Organic agriculture can pollute groundwater more than conventional synthetic chemical fertilization systems, researchers have found.

Israeli scientists measured the amount of nitrate, a form of nitrogen that plants consume, in the deep soil beneath crops grown using composted manure mixed into the soil. The team of biologists, agronomists and geologists also measured the levels of nitrate beneath crops fertilized with liquid synthetic fertilizer mixed into drip irrigation water, a system known as fertigation. Much higher levels of nitrate existed beneath the organic crops.

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Soil directly beneath organically farmed green houses contained a maximum of 724 milligrams of nitrate per liter (mg/L) of soil at a depth of 2.5 meters. At that depth, the roots of the plants were unable to grab the nutrients, so the nitrate was destined to eventually sink into the groundwater. Nitrate contamination beneath organic crops averaged 357 mg/L. The results have been published in the journal Hydrology and Earth System Sciences.

Nitrate levels under crops receiving synthetic fertilizer averaged 38 mg/L. However, these fertigated crops had higher levels of nitrate in the upper soil where the plants roots could actually feed on the nutrients. Nitrate levels averaged 270 mg/L in the root zone of fertigated crops, compared to 109 mg/L in the organic crops’ root zones.

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The researchers suggested that the increased contamination beneath organic crops may result from compost being applied to the soil before the plants are sown. The compost provides nitrate to fertilize the crops. However, after the seeds sprout, the tiny plants can’t absorb much nitrogen. Irrigation water leaches the unused nitrate into the deeper soil before the baby crops can devour the fertilizer. The synthetic fertilizers, on the other hand, are added incrementally to the irrigation water in amounts that the plants can absorb at different life stages.

The results of this study don’t mean that organic agriculture necessarily pollutes more than conventional systems, however. Run off from synthetic nitrogen fertilization of lawns and crops leads to massive pollution of waterways and marine dead zones, such as the oxygen starved waters at the mouth of the Mississippi River in the Gulf of Mexico. The nitrogen fuels blooms of algae that suck oxygen from the water when they die and decompose. Fish and other marine life suffocate in these waters.

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The Israeli study does add to the evidence that organic agriculture doesn’t provide a flawless solution to agricultural pollution. Besides pollution, the demand for organic produce led to farming of the parched Baja Peninsula of Mexico, reported the New York Times. The irrigation for the organic crops sucked water from scarce sources in the arid region and shattered the fragile ecology of the desert.

In rainier areas more suited to farming, organic agriculture can improve soil quality and foster a healthy community of microbes in the soil.