Wednesday, June 30, 2010

Some thoughts about protein and nitrogen

As a food-loving, Peak-aware, agricultural scientist I worry a lot about a likely future of world-wide food scarcity.

And, as an ex-vegetarian, I have real (but limited) sympathy for the increasingly popular "save the planet, go vegetarian" movement. (My sympathy is limited because, environmentally, vegetarianism is just an efficiency measure that can't possibly "save the planet". To really save the world as we know it, we need to reduce the number of humans on the planet, quickly).

Nevertheless, although vegetarianism is limited in what it can achieve, I've been surprised and disappointed by what seems to be an increasingly vitriolic anti-vegetarian backlash, led by people like Lierre Keith with her 2009 book, The Vegetarian Myth.

The main problem I have with Lierre Keith is that she's a patronizing ignoramus who apparently knows remarkably little about the subjects she wrote an entire book on.

One of Keith's central tenets is that humans cannot live by eating plants alone. She's correct on this point, for the simple reason that vitamin B12 is an essential nutrient not produced by plants. The problem is that Keith's book is a 321-page rant, in which she bases many of her specific arguments on notions that have no basis in reality (I'll provide some specific examples in a minute).

However, The Vegetarian Myth has received an average rating of 4 out of 5 stars on Amazon, indicating that most of Lierre Keith's readers are so completely unaware of the basic facts about plants, animals, and nutrition that they find her arguments completely plausible. This is profoundly worrying to me.

If people eagerly absorb false beliefs about, for example, how many chickens can live sustainably on one acre of pasture; and if they have no idea where nutrients actually come from - then how can they possibly hope to feed themselves when relocalization is forced upon us all, courtesy of Peak Oil, and whatever other global catastrophes may await us?

The subject of human nutrition is way too big for one blog post, so I thought I'd focus for the moment on the subject of protein (which is probably still too big for one blog post).

In The Vegetarian Myth, Lierre Keith carries on about "poor-quality plant protein". She repeatedly compares animal protein with plant protein, and declares that animal protein is "better".

To say that animal protein is "better" than plant protein is to express a personal opinion. But, because many of Lierre Keith's beliefs and opinions are based on misinformation, I would argue that she has arrived at her conclusion via seriously flawed "reasoning".   

In fact, animal protein (from meat, milk, and eggs) is "complete", meaning that it contains enough of all the essential amino acids to support a human's total protein needs. However, complete protein can also be obtained from plants, very easily. A small number of plant foods (such as amaranth, buckwheat, soybeans and quinoa) provide complete protein on their own. Other plant foods have to be combined to provide complete protein. For example, common grains such as rice and wheat can be combined with legumes, vegetables, or nuts to yield a highly nutritious, complete protein meal.

Hence, for Lierre Keith to make the blanket statement that all plant proteins are "poor quality" is either ignorant or foolish (the irony here is that Keith delights in telling her readers - over and over again - how terribly ignorant vegetarians are).

Where does protein come from?

Proteins are made up of amino acid building blocks. Amino acids are molecules based on the general formula: H2NCHRCOOH. In other words, proteins are based on the elements:

Hydrogen (H),
Nitrogen (N),
Carbon (C), and
Oxygen (O).

Carbohydrates and fats are also based on hydrogen, oxygen, and carbon. Hence, the characteristic element that differentiates protein from the other macronutrients is nitrogen. Indeed, it is the nitrogen component of protein that enables scientists to use the so-called Kjeldahl method to estimate the amount of protein in a particular food, and "on food labels the protein is given by the nitrogen multiplied by 6.25, because the average nitrogen content of proteins is about 16%."

Lierre Keith actually mentions the importance of nitrogen, albeit in rather purple prose:
"This is the chemistry we should learn like a liturgy: life is spoken in the language of nitrogen. You’ve probably heard that amino acids are the building blocks of protein. Well, nitrogen is the building block of amino acids, the alphabet of DNA." (p.104)
Keith then briefly mentions the crucial fact that particular bacteria, in symbiosis with leguminous plants, "fix" atmospheric nitrogen into a form accessible to plants, and thence to animals. She states that "this is where essentially all the fixed nitrogen on the earth started" (p.105). She isn't quite right there. For example, cyanobacteria (blue-green algae) "inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere". Cyanobacteria operate independently of leguminous plants.

Keith explains that, to grow food, soil needs adequate nitrogen. This is absolutely true. However, she then leaps into la-la land by stating, repeatedly, that adequate nitrogen can come only from animal products, such as livestock manures and "what’s left of the dinosaurs". This is bizarrely, blatantly, totally incorrect.

Keith obviously hasn't pondered where the nitrogen in animal manure comes from in the first place. It's elementary: the nitrogen in cow poo comes from the nitrogen in the plants that the cow ate. You can obtain even more nitrogen by cutting the cow out of the equation completely (because the cow, of course, uses a significant amount of nitrogen to create the protein that you find in a steak):

Below I'll respond in detail to Keith's following statement about nitrogen and protein:
"Nitrogen, phosphorus, potassium - NPK - is the Triple Goddess of gardeners, the Troika of elements that rule plant growth. [...] Nitrogen was the big one. There are plants that fix nitrogen. Wasn’t that enough for my garden? Couldn’t it be? I begged. But I was begging a million living creatures who had organized themselves into mutual dependence millions of years ago. They had no use for my ethical anguish. No nitrogen-fixing plant could make up for all the nutrients I was taking out. The soil wanted manure. Worse, it wanted the inconceivable: blood and bones." (p.19)
There are at least two important questions to ask in response to Keith's emotional paean to animal-based fertilizers:

1) How much nitrogen does Keith's garden really need? And,

2) Where does the nitrogen in Keith's animal-based fertilizers actually come from - and where does it end up?


1) How much nitrogen does Keith's garden really need?

The answer to this question depends on several variables that are unknown to me, such as what plants Keith is trying to grow; how much water she has access to; and whether her garden is limited in nutrients other than nitrogen.

However, below are some ball-park figures for nitrogen requirements (in kilograms of nitrogen per hectare, or kg N/ha) for tomatoes, corn, wheat, and soybeans:

Tomato plants use ~100-150 kg N/ha, yielding 35-60 metric tonnes of fruit per hectare (reference).

Corn (maize) uses up to 200 kg N/ha, yielding 20 tonnes of corn, fresh weight (reference).

However, one 2005 study found that the overall economically optimum nitrogen rate for corn was 125 kg/ha.

Wheat uses ~95 kg N/ha, yielding 2.7 tonnes of grain (reference).

Soy uses 312 kg N/ha, yielding 3.4 tonnes of soybeans - however, typically ~50% of this nitrogen is fixed by the soy plants in symbiosis with bacteria such as Bradyrhizobium japonicum (reference).

Hence, it seems fair to say that high-yielding food plants generally require roughly 100-200 kg N/ha.

Lierre Keith claims that "no nitrogen-fixing plant could make up for all the nutrients I was taking out" - but she provides no data to support her claim. So I went and consulted Google, again.

According to a paper published by the University of Florida, symbiotic nitrogen fixation rates of "75 to 300 kilograms of N per hectare per year are common in various combinations."

And one study found that a leguminous green manure crop, "in general, supported corn yields equivalent to those achieved with 90 to 125 kg/ha of fertilizer N".

So, contrary to Keith's claim, normal symbiotic (legume-bacteria) nitrogen fixation rates are more than enough to make up for the requirements of common food crops - even notoriously nitrogen-hungry ones like corn; and even assuming that none of the corn crop (i.e. the stem and leaves) is returned to the soil as green manure or compost.

However, it's important to note that, to maintain adequate soil nitrogen levels, a leguminous green manure crop would have to be planted in lieu of a food crop about once every two to four years, depending on which food crops were being grown. This means that a truly sustainable farm could probably produce only 50-75% as much food as a typical, modern, unsustainable farm that relies on synthetic nitrogen fertilisers made from fossil fuels.  


2) Where does the nitrogen in Keith's animal-based fertilizers really come from, and where does it end up?

Keith writes emotively about how her soil "wanted manure. Worse, it wanted the inconceivable: blood and bones."

Keith is playing games here by anthropomorphizing her soil. In reality, it's Lierre Keith who "wants" these animal products for her soil. So, my question for Lierre Keith is: where does the nitrogen in her manure, blood, and bone come from in the first place?

Nitrogen is a chemical element. Cows can't make nitrogen, nor can they metabolise the molecular nitrogen (N2) which is in the air all around us. Rather, cows must eat nitrogen in a form that is bio-available to them. Grass is rich in bio-available nitrogen, which it gets from the soil. The soil gets nitrogen from nitrogen-fixing bacteria, which in turn get nitrogen from the air.

If you remove animal manure from the pasture where that manure was produced, you deplete that pasture of nitrogen. If you deplete the pasture of enough nitrogen, it will no longer be able to grow enough grass to feed a cow, meaning that you will no longer get manure for your garden, meaning that you'll probably starve to death if you don't wake up and realise that you can't eternally rob Peter to pay Paul without eventually sending Peter broke.

As for the question: where does the nitrogen in Lierre Keith's animal-based fertilizers end up? The answer is that a significant portion of it ends up in her excreta, which she (presumably) flushes down her toilet and thence into her local municipal sewage works. (Incidentally, according to her book, Lierre Keith lives in both western Massachusetts and Arcata, northern California - an odd cross-continental living arrangement for someone who preaches about sustainability and localisation).

Anyway, as it happens, Arcata's sewage processing facility is the well-known Arcata Marsh and Wildlife Sanctuary (AMWS).

So, what exactly happens to the nitrogen in the sewage that ends up at the AMWS? Basically, it gets turned back in dinitrogen gas (that is, the atmospheric form of nitrogen that plants and animals can't use). According to EcoTipping Points, two processes are of particular importance for nitrogen cycling at the Arcata marsh:

1) Dentrification: The conversion of nitrates found in wastewater by bacteria into nitrogen gas through removal of oxygen in the anoxic conditions found in the detrital layers of the wetland.

2) Nitrification: The conversion of ammonium to nitrates (which must happen in oxygen rich regions of the soil, usually near the clumps of vegetation) which can then be converted into nitrogen gas through dentrification.

It's possible that Lierre Keith captures the nitrogen in her effluent by using a composting toilet, but she certainly doesn't mention this in her book.

I use a composting toilet, and I think it's wonderful - but that's surely another blog post.

1 comment:

  1. Whenever I hear people like Lierra Keith making up stories about this sort of thing to make themselves feel good, I think of an episode of The Simpsons where Lisa builds a perpetual motion machine. Home tells her off, saying "In this house we obey the laws of thermodynamics!".

    Right now, we live in an age where most crazy ideas have no consequences (so you can hold them without ending up dead). If Keith is wrong, then she goes to her supermarket and buys what she needs instead of getting it from her garden.

    What worries me about this kind of magical thinking is that when we reach a point where ideas like this DO matter, there will be no second chances. If your crop fails, or your livestock die, or they just don't grow enough calories to get you through the winter, then you, your family, and a lot of your community will be dead.

    You have to get this information right. So thank you Sam for doing this research and adding a little bit more to my understanding of what is and isn't possible.