of the energy in the starting material was lost during composting and what’s left is recalcitrant.  Therefore, the best way to feed soil life is to supply as fresh as possible organic matter, especially plant material, on the soil surface.  After that, it is a pretty straight forward relationship, the more organic matter added (measured by dry not wet weight), pretty much regardless of form, e.g., plant residues, manure or compost, the higher the soil organic matter will be.  However, don't expect miracles, the evidence is increasingly showing that it requires considerable amounts of additional organic matter over many years to achieve small increases in soil organic matter. 

For any gardeners currently spluttering with indignation because their vegetable beds are dark brown with organic matter, gardeners have the luxury of being able to add vast amounts of organic matter , e.g., hundreds, even thousands of tonnes / ha, that, in an agricultural situation, is both infeasible and also unsustainable.  If you could only add organic matter to your vegetable plot that was produced by the plot, i.e., no kitchen scraps that did not originate from the garden, then soil organic matter would be similar to well managed farmland. 

The importance of green cover

While crop residues are important for maintaining soil health, having a continual cover of living plants ‘green cover’ is even more important.  This is called cover cropping and is a globally recognised farming system and has been an organic aim since the earliest days.  It is not so much the protection given to the soil surface from rain and sun, rather it is the retention of nutrients, especially over winter, the continual rain of freshly dead leaves and root activity in the soil that is so valuable.  The main nutrient retained overwinter is nitrogen, which would be otherwise lost, mostly by leaching, but also by denitrification to the air.  The rain of freshly dead leaves is prime earthworm food, especially in winter when the soil is wet and earthworms are most active (some of the highest earthworm populations have been found in winter under green cover crops).  Roots are the forgotten half of plants, yet they are growing and dying just the same as the aerial parts, so are also providing continual food sources for soil life.  In short, if crop residues are good, green cover is great.  Always try to maximise it where possible. 

The value of earthworms

Among the living components of soil it would be remiss not to extol the virtues of earthworms.  There is much hype about earthworms and unlike many aspect of soil husbandry most if it is backed by evidence.  However, it is critical to note that it is earthworms, e.g., Lumbricus species, that are important to soil not genera such as compost worms, e.g., Eisenia species.  Earthworms tunnel through the soil.  Endogeic types form complex horizontal burrows, rarely if ever come to the surface, and feed by ingesting soil.  Anecic types form permanent deep vertical burrows and regularly come to the surface to collect decaying plant residues which they pull into their burrows to eat.  The effects of the feeding habits of both species are to help the breakdown organic matter (which plants cannot absorb) into both simpler organic forms and also plant available mineral forms.  Their burrowing also mixes the soil, with figures of up to 5 cm of soil a year being turned over.  This aerates and drains the soil as well as assisting with the formation of good soil structure.  This is but a snapshot of the multitude of benefits of earthworms, however, put very simply, the bigger the earthworm population, both endogeic and anecic, the greater soil health.  Therefore do everything practical to maximise earthworm populations, i.e., don't disturb soil and provide a steady supply of freshly dead organic matter, especially plant residues.

Organic matter summary

To summarise the benefits of organic matter, it is enough to say that without organic matter, soil is just inert rock dust.  All forms of organic matter are important, however, practically all soil processes are biological at some level, and most of those processes are mediated by living organisms, not dead humus.  Therefore, a soil ‘fed’ only compost is not going to contain nearly as much life as a soil ‘fed’ freshly dead organic matter, so it will not be as healthy.  Also fresh organic matter fed to soil eventually ends up as humus, while the reverse is not the case.  To re-iterate, compost is poor soil food and humus is far less important to soil health than living organic matter.  Finally, increasing organic matter levels suffers from diminishing returns, adding a small amount to a denuded soil will have a big effect, adding a lot to soil already brimming will have virtually no effect at all. 

Tillage

The plough

Moving on from soil properties and function to soil husbandry; first, that icon of farming that generates both admiration and loathing, the plough.  So what is the problem with ploughing?  Ploughing is an intensely damaging activity for soil.  Ploughing, particularly in the furrow, is a major cause of compaction, worse because it is deep compaction.  Ploughing also kills earthworms and filamentous fungi, species which are of immense help in improving soil fertility and crop productivity.  It introduces large quantities of air (oxygen) into the soil which results in rapid loss of organic matter (mineralisation), which in turns results in loss of soil structure, and, if there is water draining from the soil results in nitrogen leaching.  It also increases the risk of surface erosion, i.e., soil loss, which is effectively impossible to replace and is a major cause of phosphorous loss and pollution.  It also buries crop residues, which often results in their decomposition becoming anaerobic resulting in the production of substances toxic to soil and plant life.  In addition, the amount of organic matter destroyed due to ploughing-under residues is often greater than the amount of material ploughed under, i.e., there is a net loss of organic matter.  In pasture, especially long term pasture, a significant proportion of the fresh and living organic matter accumulates in the top few centimetres of the soil, which means that this is were much of the plant available nitrogen and phosphorous accumulate.  Ploughing will bury this out of reach of many plants.  However, on regularly ploughed or tilled soil, the nutrient distribution is pretty homogeneous.  In all, there is absolutely nothing good to say about ploughing for soil husbandry. 

Why then is ploughing so popular?  First, probably ignorance of the harm it causes.  Second, European and NZ terroirs can handle the damage, unlike those of the mid-west USA.  Third, it is an incredibly useful tool that can quickly turn a field of crop residues, weeds, pasture or just a mess, into a fine tilth ready for planting.  This is probably the overwhelming reason that ploughing persists despite its ‘dark side’.  It can also be a valuable tool for weed management as it can be used to bury weed seeds when large amounts are shed and hard to control perennial weeds.  However, it will also bring up the last lost of seed that was buried and if perennial weeds are not completely killed by inversion, they often re-establish themselves from depth, making them much harder to eradicate.  Therefore the only excusable reasons for ploughing are weed and vegetation management and preparing a tilth for the drill.  Therefore, to eliminate ploughing from a farm system, it is not so much alternative tillage techniques that are required, as these are common, e.g., subsoiling, shallow tines and disks.  Rather, it is alternative weed management techniques and drilling, planting and hoeing equipment that can handle crop residues.  For some inspirational examples of farmers achieving just this http://www.newfarm.org/depts/notill/index.shtml is a great example, just don’t expect to be able to quickly implement in on a UK terroir. 

The rotovator

If the plough gets a bad press, then the elephant in the machinery room has to be the rotovator.  It is difficult to describe just how barbaric this machine is, evil is perhaps close.  It has all the vices of the plough and practically none of its redeeming features, yet never seems to get the same criticism.  The only good thing is that it produces less compaction at depth.  It is far worse than the plough for mixing soil layers.  It mutilates earthworms and soil fungi decimating their populations.  It introduces vast quantities of air with all the problems that entails.  It smashes soil structure to smithereens, creates a thin, smeared compact soil layer that prevents root penetration, air exchange to the lower soil layers and inhibits water drainage.  It mixes crop and weed residues and weed seeds through the tilled layer, to name a few of its terrible effects.  So why is it so popular?.  I suggest, ignorance, any fool can use one (and frequently do), and it’s a jack of all trades, doing a job (badly) that would otherwise require several different machines, e.g., mower, plough, tines and harrows. 

Unfortunately for the larger home garden and small market garden and especially in protected cropping, e.g., tunnels, the rotovator, often becomes the major, or worse, only cultivation tool.  It’s almost like a drug, people get hooked, even though it causes huge damage, and then can’t wean themselves off.  The solution?  Go cold-turkey, sell the beast, use a fork or tines to loosen the compact layer below the depth of soil that the rotovator tilled, and then instigate a NTD system. 

Having thoroughly demonised the machine, the one thing I believe it is useful for, and where the damage it does is small and balanced by the practical advantages plus the harm alternative approaches can do (e.g., ploughing), is for very shallow (less than 5 cm) skim tillage to kill off vegetation, e.g., pasture and green manures, where alternatives such as disks and sweep coulters are impractical (e.g., vegetable beds) or it entails a significant reduction in the number of trips across the land, i.e., reduces compaction.  If the soil is only tilled a few centimetres deep, only a small fraction of the soil is damaged and the smeared layer is destroyed by subsequent tillage. 

Assessment of tillage machinery on soil health

With the huge number of agricultural tillage machines it is impossible to give each an individual commentary.  There is however, a few simple guides that I consider helpful in estimating how good or bad a machine is for the soil.  The first is based on depth of tillage.  Based on the premise that less is best, the shallower the tillage the better it is.  The exception is where compaction has to be relieved, when it is inevitable that deep tillage be used.  The next guide is the amount of kinetic energy imparted into the soil.  The less is best premise equally applies as it takes energy to break soil apart, and the more energy used, the more the soil will be broken down.  The amount of energy imparted to a soil is the sum of the draft to pull the machine through the soil and the power delivered by the PTO.  Low-draft, non-PTO, shallow tillers like harrows are therefore streets ahead of low-draft, high PTO powered, medium depth machines such as rotovators.  Next is the amount of vertical mixing of soil layers, with less mixing being better, so a vertical axis power harrow is better than a horizontal axis rotovator, even though both are high PTO power machines.  Finally is an analysis of how a machine breaks the soil structure apart; machines that allow soil peds (clods) to fracture along their natural lines of weakness are preferred that those that compress and/or smash peds.  For example, a tine lifts soil up allowing it to break along its weak points, while a rotovator blade vertically impacts on the soil surface, crushing and compacting soil before smashing the soil open as it is churned within the rotor.  Speed of soil engagement is am important component of how a soil will fracture, slow speeds allow sufficient time for cracks to propagate along the peds natural lines of weakness, high speeds result in the tine or blade moving through the soil faster than crack propagation speed.  The effect on the soil is very clear; soil that has been allowed to break along its natural fracture lines will be angular and blocky, like a frost tilth, while forced tilths will be round and balled up. 

The folly of soil aeration

Another misconception is that of soil aeration.  For example, there are a host of machines and tools sold to ‘aerate the soil’ and crops, such as potatoes, are espoused for their ability to ‘break up and aerate the soil’, to name but two among many badly misguided, ideas.  Soil is aerated (breathes) primarily due to wetting and drying cycles, where soil pores fill with water during rain which then drains out or evaporates, thereby sucking air into the soil.  The other main ways the soil and atmosphere exchange gasses are due to variations in atmospheric pressure as high and low pressure areas alternate and plain old diffusion.  Only severe compaction will stop these processes, certainly not pasture thatch, other surface plant residues or even plastic sheeting unless it is totally sealed to the ground.  Unless there is serious compaction, there is almost certainly no need to aerate the soil at all.  Indeed the converse is true, any aeration, especially that caused by tillage, will damage the soil.  Firstly tillage directly destroys soil structure.  Secondly aeration provides soil microbes with huge amounts of oxygen which they don't naturally get, which they promptly use to destroy (mineralise ) soil organic matter (including humus) turning it into ammonium, then nitrites and nitrates which can be lost from the soil through leaching or by nitrogen gas produced by microbial denitrification.  In short aerating the soil destroys soil organic matter and soil structure, which is a terrible toll to impose of soil, so don't do it.  The only time aeration to mineralise nitrogen may be justified is in spring arable crops when there is little soil nitrate available, and even then, only shallow surface tillage should be used, i.e., an interrow hoe. 

Putting it all together

To convert from a full inversion to minimal tillage system, on arable, vegetable or home garden systems, is not going to be a straight forward task, just as converting from non-organic to organic is no place for fools.  However, if the aim is to maximise soil health, there are overwhelming reasons to minimise tillage.  If the aim is just to increase profit or reduce back breaking labour, then why waste money on unprofitable, or even loss-making tillage and hard physical toil that can be done without?  Changing to a min-till and no-dig approach requires changes to be made to the whole farm and garden system, just as for the change from non-organic to organic production.  Alternative management techniques will have to be used or developed to replace those provided by machines such as the plough, and new and different machines are likely to be required.  New approaches will be equally required in the garden. What works and what does not will depend on terroir, farm system and management skills.  There is no more a recipe for great soil husbandry than there is for organics.  The ‘principles’ are the foundation on which to build knowledge, experience and ultimately, success.  Fortunately the principles are pretty straight forward, minimise tillage and digging to the essential minimum, avoid all forms of compaction and only use deep vertical loosening to relieve it, feed the soil with fresh organic matter maintaining a residue cover whenever practical and always try to have growing plants in the ground. 

I believe it is essential that ‘soil husbandry’ is retrieved from its neglected status and rescued from ‘recipe’ based approaches that are often only applicable to a limited range of terroirs, and moved back up the organic agenda, ideally to the position it held for founders of organics.  If it is not, I believe that organics could be left behind by the increasing strides of the newly integrated science of soils and government realisation that soil is an irreplaceable resource and needs the strongest protection.  This would not be good for a movement that has healthy soil as its foundation. 

1.         Baker, C. J. and Saxton, K. E., eds. No-tillage Seeding in Conservation Agriculture, 2nd Edition. 2007, Food and Agriculture Organization of the United Nations: Wallingford, UK. 352.

2.         Davies, B., Finney, B., and Eagle, D., Resource Management: Soil. 2001, Tonbridge: Farming Press Books. 280.

 

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Copyright 2008 Charles N. Merfield.