Ξ March 1st, 2010 | → 5 Comments | ∇ A Day at a Time, Technology, Wine News |
Soil science is a very complex, elegant discipline. And having everything to do with the feeding of the world’s hungry populations, it can also be highly contentious. Though not overtly political, rival research programs within soil science nevertheless often butt heads against one another. Witness, for example, the heated debates, still underway, over the consequences of the Green Revolution, a massive post-war transformation of agricultural technological practice that led to very significant, if short-term gains in the ability of developing nations to feed their populations. Though initially successful in Mexico and subsequently exported throughout the world, a look at the remains of that model today reveals a Mexico teetering on the edge of collapse, its agricultural sector further strained under NAFTA’s relentless weight.
Now, of course the reasons for Mexico’s economic and social troubles are as multiple as they are tangled, but it is undeniably true that the soil science, as understood mid-century, played a significant role in the optimism energizing the Green Revolution.
All will agree that the ’success’ of the Green Revolution relied a host of social and scientific technologies formerly limited to industrialized nations: the zealous use of broad spectrum pesticides, often without significant independent scientific review; the insistence on monoculture at the expense of indigenous polyculture, and biodiversity generally; a structural necessity of greater petrochemical inputs; irrigation projects resulting in both reallocation and massive new drafts on local water reserves; displacement of underperforming farming populations in favor of mechanization; the planting of hybrids at the expense of traditional varieties, hybrids the farmer needed to purchase each year. These were but a few of the technological requirements imposed upon developing nations in the post war era. The upshot is that food production, its promise, would eventually become an instrument of foreign policy. I will pass over in silence the profound environmental consequences.
More narrowly, on the matter of new hybrids, they were selected because of their higher yields. Higher yields require greater amounts of Nitrogen (N). In this way did heavy applications of synthetic N become the order of the day. I will limit the balance of my post to this topic alone.
Formerly farmers were limited in how much they could grow by the need to replace the N their crops removed from the soil. Even the gardener knows how important it is to grow cover crop, to hustle up manure from a local ranch, at the very least to turn the soil so as to incorporate seasonal plant waste. The basic tenant of organic farming is ‘feed the soil’. It is no different for the large scale operation, at least it wasn’t until the rise of the synthetic fertilizer industry many years ago. With the mass production of synthetic N it became possible to use this to supplement the seasonal reduction of N reserves, but now in a more limited combination with plant waste, green and other manures. Further, it has long been believed that with these appropriate Carbon, Potassium, Phosphorus etc. additions along with judicious applications of synthetic N, soil health, including ‘relevant’ microbial populations, could be maintained for the long haul. As Ron Jackson puts it in his industry standard text book, Wine Science,
“Until the use of inorganic nitrogen fertilizers, vineyard nitrogen supply was dependent primarily on the activity of free-living nitrogen-fixing bacteria in the soil, nitrogen fixed by endosymbiotic bacteria in the nodules of legumes, and the addition of manure. Unlike other soil nutrients, nitrogen is not a component of the mineral makeup of the soil. Its availability, unlike that of potassium, phosphorus, and magnesium, is particularly dependent on the effect of seasonal factors, such as soil moisture, aeration, and temperature, and on how these factors affect the activity of soil microorganisms and cover crops. [....] The lower cost of urea and ammonia salts, combined with ammonia’s ready sorption to soil particles, generally makes it the preferred form of nitrogen fertilizer.”
And this approach is consistent with the broad research program of established soil science since the post-war era. But there is another parallel research program of similar historical pedigree. Often called organic, though well developed before its eviscerating codification in our era, it is properly explained, with an updated lexicon, by Peter Schmidt of the Delinat Institute.
“Just one cubic meter of good soil is home to nearly 60,000 species of microorganisms. They are all interconnected in the so called soil-food-web. All have different functions and maintain through their functional biodiversity the stability of the soil-plant-system. Each plant is symbiotically integrated in this very complex system. The plant offers to the microorganisms carbohydrates through their roots exudates and gets phosphates, nitrate, oligo-elements and water in exchange. The whole process is in an ingenious balance between give and take, fixating and releasing. If we intervene into this process with mineral fertilizers, the whole system gets out of balance as we favour some few species over others. It’s in fact a negative selection. As the plant gets easy fast food through the fertilizers it has no need to maintain the symbiosis with the microorganisms and it stops nourishing those microbes that usually fix nitrogen, carbon, phosphates and all the other aliments for the soil-food-web.
“And there is another point. Mineral fertilizers are salty which means that the most of the 1 billion microorganisms that one can find in 1 gram of good upper-soil dry up and die. Those that survive feed on the nitrate and ammonium of the fertilizers and on the carbon of the soil organic matter. The function of soil-food-web is surely as complex as the function of the brain, but it does not need magic to explain why nitrogen-fertilizers provoke the diminishing of soil carbon and the increase of greenhouse gases.
“To increase the functional biodiversity of agricultural systems is the most efficient and cheapest method for sustainable agriculture and resistance to climate change.”
It is by design that I select these two comments centering, as they do, on the question of synthetic N. There are other pressing distinctions between organic and industrial farming, and Mr. Jackson cannot fairly be said to be squarely in the latter camp. The point is that the organic community, broadly understood, has been historically critical of synthetic N; the industrial community broadly supportive. And for the past three score years this is where things have stood. Until now. Very important new research has recently appeared, research from within the university establishment itself. In a paper, titled Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma For Sustainable Cereal Production [click on right sidebar link for free download] by R.L. Mulvaney, S.A. Khan, and T.R. Ellsworth of the University of Illinois, the evidence from a decades-long project shows, according to the fine gloss of the paper by Tom Phillpott writing for Grist:
“[T]he net effect of synthetic nitrogen use is to reduce soil’s organic matter content. Why? Because, they posit, nitrogen fertilizer stimulates soil microbes, which feast on organic matter. Over time, the impact of this enhanced microbial appetite outweighs the benefits of more crop residues.
“And their analysis gets more alarming. Synthetic nitrogen use, they argue, creates a kind of treadmill effect. As organic matter dissipates, soil’s ability to store organic nitrogen declines. A large amount of nitrogen then leaches away, fouling ground water in the form of nitrates, and entering the atmosphere as nitrous oxide (N2O), a greenhouse gas with some 300 times the heat-trapping power of carbon dioxide. In turn, with its ability to store organic nitrogen compromised, only one thing can help heavily fertilized farmland keep cranking out monster yields: more additions of synthetic N.
“The loss of organic matter has other ill effects, the researchers say. Injured soil becomes prone to compaction, which makes it vulnerable to runoff and erosion and limits the growth of stabilizing plant roots. Worse yet, soil has a harder time holding water, making it ever more reliant on irrigation. As water becomes scarcer, this consequence of widespread synthetic N use will become more and more challenging.”
I contacted the lead author, Prof. R.L Mulvaney, with supplemental questions specifically related to viticultural practice.
Admin Does the long-term degradation of soils with the use of synthetic nitrogen fertilizer also lead to other mineral deficiencies? I’m thinking of phosphorous, potassium, calcium, boron and manganese in particular.
Richard Mulvaney Yes, organic matter depletion will adversely affect numerous soil functions that impact nutrient availability. The most obvious effect is on the supply of mineralizable N, P, and S from organic sources, but most of the other nutrients are also affected. Because of its high cation-exchange capacity, organic matter plays an important role in holding Ca, Mg, and K in exchangeable forms that are protected against leaching, and has a similar effect in stabilizing the supply of micronutrients. There are important effects on the soil’s physical properties, such as water-holding capacity, aeration and drainage, structural stability, and resistance to erosion and compaction. Soils with ample organic matter provide a good rooting medium that promotes plant uptake of immobile nutrients such as P and K, and of course also water. Not surprisingly, the world’s most productive soils in such areas as the U.S. Corn Belt and the Ukraine are known for having a high organic matter content.
Would the accelerated loss of organic material associated with synthetic nitrogen play any role in increasing levels of salt in soils? I’m thinking of the Salinas Valley in California. Another question following upon the first: Would changing the soil profile exacerbate problems associated with salt water intrusion? And would additions of organic matter help slow the destructive effects of salt on crops?
RM By impeding drainage, a loss of organic matter would exacerbate salt accumulation through evapotranspiration. Depending on irrigation water quality, the salt buildup could reduce productivity and restrict cropping plans.
Do irrigation methods make a difference? Perhaps an obvious question, but I’m thinking of a perennial crop, such as wine grapes. Does drip irrigation, often the synthetic nitrogen delivery tech of choice for large and small scale grape growers, ultimately have a deleterious effect? With drip irrigation the vine root system is encouraged to remain near the soil surface. So I’m wondering for established vines, whether synthetic nitrogen fertilizer applications would, over the life of the vine, result in the selective degradation of it’s immediate soil, the few square feet the vine inhabits.
RM Drip irrigation is the most efficient option for supplying water, and would also increase nutrient uptake efficiency with lower fertilizer rates in close proximity to the rooting zone. Under these conditions, C depletion should be minimized by synthetic N fertilization. Without long-term data on drip irrigation, any further comment would be speculative.
What are your recommendations for the rehabilitation of degraded soils? I realize it varies from crop to crop. Corn is a heavy nitrogen feeder; wine grapes. less so. But given the recognition by a grower of a degraded farm soil, what steps might be taken to begin to re-establish soil health?
RM The Morrow Plots and other long-term experiments have shown that mixed legume rotations and the use of manure are conducive to soil C sequestration, as opposed to synthetic N fertilization for continuous grain production. The damage in the latter case will escalate if residues are harvested for ethanol production.
What is you opinion of biochar as a method of carbon sequestration in agricultural soils?
RM Biochar can be a valuable amendment for soils that are very low in organic matter, and has been particularly useful in managing tropical soils subject to deforestation and shifting agriculture. Soil C will be sequestered, and plant growth will benefit from deeper root penetration with improved soil structure, higher water-holding capacity, etc.
Thank you, Professor Mulvaney.
RM Thanks for your interest in our work on this topic. I hope these comments will be helpful.
Apologies to the reader for the breezy, rapid presentation of such a complex issue. I will post additional remarks on this important topic in the coming weeks.