The end of World War II marked the beginning of a new era in farming with a multitude of revelations in mechanization and the use of chemicals. Not the least of these man made miracles was the availability of commercial fertilizers. Their use over the last 60 plus years has probably consistently contributed, more than any thing else, to the phenomenal crop yields so often taken for granted today.
Nitrogen is probably the one element of fertility that drives yield more than any other. The most widely used commercial nitrogen sources are anhydrous ammonia, ammonium nitrate, ammonium sulfate and urea. All of these sources provide surplus hydrogen in conjunction with supplying nitrogen. That surplus hydrogen contributes to soil acidity.
There are some people who feel soil pH is not as important as calcium and magnesium levels. It’s interesting to note those folks often farm naturally high lime soils. While calcium and magnesium levels are important, do not underestimate the importance of soil pH. As soil pH nears the window of 6.6 to 7.4 soil microbial activity increases. As we go in either direction away from that window soil microbial activity decreases. As soil life increases so does the tendency for soil productivity. Also, the availability of nitrogen, phosphorus, potassium, sulfur, calcium, and magnesium tend to be greatest within this window. Therefore, we often pursue a pH close to neutral, between 6.0 and 7.0 for many crops.
As we apply limestone, the most common soil amendment to neutralize soil acidity, we are adding calcium and magnesium to the soil. Along with these two elements we are also adding carbonate (carbon and oxygen). It is the carbonate portion of the molecule that ties up the hydrogen in the soil to neutralize it. Herein is the potential quandary on the horizon for our current agriculture.
We use commercial forms of nitrogen to increase crop yields. The commercial fertilizer lowers our soil pH which retards crop yields so we add limestone to correct the soil pH.
Just how much limestone does it take? For every 100 pounds of nitrogen applied from ammonium nitrate, anhydrous ammonia, or urea it will take 225 pounds of limestone with an 80% relative neutralizing value (RNV) to correct the acidity caused by the commercial fertilizer. If ammonium sulfate is used for a nitrogen source it will take 669 pounds of limestone (80% RNV) to correct the situation. As long as we rely on commercial nitrogen sources we also have to rely on the use of another material to neutralize the effect of the fertilizer.
There are alternatives
Sludge, from municipal waste, is a possibility; however there is a potential, possibly significant, risk from industrial contaminants. While some sludge might meet the U.S. standards for safety, it should be noted that the U.S. standards are among the most lax of the industrialized nations. Wood ash provides another possibility but the impracticality of this makes it highly improbable. Limestone remains the most prevalent practical material for balancing soil acidity.
Utilizing non-commercial forms of nitrogen reduces or eliminates the negative consequences. Amending the soil with animal manures, composted is best, not only ads all the major elements but can actually increase organic matter, soil life, and pH. Legumes can conservatively contribute 20 to 80 pounds of nitrogen per acre following the year of establishment. When all the attributes of clovers were also considered the Noble Foundation credited an over-seeding of clover into grass with the equivalent of 150 pounds of nitrogen per acre on a cow-calf pasture.
Cover crops offer yet another approach with results dependent upon each particular situation. Who has considered searching for valuable crops that thrive in acid soils?
So why the concern?
Our stores of limestone are finite. Limestone is also used for other purposes such as concrete. Currently, the unprecedented growth in China is consuming more concrete than the rest of the world combined while construction throughout the world is escalating.
How many of you remember “gas wars”? Gas wars drove gasoline as low as 15.9 cents per gallon in the 1950s. Oil was plentiful – there was no reason to worry, or so we were told. According to the U.S. Geological Survey, our limestone supplies are, “adequate” – whatever that means.
Mankind has a history of squandering material based on its current economic value. Oil is just one example. Some of the best crop ground in the world is being paved over every day for new construction. Economics, alone, is a poor substitute for vision and wisdom.
Chemical fertilizers are just one of the modern crutches we have become accustomed to.Who, in there right mind, wants to walk on crutches?