With the escalating prices of commercial fertilizer and the pressures from environmental regulations, more growers are turning to organic sources for plant nutrients.
It is about time! Trends in follow up plant analysis on fields that have a history of manure and those that do not show significant benefits. It is true that we can manage the nutrients using commercial fertilizers, Urea, ortho-phosphate, etc. Growers have been doing this successfully for many years. However, I have observed differences in follow up plant tissue analysis in ranches that have had supplemental organic sources applied compared to those ranches that continue on the commercial “inorganic” fertility regimes. Petiole results from the latter have a more spurious nature, the petiole graphs look more like nitrogen “EKG’s“. Conversely, the petiole graphs with the supplemental organic program appear to transition more smoothly from sampling to sampling over the season.
The problem is predicting when the nutrients become available from the organic sources. You may remember from your basic soils class that nitrification or mineralization occurs only in the presence of nitrifying bacteria, adequate moisture, oxygen and favorable temperatures. A complex system is at play here. I have observed adjacent fields in comparable crop rotation, manure application and soil type display very different petiole nitrate levels early in the season. One shows excessive nitrate, the other low nitrate. One requires no nitrogen application, the other a significant application.
Other observations: care should be taken when managing nitrogen with the sap nitrate quick tests. I did a petiole program on a durum wheat field about 10 years ago. The crop was grown on soils reclaimed from an old feedlot. About late jointing the basal stem nitrate levels dropped quickly and I recommended a relatively high application of UN-32 (60 lbs N/ac) to be applied in the next irrigation. I called the grower and gave him my recommendation, he was unsure of the results. He said the crop looked fine to him. We re-sampled the stems and the lab had comparable low N results. The grower applied the N and got a very good response, both visually and in the plant test. I found out later the grower was using sap nitrate quick tests on his own. Those analysis had not shown the low plant nitrogen. Our best guess was that there may have been some organic interference in those plants that gave a false positive for Nitrate in the quick tests.
Organic sources have benefits, most agronomists are schooled in the three year Nitrogen release rate of Bovine manures (33%, 33% and 25%) in the desert. Phosphate has a 2 year release rate of (50%, and 40%). The trick is to know the timing of the release of these nutrients to the plant system.