Hi, Linda.toggle quoted messageShow quoted text
Here's my understanding. Nitrate is the most plentiful and available form of nitrogen that plants use. Remember the nitrogen cycle from 8th grade science? Plants can absorb or use only a mimimal amount of ammonium nitrogen (urea) or atmospheric nitrogen. The vast majority of the nitrogen used by plants must first be processed by soil organisms... into nitrate.
The problem of nitrate accumulation in plants is one of incomplete nitrate utilization within the plant tissue, not of the application of nitrate to the surface of the plant or the manufacture of nitrate within the plant. IOW, nitrate enters the plant's vascular system after being absorbed through the roots (mostly) but is interrupted -- or delayed -- in the process of transformation from nitrate into plant proteins.
Delay can be caused by the lack of availability of other essential nutrients, the most basic and common of which is water. Plants require ample water to process nitrogen (and just about anything else). Soil type also promote nitrate accumulation: heavier, clay soils don't leach out (wash away) nitrate the way sandy, loose soils do, so there can be an over-supply of nitrate in heavy soils. Over-application of nitrogen fertilizer or manure without adequate rainfall or irrigation is another possibility. Plant roots absorb nitrate through their roots if it is there in solution in the soil, even if there isn't sufficient other water to complete the process of nitrogen utilization within plant cells. Once within the plant's vascular system, other key nutrients (like calcium) must be available for the plant to effectively utilize the nitrate. Even temperature changes affect nitrate uptake: cold soils don't allow it, warm soils do.
A key quality of nitrate accumulators is that the plant is invariably very drought-resistant and a superior harvester of available soil nutrients (like nitrate), even in poor soils under poor conditions. IOW, it's usually a weed. Too much nitrate will actually kill many plants. Have you ever seen yellow stripes on a lawn fertilized heavily in high heat? That's grass killed by nitrogen fertilizer applied with insufficient available water. Nitrate accumulators survive in conditions that would kill other plants: heat, drought, shade, lack of balanced nutrients. Even normal forages have different amounts of nitrate within their plant tissues: closer to the roots, more nitrate.
Here's a great recap of the whole story.
Blaming manure application for high nitrate forage is really only part of the story. There is nothing wrong with manure except that it is not a complete fertilizer, and, like any source of nitrogen, requires ample water and other essential nutrients for effective utilization within the plant. It seems obvious that manure-sprayed forage should never be fed until after overhead irrigation or rainfall has soaked the manure into the soil. I doubt sprayed manure (like from dairy ponds) is a common source of nitrate contamination in hay. You'd smell it.
Cass for Satra (who doesn't know much about plants)
Sonoma County, Calif
--- In EquineCushings@yahoogroups.com, "Linda" <PapBallou@...> wrote: