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Legume Nitrogen: The Generator of Crop Profit

The Problem; N removal in crops has left a hidden “black hole”.  


Common crop centric farming systems of the last 20 years have severely depleted soil N reserves, as rate of N removal in grain has far exceeded replacement via fertiliser or N fixing legumes.  If that reserve run-down were valued at cost of effective replacement using fertiliser N at 2008 prices, we would be looking at a price tag of roughly A$25 billion for the wheat-sheep zone. 

This hidden cost of crop production can be stated (and will be eventually assessed) as a reduction in real land value; approaching A$1 million for the average farm over the last 20 years.  That is more than the total net farm income generated from crops in that time; ie if this N depletion were accounted for at today’s fertiliser prices, virtually all of the farm profits from growing cereal crops over the last two decades would be wiped out.  This reserve rundown is thus unsustainable biologically, and now economically as well. 

The Answer; Include Pristine pasture or forage legumes in the rotation. 

This will generate direct profit and return vital N to the soil bank at the same time.  That effectively free N will dramatically increase real crop profits, restore sustainability, improve soil health and safeguard fragile and very limited soil resources. 

  1. Why do we need nitrogen?

Nitrogen is vital for all the functions that distinguish “life” from the inanimate.  It is the key part of all the amino acid building blocks for the enzymes and proteins that make life possible.  Low availability of N means less cellular protein.  This ultimately slows all cellular activities; for growth, photosynthesis, cellular division, maintenance of cellular integrity, pathogen resistance, etc. etc.  In crops, low N manifests itself as reduced crop growth rates that will also eventually reduce yield, plus also lower protein and quality in the harvested product.  In forages and pastures, low N severely impacts on growth rates and therefore on harvestable forage yield and quality, and/or on stock carrying capacity. 

  1. N sources.

Except for those few families of plants (such as the legumes) that can “fix” their own nitrogen from atmospheric N (see below), all plant usable N comes from the soil “bank” of nutrients, organic matter, etc.  N can be added to that bank via fertiliser or via organic residues from plants of previous crops or pastures.  However, unless those plants are legumes, then they also derive their N from the soil N bank.  Legumes are able to take atmospheric N and “fix” it; ie convert it into a plant usable form via a unique and complex symbiotic relationship with Rhizobium bacteria contained in nodules on the plant roots.  Cereals, grasses and other major (non-legume) agricultural crop and forage species and families are not able to do this.  Some free-living soil microbes can also fix their own N, but this is a very minor contributor to the pool.  Therefore, the ultimate source of N in our agricultural systems is almost entirely either fertiliser N or N that derives from fixation by legumes.

  1. Crop N; how much is needed and what does it cost?

Every tonne of cereal grain with 10% protein contains about 16kg (units) of N, and when this is sold off farm, that N goes out with the grain.  The crop residue from that tonne of grain (straw, chaff etc.) contains around a further 15kg.  Therefore, an absolute minimum of 30kg of N is needed to grow every tonne of grain, and over half of that is then removed and lost from the system.  To replace this via fertiliser (some of which is inevitably lost in the process) will currently cost $50-$75 per tonne of grain; whereas if this N is supplied via legume pastures or forages, that $50-$75 can be added to real bottom line profit.  (Note; legume crops or forages that are harvested and removed do not add much N to the system, as most of the N fixed is taken off with the harvested product).

  1. Soil N bank depletion; how far can I, or should I go?

Soil N bank reserves in cropping zones have generally declined dramatically over the last 20-25 years.  Prior to about 1985, even with fertiliser N being at a small fraction of the price it now is, addition of N fertiliser gave no consistent yield response and was generally uneconomic.  By contrast, the yield responses are now so profound that very few crops are grown without significant amounts of N fertiliser.  The typical application rate is enough to replace about half of export loss with grain; thus there is obviously still sufficient N in the soil bank to supply significant amounts of the crop needs, and there is likely to still be years, or even decades before soil N banks are completely exhausted. 

However, the problem is that as total N reserves decline, soil health and system resilience parameters decline far more dramatically.  For example, C/N ratios rise significantly (from say 20 or 30 to 1 to 80 or 100 to 1).  At these lower C/N ratios there is not sufficient N for good microbial activity, and that impacts negatively on cycling and availability of all nutrients, and on the entire biological system.  Fertiliser addition can become considerably less effective, as there is less biological activity to capture and soak up the added nutrients before they are lost to the system.  Root disease organisms can become more prevalent, and far more devastating in their impact.  Often there is then an increase in soil microbes that do fix their own N; which may at first glance appear to be good.  However, such N fixation will require consumption of very large amounts of soil organic matter for only a few kg of N being fixed; and may be effectively correcting a critically low C/N ratio by reducing the organic C level more so than by increasing the organic N.  In other words, this is symptomatic of the soil starting to feed on itself, cannibalising soil organic matter in an attempt to generate new N.  This C/N ratio will then continue to bounce around the ratio of 80 to 1, while the vital organic carbon component of the soil (and the remaining nutrient banks it holds) progressively dwindle away. 

No soil should be pushed to this point, as there is no beyond it, and there may be no further major indication of impending collapse before the system finally reaches the point where it can no longer economically support any agricultural activity.

  1. Using pasture and forage legumes to replace N and build soil health.

Pasture legumes can fix very large quantities of N.  For example, a 10kg/ha sown medic stand can fix 200kg of elemental N (equivalent of about 450kg of urea) per hectare in 5 months of growing season.  As current international fertiliser prices and the A$ are so volatile, it is pretty difficult to assign an actual dollar value to that, beyond saying that while the value of that N was about $300 last year, this year it is peaking beyond A$600/ha.  In either case, farmers using legumes as their N source are making huge cost savings (and more profit) on their crop production and protecting the value of their investment in land.

  1. Basics for maximising N fixation.

The key thing to remember for legume N fixation is that a legume will only fix what it needs.  If it can get that from the soil, then it won’t fix anything.  Fixation in many pastures is very limited because the low number of legume plants present can get what little N they need directly from even N deficient soils.  An average (but still N deficient) wheat-sheep zone soil may have enough available N/ha to grow about 1t of legume biomass, but doesn’t have enough legume in it to reach that figure.  By comparison, a dense legume may produce 6-10 t of top biomass on the same hectare, and that would see fixation of 200 to 300kg of N/ha over a full season.  In practice, fixation rates above 200 kg/ha are not common, as a soil with low N is usually run down in other ways as well, and will need to be built up over several years to reach the 10t mark; by which time, soil available N is also rising.
However, the point is that below a certain threshold of legume numbers and consequent biomass, there will be little N fixation, even on N deficient soils.  On the other hand, correcting that lack of legume by sowing a good legume stand and managing it for good production will more than pay for itself just in the net addition to the N bank in the soil.  Further, a 6t biomass legume stand is more than twice as good as a 3t stand, and that in turn will be immeasurably better than many typical pastures with their current low legume content.

  1. The impact of grazing on N fixation

While other crop and forage legumes can suffer serious production and fixation losses under grazing, Pristine legumes are resilient and very well adapted to it, and judicious grazing may even assist growth and increase total N fixation.  By using this as feed, some of the fixed N will be converted to animal product and some lost (eg through volatilisation) from both faeces and urine.  Nevertheless, most N is returned to the system.  Further, grazing of good stands is often beneficial, as it has the side benefit of “calming down” the large amounts of N added, by converting it into forms that are mineralised more slowly over future years.  If instead of being grazed, a good legume stand were to be ploughed straight back into a light soil with low organic carbon (ie limited nutrient buffering and storage capacity) and sown to a crop, this crop can be very lush and over leafy, hence using up too much moisture too early in the season.

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