A Primer on Soil – Agricultural Inputs and Their interaction with Soil Ecology
November 27, 2012 | Helen Weatherall
For most of us soil, or “dirt”, is something to get rid of, even to fear. Millions are spent on television ads to tell us how to fight dirt and win. A farmer’s relationship with the stuff that gets under fingernails, however, is starkly different. To reap a harvest and turn a profit, a farmer must work with dirt, or as one crop advisor calls it, “that black box we call soil.” All inputs, chemicals included, are applied to optimize harvests and net profit. But earth, from which seeds sprout, though seemingly simple in its brownness, is not a single entity; rather it is a complex ecosystem as full of mystery as the depths of the sea. Since it is from the earth that all terrestrial life springs, it is worthy to ask: what of crop plants, agricultural chemicals and this little known realm that absorbs them?
We know that plants grow in soil but few of us know why or how. Soil helps plants stand up, or so it seems. The answer as to how is found in at least two of soil’s ingredients, calcium and potassium. Roots grab the soil and provide a base for plants, but without calcium and potassium, which are needed to make cellulose and lignin, or what make a corn stalk rigged and a snap pea snappy, plants would be mushy and lie limp on the ground. And they would lack the armor that protects them from insects and disease.
In all, soil contains thirteen nutrients vital to plants. The three needed in greatest quantity – nitrogen, phosphorous, and potassium – represent the macronutrient group. The group containing secondary nutrients includes calcium, magnesium and sulphur. The remaining seven nutrients – boron, chlorine, copper, iron, manganese, molybdenum, and zinc – are referred to as micronutrients. They are found in trace amounts in the soil and though essential to plants, are needed in relatively small amounts. Plant species have overcome obstacles to acquiring these nutrients through evolutionary adaptations but all plants require these nutrients to survive, thrive and reproduce.
Understanding that soil is in part made up of essential nutrients begins to crack the mystery of what soil is, but only just.
“We know quite a lot about soils,” Dr. Johannes Lehmann head of the Soil Biogeochemistry & Soil Fertility Management Program at Cornell University’s Dept. of Crop and Soil Science answered when asked the degree to which we understand what soil is about.
“But soils are very heterogeneous and vary in properties between soils and within,” he said.
“Where we do indeed know very little is the soil biology. Here we only begin to ask the question, “who is there?”, and know mightily little of what they do,” Lehmann confided. And in this he is in agreement with his colleague Dr. Patrick Brown of the University of California, Davis.
“We certainly don’t understand it at all. But we understand that soil health matters,” says Brown, who is a specialist on plant mineral nutrition and root soil interaction.
What scientists do know about soil biology, however, is that soil contains a vast quantity of both macro and microorganisms. According to soil microbiologist David A. Zuberer of Texas A&M University their combined biomass can amount to thousands of pounds per acre of soil. A sample of soil weighing one gram could contain billions of microbes says Zuberer. Measured another way, one 1,000 square foot plot of earth contains twelve pounds of these tiny but highly significant organisms. Of these microbes most would be forms of bacteria. The other microbes found would be fungi, algae (including) cyanobacteria, and protozoa.
Digging in any healthy sample of soil, in addition to microbes, one also finds soil animals- earthworms, millipedes, and potato bugs to name three commonly known to anyone who has ever turned over a rock in North America. Recent research from England’s Rothamsted Experimental Station suggests that fertile farmland may have up to 1,750,000 earthworms per acre. In other words, the weight of earthworms beneath a farmer’s soil could exceed that of the sheep, cattle or other livestock grazing on its surface. These hidden but ubiquitous animals munch on dead leaves and other organic debris thereby facilitating further decomposition by microbes.
Decomposition of dead matter by insects and microbes is important in two ways: one it prevents the piling up of bodies; and two it recycles nutrients. Microbes are all but entirely responsible for making nitrogen available to plants through nitrogen fixation – a process by which bacteria combine nitrogen with hydrogen to produce ammonium, or with oxygen to produce nitrate that plants then convert to the proteins and chlorophyll they require to grow.
Given that the pathway to photosynthesis originates in the soil, and that photosynthesis is the means to sustaining all plant life and by extension all land-based animal life on the planet, it is startling to learn that in developing synthetic agricultural chemicals minimal consideration was given to their potential effect on soil biota.
“Any changes induced by xenobiotics such as glyphosate is not really known,” said Dr. Lehmann when asked if he agreed with this assertion made by plant pathologist Dr. Don Huber professor emeritus of Purdue University.
“There are a lot of xenobiotics whose fate and effects have not been tested in soils,” Lehmann confirmed.
Xenobiotics (substances taken up by plants but not otherwise occurring in them naturally) known in this case as synthetic herbicides, pesticides and fungicides are applied routinely every year on agricultural land throughout the growing season in America and abroad. As farming practices have changed and technology has evolved so too has the use of agricultural chemicals. One change is that in the current generation of herbicides the active agents are taken up by plants and incorporated into their tissues from root to seed. Because of this they are known as systemic chemicals. Glyphosate, commonly known as RoundUp – agriculture’s most favored chemical- is one such product.
“Glyphosate kills by giving plants a bad case of AIDS,” Dr. Don Huber said in advance of explaining the science behind what makes RoundUp so effective.
“Glyphosate was originally patented as a chelator. It immobilizes minerals, particularly manganese, but also iron, nickel, zinc and calcium. So it prevents plants from being able to take up these nutrients.”
“It is also a powerful antibiotic,” Huber added as he went on to explain that by killing certain soil microbes, glyphosate fosters the growth of pathogens such as fusarium, a fungi that produces mycotoxins in cereal crops.
“We have observed a 500% increase in root colonization (of fusarium) in RoundUp Ready soybean,” said Huber.
Because RoundUp is engineered to interfere with a plant’s metabolism by inhibiting an enzyme known as EPSPS in what is called the “Shikimic acid pathway”- something that plants have but mammals do not- glyphosate is roundly deemed harmless to humans.
But Huber and others, who have looked behind the curtain to better understand glyphosate’s action on the micro and macro level, have found reason for concern.
“What determines the success of our (agriculture) is the interaction of all the components. (It’s about) managing an ecology,” said Huber adding, “We have relatively limited knowledge of the system.”
“Degradation of glyphosate in most soils is slow or non-existent since it is not biodegradable,” explains Huber who notes that any degradation that does occur is attributed to microbial action. Regardless, glyphosate that has been immobilized can be reactivated by phosphorous whether deposited on the ground by a passing cow or by machinery spreading commercial phosphorous fertilizer. Made soluble in this way, glyphosate can readily leach into water supplies or be taken up by plants – gmo or otherwise.
“80% of glyphosate (taken up) will remain for the life of the plant,” said Huber. The rest may be exuded into the soil through the plant’s roots. Some too is released back into the environment through pollen.
According to Dr. Huber, at the root level glyphosate goes to work as a powerful microbiocide. Here where legumes such as soybeans and peas fix nitrogen with the aid of bacteria, glyphosate obstructs the nitrogen cycle. Within plants through its continuing chelating action glyphosate disrupts the creation of proteins. This means that though glyphosate doesn’t kill RoundUp Ready plants it does reduce their vigor, and likewise, their ultimate nutritional value.
Despite agreement amongst scientists such as Dr. Huber and Dr. Michael J. McNeill, President of Ag Advisory, Ltd. that “we don’t know dirt about dirt”, and acknowledgement that excessive use of glyphosate has led to an increase in previously insignificant plant diseases, lower crop yields, and the emergence of “Super Weeds”, the use of RoundUp increases every year.