Inaugural Newsletter

‘Conventional' and ‘Biological' Fertiliser Perspectives

It was great to see 100+ people turn out for the first day in a new series of extension events in the Tararua - thanks to all who attended!

We bravely tackled two topics which can be a source of endless debate: aerial cropping/re-grassing, and ‘conventional' versus ‘biological' approaches to fertiliser.

Although the day may not have changed your philosophy on either of these areas, hopefully you came away from the day with a better knowledge of both topics.

There are PDFs available of each speaker's notes and/or powerpoint presentation; these are available from Richmond Beetham.

This newsletter focuses on the soil and fertiliser part of the day. Some attendees may have been disappointed that there was limited debate between the two sides and perhaps inadequate coverage of some areas, particularly with regard to some of the concepts that are currently being promoted in ‘biological farming'.

So below you will find a list of the topics covered on the day, with a summary of each speaker's perspective on these; this has been expanded on where necessary with help from some readily available NZ soil science references, plus some more recent coverage of biological topics. You will note that there is much crossover between ‘conventional' and ‘biological'; both our presenters noted this, and it should be borne in mind that the two approaches are not independent of each other.

I'm sorry that this is such a big newsletter! But given the high degree of interest in the subject, and that some areas weren't adequately covered on the day, we wanted to provide you with a more complete resource.

Thanks for your interest, and if you have any feedback, be sure to let us know.

Ginny :-)

Ginny Dodunski, on behalf of the facilitators, Baker & Associates and Totally Vets Ltd


‘Conventional' v. ‘Biological' Fertiliser Perspectives


We deliberately chose keynote speakers for this topic who were not affiliated to any particular fertiliser sales entity. So we had:

‘Conventional': Ian McNabb, AgKnowledge Manawatu

‘Biological': John Turner, Independent Fertiliser Consultant, provides advice to Hill Laboratories

As part of the exercise we also asked each party to undertake a soil test and assessment on the same block of Tararua Hill country, and present their recommendations. For this section, Jim Macmillan from Outgro Ltd presented the ‘biological' recommendation.

The following table summarises each speaker's ‘take' on the various topics that were covered.




Soil structure and physical attributes

The performance and resilience of a soil is a function of its physical, biological and chemical properties, and these all impact on each other.

A typical NZ pasture soil will be about 50% pore spaces (containing air and water) and 50% solid soil. Around 10% of the solid fraction (5% of total) is organic matter (OM).

‘Conventional' soil science concedes that liming can have a beneficial effect on soil structure and physical properties, but that currently in the NZ pasture situation there is no evidence that other applied soil amendments will have a beneficial effect on the physical attributes of most soils.

Agree. Although the organic fraction is relatively small, the reactions and activity within it have a big impact on nutrient availability and resulting pasture performance. Biological fertiliser proponents argue that the activity and health of the soil biomass can be strongly influenced by what is added or not added to the soil as fertiliser, and other ‘soil amendments' applied concurrently.

While John Turner did not disagree with this, he stressed that issues  affecting the whole soil mass, such as drainage, subdivision and prevention of treading damage  should be given equal importance to the type, form and rate of nutrients being applied.  John showed a soil test result with excellent P levels, good pH and cation exchange capacity, which was in fact from a hard clay soil with major structure and drainage issues that ‘grew virtually nothing for most of the year'!

Nutrient availability

Nutrients are taken up by plant roots as the dissolved ion form in soil water. They can leach away from the root zone in this form. Note that of the major nutrients, this mostly applies to N and S. The concentration of P in the soil solution at any one time is very low; see below.

Root mass and depth have a major influence on nutrient availability. The environment directly around the root (rhizosphere) can be quite different from the environment of the bulk soil due to the activity of microorganisms living off the products of root sloughing. See below for some examples of this.

Phosphate and P requirements

Phosphate is the element needed in most quantity by clover. Clover is less competitive for P than grasses. If you are running a clover-based pasture system you must supply it with P.

Phosphate is held to or within soil particles via various chemical bonds; its availability is driven mostly by inorganic reactions within the soil.

There is also P in the organic fraction of the soil, and rapid organic cycling of P can contribute a variable amount of P to plants. Increased soil microbial activity will not necessarily improve P availability to plants, as much P released by this process is quickly recaptured by the soil microorganisms, which also have a high demand for P.

The Olsen P test is the test of choice in NZ because it has been most extensively calibrated with pasture production. It measures the concentration of available inorganic P.

Most applied P remains in the topsoil layer and does not leach; since the concentration of P in the soil solution is very low as described above.

Use the least-cost form of P for annual pasture dressings. ‘Standard' maintenance rate for P is 1.5kgP/su/ha/yr; derived from a large set of NZ data.

It takes 5kg of P/Ha to lift Olsen P by one unit; an extra 2-5 kg  above this on high retention soils.

Phosphate is also present in the organic fraction of the soil. The P in the organic fraction is in equilibrium with the soil solution. Where there is poor earthworm and/or microbial activity to break down dead and decaying litter, P availability can be reduced. In these cases management practices that improve nutrient cycling in the organic fraction of the soil can improve P availability.

The combined results of 17 long-term NZ field studies (NZJAR 1997) indicated that ‘at low Olsen P, relative pasture yield may range from low to high, but at high Olsen P levels the chance of a low relative yield was decreased': So in some cases high relative pasture yield is possible in the face of low Olsen P levels. More work is required to define the drivers of this phenomenon.

The ‘standard' Olsen P test sample is taken to 7.5cm soil depth. Many pastures have a root zone depth double this or more and a ‘lower' P level may still result in good yields. Outgrow use a soil sampling depth of 15cm.

John Turner: ‘There will be more plant-available P where there is a good healthy layer of topsoil and an Olsen P of 18 versus a shallow topsoil and Olsen P of 30'.

Nitrogen Nitrogen is the main nutrient driving pasture growth.

95% of the nitrogen in soil is present in the soil organic matter and unavailable to plants. To become plant-available these organic compounds need to be broken down (mineralization and nitrification)) to nitrate by microbial activity in the soil. The drivers of the rate of this are all the factors that influence microbial activity - pH, moisture, aeration, temperature, nutrient status. Once mineralized, N is prone to leaching and volatilization.

Much of the N taken up by pasture plants is derived from Rhizobium bacteria that live within the roots of clover in the sward. This system can fix 50-200kg N/ha/year. However, clover-derived N often sets a ceiling on pasture growth and increased annual yields can be obtained by applying fertiliser N.

There are free-living bacteria that can also fix N but this amount is minimal compared to clover-derived N - 0.3kg/ha/year.

A biological approach would not apply nitrogen on its own.

The commonly used N fertilisers are regarded as acidifying because when they are converted from ammonium to nitrate in the soil there is a release of H+ ions. Also when nitrate (N03-leaches from the soil, it takes an ion of opposite charge with it, often calcium. Many biological fertilisers contain N as the dominant element however.  The N in soluble fertilisers can be directly absorbed in small amounts by the leaves of the plant.

Free living N-fixing bacteria can thrive in the rhizosphere (see ‘nutrient availability', above) and may contribute N to plants without the N being apparent in the bulk soil? To what degree this may happen in NZ pastoral soils has not been well studied or described

Other nutrient requirements

After ensuring optimum P levels and appropriate pH, focus on achieving plant-sufficient amounts of potassium, sulphur, magnesium, calcium and sodium plus check that molybdenum is not limiting to clover. Inadequate potassium is common on sheep & beef farms and will limit clover production.

Clover tests are important as well as soil tests. Some elements are applied in excess of plant requirements because of animal health reasons; e.g. selenium, magnesium.

Micronutrients other than selenium and in limited circumstances, cobalt, are rarely applied to soils to prevent animal deficiencies, because of the many interacting factors that can render these unavailable to stock.

A ‘biological' approach considers calcium to be very important and would advocate that it be applied frequently. Outgrow monitoring work shows a seasonal pH drop each autumn.

‘Biological' fertiliser mixes will frequently contain nutrients such as Zinc, Iron, Copper and Iodine.

Soil pH, Calcium and lime requirements

Acidification occurs in most soils over time. Acidification will proceed more rapidly in a highly fertile and productive pasture soil; in large part driven by increased mineralisation of N and S by soil organisms, this being a consequence of the increased clover activity, pasture growth (and therefore litter and animal waste deposition) that result from improved fertility.

Optimum pH is 5.8-6.0

Liming corrects this process of acidification and does not need to be carried out every year. The most economic rate of lime for a farm may not be that which results in the highest pasture production.

Liming also improves the ‘wetting' ability of soils with hydrophobic properties. Research is ongoing in this area.

Calcium per se is almost never deficient in NZ pasture soils and is in fact applied in large amounts each year wherever superphosphate is being used. Calcium levels in NZ soils are never low enough to advocate that it be added to improve clay structure.

The leaching of basic cations from the soil (e.g. Calcium, magnesium and potassium) is a leading cause of soil acidification. These are usually replaced by the acidic hydrogen or aluminum ions, resulting in a drop in pH.

Liming therefore increases the ‘store' of basic cations in the soil which is an additional buffer to pH over and above the initial release of hydroxyl ions which provides the initial pH raising effect of lime.

A ‘biological' approach considers calcium to be very important and would advocate that it be applied frequently. Outgrow monitoring work shows a seasonal pH drop each autumn.

Calcium is thought to encourage earthworm activity and thus aid in aerating the soil.

A high level of calcium is associated with a stable soil structure.

Other cations and CEC(cation exchange capacity)

Cation is a fancy word for an ion with a positive charge. With regard to soil fertility the important ones are: Potassium (K) Sodium (Na) Calcium (Ca) and Magnesium (Mg)

For NZ pastoral systems it is important that both K and Mg levels in soil are adequate.

Note that excess K can precipitate metabolic problems in cows around the calving period

Cation exchange capacity (CEC) is a measure of a soil's ability to hold these positively charged nutrients and is a function of how many sites there are on the soil where these nutrients can bind.

The number of CE sites is determined in part by the number of clay particles in the soil, and also the amount of humus present. pH also has an effect.

CEC for agricultural soils is ideally between 10-30meq/100g.

A conventional approach would advocate that for pastoral soils, as long as pH is OK and Mg and Ca are not deficient, worrying about CEC and base saturation is unnecessary.

A biological approach advocates that CEC can be improved by improving the level of humus in the soil.

Base saturation is regarded as an important measure relating to cation exchange, it expresses what percentage of the cation exchange sites are taken up by ‘basic' (non-acidic, ‘good') cations. In most NZ soils calcium would account for over 80% of the basic cations.

Soils with low base saturation will have more H+ and aluminium ions on the exchange sites, which can negatively affect productivity and health of soils.

Low pH (acid) soils will have a reduced CEC and base saturation.

pH, CEC and Base saturation are considered key indicators of soil health.

Humates and other soil conditioners

NZ soils are relatively rich in organic matter; on a site producing 10T of pasture/year another 10T of litter is being returned to the system that we don't ‘see'

Soil humates are the soil's ‘mature compost' - organic matter which has been degraded to a point where it is highly resistant to further breakdown

Humates in soil improve: Water-holding capacity, CEC, soil structure, soil biological activity and plant root growth.

Addition of humates may improve performance of soils which are very low in organic matter or very low in clay particles E.g. raw sands or podzols.

Most NZ pasture soils are relatively rich in organic matter and the amount of humate able to be added in fertiliser blends is highly unlikely to have any measurable effect.

Ian McNabb showed that in a typical NZ pasture soil there may be 15 tonnes of fulvic acid (a major soil humate) per hectare in the top 10cm.

Many ‘biological' fertiliser blends will include humates as part of the mix.

The fertiliser recommendation presented by Outgrow applied total product of 315kg/ha/year; an unspecified amount of which was humic and fulvic acids.

Jim MacMillan commented that there is international research to show that the application of humates at the same time as other nutrients can enhance their uptake.

Gibberellic acid

A naturally occurring plant growth regulator that increases leaf & stem growth. The window of application is 1-5 days post grazing; before plant starts to produce its own gibberellins; therefore best suited to a paddock-by-paddock application system.

Gibberellic acid was a component of the ‘Outgrow' fertiliser recommendation.  Any growth response will be additive to any N effect.


Adding earthworms to a site where they are absent can increase pasture production by 10-30%.

Earthworms under NZ pastures are accidental introductions from European settlement and their distribution is patchy.

Whether earthworms are present in pasture soils or not is more to do with whether or not they made it to your patch by accident or not rather than what has happened to your soils since

That said; the things earthworms dislike is treading damage, excessively acid soils, constant saturation and lack of litter to eat! Higher fertility sites producing more pasture litter are a better environment for earthworms than low fertility sites.

All the outcomes advocated by a biological approach will improve the health and number of earthworms in a soil, and this will positively feed back as further improvement in the health of and nutrient supply from the soil organic matter.


Soil test and fertiliser recommendations


Ian McNabb (Agknowledge) and Jim Macmillan (Outgrow) presented fertiliser recommendations for a block of quite intensively farmed, well subdivided Tararua Hill country running 12.5 su/Ha which like many sheep and beef farms has had sub-maintenance fertiliser applied in the past 2-3 years. These assessments were completed in late August 2011.

The owner's aims are: Make more profit, grow more grass in winter and maintain more ryegrass in the pasture (browntop is increasing).

The soil test result is below.




   Olsen P









Plot 1











Plot 2























A visual assessment completed by the Outgrow team showed:

  • Rooting depth of 186mm
  • Minimal thatch, earthworms present; average size
  • Scored 3/10 for flatweeds, 4/10 for moss
  • Scant clover, small leaves (time of year a factor?)

Outgrow also ran a herbage analysis and animal dietary report from pasture samples. Of note from these samples were low selenium levels and very high iron levels (which can interfere with the uptake of other elements by animals; notably copper). Zinc and sodium may have been ‘marginal' but it is important to note that a production response to supplying extra of these would be very unlikely, especially in a sheep and beef system (ed).

Agknowledge recommendation:

  • Capital P and S required, plus maintenance K:
    • To lift Olsen P to 25 would take an extra 50kg P/Ha above maintenance; therefore require 80kgP/Ha
    • 50kg K/Ha
    • The above could be supplied by 500kg/Ha ‘Superten 5K' in 2 applications, which would also supply the extra sulphur needed; total $500/Ha (applied)
    • When questioned, Ian commented that lime could be applied the following year to correct any further drop in pH, since the current pH is at the lower end of optimum

Outgrow recommendation:

  • Spring: ‘Outgrow Pasture Boost' 65kg/Ha: Contains N (9.6kg), S (0.12kg), Na (2kg), Zn (0.15kg), Se (0.01kg), plus gibberellic acid, humic acid, fulvic acid, Ulmic acid, molasses
  • Autumn: ‘Outgrow Soil Revitaliser' 250kg/Ha: Contains N (9.4kg), P (4.1kg), K (0.2kg), Ca (51kg), S (3.8kg), Mg (1.5kg), Na (6.7kg), Fe (0.1kg), Mn (0.15kg), Zn (0.15kg), Cu (0.06kg), B (0.14kg), Mo (0.004kg), Se (0.01kg), plus humic, fulvic, ulmic acids, molasses and ‘Outgrow Microbial Inoculum.

Cost of both applications (applied) was $204/Ha.

Editorial comment:

There are major differences between these two recommendations! It is then unsurprising that we have such a level of confusion and skepticism in both directions at present.

The Agknowledge recommendation is based on the premise that ‘clover is king' and is supplying capital amounts of nutrients that clover requires in amounts greater than grass does, based on the science datasets for pasture production that exist in NZ.  Lime would be applied if monitoring showed pH to drop much more after this application. For some there may be a degree of faith required that soil chemical fertility is the major factor holding back pasture production on this site, although given that it is already well subdivided and running a high stocking rate, this has to be very likely.

Alec Mckay (AgResearch) is on record as saying that our high performing, high stocking rate sheep farms need to lift their expectation of adequate Olsen P closer to that of dairy farms (if you can achieve a similar EFS!), which could put an Olsen P of 17-18 at below optimum.

However for this example, an Overseer econometric model output based on NZ science would recommend maintaining Olsen P and supplying S. This could be achieved by applying 200 kg/ha super/ha/yr +/- 1 kg/ha Se prills (see below). Cost: $100/ha applied by air.

The Outgrow recommendation supplies some N in the spring application which could reasonably be expected to generate some extra feed (although it is a low application rate). The autumn application supplies another small amount of N, much less P than would be supplied in a conventional fertiliser recommendation, and a relatively high amount of Ca compared to other nutrients (although less than what would be supplied in a conventional liming programme). Beyond this it requires faith that the application of low rates of a range of macro and micro elements, plus the various organic acids, sugars and microflora is having a quantum effect on the life and processes within the topsoil. This is still required to be demonstrated other than by anecdote.

Note that neither of these recommendations supply enough Se to adequately lift supply to stock if animal testing confirmed that there was indeed a deficiency.

If you accept John Turner's premise that Olsen P is ‘not the whole story' and that high pasture production is possible without high Olsen P levels, although he concedes that this needs to be better defined, (it could be as simple as having a system driven more by N fertiliser in environments where moisture or other factors limit clover production), then a recommendation that would maintain soil P, add some K and improve pH could be:

  • About  20 units of each of P, K, S provided by ‘superten 7K' at 250 kg/ha, cost $106.50/Ha + spreading costs
  • Lime at 500 kg/ha to lift pH away from bottom of adequate range (given that an autumn drop in pH may occur); lime cost around $10/Ha + spreading costs
  • Add Se prills if deficiency was confirmed by animal sampling and there are not cheaper ways of doing it  - for example if lambs always get a selenised drench then it could be cheaper just to dose ewes with selenium at key times instead of applying prills


The more you understand about soils and fertilisers, the better position you are in to ask the important questions of the people who you purchase fertiliser from. 

It may be as simple as a refocus on where lime fits into your fertiliser programme. It may require a shift to ‘thinking bigger' in terms of where your Olsen P needs to sit relative to the level of production.

Or you may be comfortable with a paradigm shift to a system that applies much lower levels of conventional nutrients but looks to ‘stimulate' the soil  in other ways.

As advisors, we need to see a body of properly designed trial work to validate this. We hope that this work will be forthcoming, and are looking forward to seeing what comes out of Ballance Agrinutrients' current work in this area. Finally, we thank them for funding the soil testing for this day.


  • Presentations and speaker notes from Tararua FFP day, 14 Sep 2011
  • ‘Soil Science'; McLaren and Cameron
  • ‘From the ground up'; summary of a presentation given by Dr Alec Mckay, May 2011
  • ‘Agletter' 7th November 2011; Biological Farming
  • Outgrow website
  • Agknowledge website