Soil Nutrient Management for Irish Organic Agriculture | Organic Trust Ltd

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Soil Nutrient Management for Irish Organic Agriculture

From Dr Charles N Merfield, Teagasc, Johnstown Castle We are delighted to present the following which is the first part in a 2 part article by Dr Charles Merfield

This work gives an overview of soil nitrogen (N) phosphorus (P) potassium (K) and pH management in Irish organic agriculture and horticulture.  As for many other aspects of organic agriculture it is not ‘a recipe’ of how to grow organic crops, but more of a guide on ‘how to farm’.  It must therefore be combined with other sources of organic production information as well as your own experience and tailored to the climatic conditions and soils of your own farm or holding to determine the best approach to meeting your farm’s optimal nutrient and agricultural management. 

This work will also be the basis for a future Teagasc booklet and an organic section in Teagasc’s third edition of the ‘Green Book’ officially called ‘Nutrient and Trace Element Advice for Productive Agricultural Crops’.  These future publications will include more detailed information and cross referencing to national and international legislation and environmental protection schemes and should be used in preference once they have been published.

The organic dichotomy: law of return vs. closed cycles
Organics contains something of a dichotomy regarding nutrient management, which reaches all the way back to its earliest founders.  One view is the ‘law of return’ where it is considered essential that any nutrients removed in crops or livestock must be returned to maintain fertility i.e., a balanced nutrient budget.  The other view considers the farm to be a ‘closed system’ for nutrients and that they should be carefully (re)cycled within the farm and the need to ‘import’ nutrients is considered a system failure.  However, the latter approach does not consider the removal of nutrients in crops and livestock or losses from leaching or to the atmosphere.  The scientific evidence is now overwhelming that both must be followed: the law of return (balanced budgets) must be coupled with efficient cycling of nutrients around the farm.  However, most English language standards are based on the ‘closed system’ concept which has resulted in standards setting high barriers for importing ‘non-gaseous’ nutrients such as phosphorous and potassium, even though ‘importation’ of the gaseous nutrient nitrogen via legumes is considered indispensable.  The scientific evidence is clear that it is equally essential to replace P, K and other ‘non gaseous’ nutrients using approved substances such as biological materials and mineral fertilisers.  Therefore, modern organic agriculture takes a similar high-level position to non-organic agriculture in that all nutrients removed or lost from the farm system have to be replaced, the differences are in the details of what forms of fertiliser are appropriate, the emphasis placed on maximising on-farm nutrient cycling and whether nutrients should be used to optimise soil quality and health or used to directly feed crops.

Feed the soil to feed the plant
The aim of fertilisation in organic farming is to ‘feed the soil to feed the plant’.  Soluble fertilisers must be avoided as the nutrients they contain can be directly absorbed by plants, which can result in excessive uptake with resulting problems, e.g., lodging, reduced dry matter content, lower nutritional quality and increased susceptibility to pests and diseases.  Soluble fertilisers are also more prone to leaching from the soil than less soluble fertilisers, leading to eutrophication of water bodies and atmospheric N pollution.  The unprocessed (raw / rock) forms of mined mineral fertilisers (P and K) also contain small amounts of some minor and trace elements which are removed during processing into the more ‘pure’ soluble forms.  This means that minor and trace elements often have to be applied separately when using processed fertilisers while they are supplied as part and parcel of unprocessed mineral fertilisers.

The aim for organically approved fertilisers is to allow biological soil processes (microbial activity) to progressively release the nutrients contained in the fertiliser so plants get a more balanced and continuous supply.  Many of these biological processes are temperature dependent, so more plant available nutrients are released during the growing season when the soil is warmer and when plants need them, while less are released in the cold of winter when there is a greater risk of nutrient leaching and most plants are hardly growing.  This also means that it is not normally possible to get a ‘quick response’ from organic fertilisers, so if deficit occurs it will take some time to correct.  This means that it is essential to have a long-term nutrient strategy, which is also a requirement under organic certification standards.  In a nutshell, a nutrient strategy is based on regular, ongoing soil nutrient analysis, coupled with nutrient budgets, which are used to determine the need for the application of manures, composts and permitted fertilisers.

The wider aim of soil management in organics is to create a ‘healthy’ biologically active soil flora and fauna by maintaining a high level of soil organic matter and minimising soil disturbance caused by tillage.  Changing from a synthetic fertiliser regime to one based on legumes for N fixation, manures and mineral fertilisers can have a considerable impact on soil biology.  There is considerable scientific evidence and farmer experience that this change takes several years, even decades, and can result in an initial drop in some crop yields until the soil biological processes have adapted to the new types of fertilisation and yields increase again.  This effect has also been well documented when changing other production practices, for example from tillage to no-tillage.

Soil analysis is your best guide
Whatever type of fertiliser is to be applied, e.g., mineral or manure, application rates must be guided by soil analysis coupled with field-by-field nutrient budgets.  It is not appropriate to just apply fertilisers ad hoc, as this has the potential to waste valuable and sometimes expensive fertiliser resources, cause leaching or run off which pollutes waterways, and cause an imbalance of soil nutrients e.g., a high K index with a deficient P index.  Soil tests should be viewed as a long term strategy with each field being tested every 3-5 years depending on intensity of production.  They should be taken at the same time of year, ideally at the same stage in the rotation.  These should be cross referenced with nutrient budgets for each field which will give a useful double check if excessive or insufficient amounts of nutrients are being applied.  If the soil tests and nutrient budgets are in agreement, e.g., more K is being applied than removed and K soil levels are increasing then the action required is clear (don’t apply any more K until the index drops and budgets balance).  If they are at odds, e.g., more P is applied than removed but the P index is decreasing, this may indicate loss from the system which requires further investigation.

Soil testing is not however a totally precise science.  There are a range of tests for the same soil nutrient, they often give different results and they also vary in their suitability for different soil types.  Therefore swapping among soil tests or testing laboratories will result in unusable information.  Consistency is the watchword: choose your soil testing laboratory and nutrient advisers / consultants and stick with them.  Only change if you are seriously dissatisfied.

There are also considerable differences in how well different soils provide nutrients to plants, what their optimal pH is, and how they respond to fertilisers and lime.  There is also wide disparity across Ireland on minor and trace element availability with some soils being deficient while others are at potentially toxic levels.  In all cases it is essential to have the tests and advice tailored to your particular soils, which re putable testing services and advisers will do automatically.  Where you have a number of different soil types on your farm, especially if some are mineral and others peat (organic) based, then individual soils types are likely to require quite different fertilisation and liming programmes and this must be taken into account when working out a soil sampling strategy.

‘Advanced’ soil and plant analysis
Internationally, particularly in the United States and Australasia, considerable emphasis is given by some organic famers, their advisers and scientists to trace elements, cation exchange capacity and the ratios between specific nutrients.  There are also specialist soil testing laboratories which undertake highly detailed soil tests and give very comprehensive fertiliser advice, often for an equally comprehensive fee.  While there is scientific consensus that exchange capacity and the ratios of various nutrients (and pH) are real and important aspects of soil quality and performance, what the optimum exchange capacity, nutrient balances, etc. are, is hotly contested among the various proponents.  In addition, the importance of these effects compared with ‘just’ optimising NPK and pH is even more contentious.  To further confuse matters, many of the soils on which the research has been undertaken are quite different to Ireland’s soils and climates.  Until such ideas are authenticated under Irish conditions it is recommended Irish organic farmers and growers continue to use the same nutrient indexes as non-organic producers and focus on optimising major nutrient levels and pH while ‘keeping an eye’ that minor and trace nutrients are within prescribed ranges.

Organic, national and international compliance rules
Organic certification has three ‘classes’ that it puts farm inputs into, ‘permitted’, ‘restricted’ and ‘prohibited’.  Permitted are allowed to be used without restriction.  Restricted are allowed to be used but normally only after permission has been given by the certification body, and prohibited are completely banned.

It is equally important to check that all planned fertilisation activities comply with national and European Union (EU) legislation and environmental protection schemes, e.g., the Rural Environment Protection Scheme (REPS).  Many of these policies are now enacting the types of environmental protection that the organic movement has been advocating for many decades.  With the rapid advancement of some of these policies compared with the comparative slowness in progressing EU organic standards, national and EU legislation can now have stronger environmental and societal protection than organic standards, i.e., meeting organic standards does not guarantee cross-compliance with legislation.

In any situation, if you are unclear if an activity is allowable under organic or other regulatory systems, it is essential to consult with the appropriate authorities before undertaking the activity.

Nutrient sources
The sources of the main and minor plant nutrients for organic farms depend on the nutrient that needs to be replenished.  Most are applied as biological material such as manures and composts and/or approved types of ‘mineral’ fertiliser.  Most of the mineral fertilisers are minimally processed, mined, fertiliser rock, for example, ground phosphate rock (GPR).  Normally chemical processing is not allowed, but the rock is ground to a sufficiently fine particle size that biological soil processes can release all the nutrients over a few years.

Nitrogen (N) Introduction
The main source of N in organic farming is the fixation of elemental atmospheric N into ammonia by the bacterium Rhizobium which live in the root nodules of leguminous plants and also in the soil.  Ammonia is rapidly converted into other mineral N forms, e.g., ammonium and nitrate, by other soil microbes before its ‘final’ conversion to nitrite which is the only form plants can take up N.  The mineral forms are also converted into organic matter, by both microbes and plants, which is the form in which N is ‘stored’ in soil.  Therefore, growing legumes, in pasture, as green manures and as cash crops is essential for successful organic N management.  For example, red clover can fix up to 450 kg N/ha/year and white clover 420 kg N/ha/year.  A balance therefore has to be struck between exploitative and restorative phases of the rotation to ensure that while soil N will be reduced through the exploitative cropping phase it is replaced by restorative crops and pasture so maintaining N levels averaged over the rotation as a whole.

There are legislative and other national and EU level restrictions on the total amount of N that can be applied to the soil as manure, compost and other biological and mineral forms.  This is currently 170 kg of N per ha per year of agricultural area used, which includes both applied manures and that produced by livestock while grazing.  Figures are provided by government agencies and in organic standards detailing the amount of manure and its nutrient content produced by different livestock types which when combined with stocking rates and amounts of manure applied give the total N application.  These are likely to be revised and updated so it is essential to use the most recent versions.

Organic certification also limits the amount of biological material brought onto holdings to replace N.  Greater leeway is made for smaller dedicated horticultural units where the lack of livestock reduces the ability to move nutrients from the restorative phase to the exploitative phase via manure.  However, horticultural units still face the same national and EU application limits as less intensive operations.  There is also the possibility of using more soluble forms of N in intensive horticultural operations, for example, seaweed meals, where a proven need can be proven to the certification body.  However, such products are also often expensive so their use is only viable on high value crops and the most economical long term strategy is likely to be a restorative clover and pasture phase as part of a rotation.

In all cases if you are uncertain if what you plan to apply is within the limits please consult a qualified agricultural adviser, your certification agency and/or government body as appropriate. 

Nitrogen sources Mixed clover and grass pastures / leys
In mixed and livestock only farming systems, the main source of N is the mixed ley, which is typically based on grass and white clover due to its persistence under regular grazing.  A two or more year mixed ley is far more effective at accumulating N and increasing soil organic matter,  plus
improving soil structure than any alternative, e.g., green manures.  For high quality silage production red clover can be a better option, however, it does not perform so well when grazed as it is less persistent than white clover when regularly defoliated.  Teagasc has conducted extensive research into white clover management in pastures the results of which along with detailed practical guidelines can be found in the “Moorepark Dairy Research Update:  A Guide to Management of White Clover in Grassland” most of which is directly applicable to organic farms. 
A key issue not addressed in the white clover guide is the destruction of the pasture at the start of the cropping phase to retain as much N as possible.  A vigorous pasture can accumulate considerable amounts of N, e.g., 400 kg N/ha/year, over its lifetime so considerable care must be taken when terminating the pasture that large N losses do not occur.  A typical scenario to cause this would be tillage using a mouldboard plough followed by several secondary cultivations in early autumn when soils are still warm and biologically active but freely draining water, and then leaving fallow over winter.  Losses of 150 kg N/ha or more could be expected under this situation.  Better options include leaving the pasture overwinter, then shallow cultivated in spring and planted to crop as soon as possible.  If cultivation at the end of the season is unavoidable, then the cultivations should be minimal and a nutrient trap crop (cover crop), e.g., a winter cereal or cereal and legume mixture should be sown.  There are national restrictions on autumn ploughing aimed to address this exact issue and these must be complied with.
Charles Merfield