Author: Nick Andrews, Small Farms Program, Oregon State University & Dan Sullivan, Oregon State University

Publish Date: Fall 12

Organic vegetable farmers use cover crops, compost, manure and other amendments to increase soil organic matter (SOM) and improve soil quality. These practices also supply large amounts of nitrogen, but the amount and timing of plant-available N (PAN) released for crop uptake is difficult to quantify. Without reliable estimates it is difficult to adjust N fertilizer rates to account for soil building efforts. We are working with farmers in the Willamette Valley to develop practical methods to estimate PAN release from cover crops and SOM. Use caution if using our approach in areas that are dissimilar to Western Oregon (i.e. east of the Cascades).

Decision process

We recommend combining site-specific planning tools with monitoring methods to fine-tune N management practices (Figure 1).

The total amount of N input needed by a crop varies depending on the kind of crop (Table 1), your yield goal, and the planting density. Information is available in University nutrient management guides (see “For More Information”). Nitrogen can be supplied to the crop by decomposition of soil organic matter or other inputs like cover crops or organic fertilizers, and by irrigation water. The amount of N supplied by each source depends on the management history of a field, cover crop species and biomass, soil temperature, soil moisture, the N content of irrigation water and fertilizers, and other factors. With so many variables, available planning tools only provide a rough guide to N availability in a particular field. After making N fertilizer decisions, you can monitor the results so that you can customize the fertilizer program.

Irrigation water PAN

If irrigating from a well in an area where nitrate contamination is likely, test well water nitrate levels during the irrigation season. If results are higher than 3-5 ppm, the fertilizer value of this irrigation water can be estimated with:

Nitrate-N (lb/ac) = inches H2O applied x ppm nitrate-N x 0.227

For example, if you apply 12” water per year and the water contains 10ppm nitrate-N, you will get 27 lbs N/acre from the water.

Cover crop PAN

During decomposition a cover crop can increase or decrease the N fertilizer requirements of the following crop. In general, legumes have higher N content than cereals and leafy plant tissues have higher N concentrations than stems and more mature plant material. For legumes like common vetch that are high in N (e.g. 3%), about half of the cover crop N is released as PAN, because the cover crop has more N than needed to “build” soil organic matter. For non-legumes like cereal rye that are low in N (e.g. 2%), the release of PAN is small, because most of the cover crop N goes into soil organic matter. As cereals mature and start heading, their N content drops (e.g. 1%) and PAN is immobilized (negative PAN) during decomposition. Most of these effects occur in the first 4-6 weeks after plowdown.

In order to estimate cover crop PAN we recommend field sampling and analysis of a whole-plant aboveground sample. The cover crop is harvested from a known area in the field, weighed wet, then subsampled. The subsamples are sent to an analytical lab for determination of % dry matter and total %N. Detailed sampling instructions are available in a new PNW Extension publication due for release in October (PNW 636, Sullivan and Andrews 2012). PAN release from cover crops can be estimated using Table 2 or the OSU Organic Fertilizer and Cover Crop Calculator (Andrews et al., 2010). The Calculator also compares the cost of PAN sources (fertilizer, cover crop and compost) and can help match N-P-K rates with crop requirements.

Soil Organic Matter PAN

In western Oregon, nitrate-N is leached from soil during heavy winter rains from November to March, so spring soil nitrate-N levels are consistently below 10ppm. PAN is released by microbial activity from SOM when soil temperatures warm in the spring. The amount of PAN supplied per day from soil organic matter decomposition usually doubles between spring and summer, because soil is warmer.

Biologically-active or “young” organic matter is derived from inputs during the previous five years or so. This young fraction of soil organic matter is a major contributor to PAN release. Older organic matter releases PAN very slowly. Soil tests report total organic matter, the sum of young and old organic matter. Numerous research studies have reported a lack of correlation between total soil organic matter levels and PAN release. So, although some soil testing labs measure total soil organic matter and then report an “estimate of PAN release”, we do not have much confidence in such estimates.

Ongoing N mineralization research is being done to see how much variability exists between fields. We are incubating moist soil at room temperature in the laboratory and comparing the results with crop N uptake from unfertilized plots. Preliminary results from these incubations show that soil N mineralization rates vary by a magnitude of up to six (Figure 2), indicating that there are opportunities to use fewer N inputs on fields with high baseline soil N mineralization rates. Variables such as soil temperature and moisture in the field and the timing and efficiency of crop N uptake influence how these results could be used. Ongoing work will help us determine whether these tests can support nutrient management decisions in organic vegetable production systems.

http://smallfarms.oregonstate.edu/calculator.

 

Gaskell, M., R. Smith, J. Mitchell, S.T. Koike, C. Fouche, T. Hartz, W., Horwath and L. Jackson (2007). Soil Fertility Management for Organic Crops. University of California Extension Publication 7249.

Hart, J.M., D.M. Sullivan, J.R. Myers, and R.E. Peachey (2010) Sweet Corn Nutrient Management Guide (Western Oregon). OSU Extension Publication EM 9010-E. https://catalog.extension.oregonstate.edu/em9272

Sullivan, D.M. and N.D. Andrews (in press). Estimating plant-available nitrogen release from cover crops. PNW Extension Publication 636. https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pd...

Sullivan, D.M., J.P.G. McQueen and D.A. Horneck (2008). Estimating nitrogen mineralization in organic potato production. OSU Extension Publication EM 8949-E. https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pd...

Acknowledgements: this work is financially supported by partnerships with Oregon Tilth, Inc and the West Multnomah Soil & Water Conservation District, and by grants from USDA Western Sustainable Agriculture Research & Education