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.