May 28, 2015 | Source: Oregon Tilth | Format: Lessons Learned

Lessons Learned: Nutrient Management

Sauvie Island Organics is a 25-acre fresh vegetable farm – certified by Oregon Tilth since 2009 – located 15 miles from the hubbub of downtown Portland. Farm manager Scott Latham and the farm crew thought they were already maximizing their standard organic practices. Cover crop rotations were strategically aligned with harvest plans. Soil amendments were designed to protect surface water quality. Succession planting was planned to increase harvests.

But Scott knew that Sauvie Island Organics would enhance their farm practices if there were less mystery and more calculated data on the nutrient profile of their soil.

Understanding nutrient management

Sauvie Island Organics joined up with Oregon State University to utilize several on-farm tools developed with support from Oregon Tilth and Western Sustainable Agriculture Research and Education to create a farm nutrient management plan. What does that mean?

The basics of any crop fertility needs – nitrogen, phosphorus, potassium and more – are often met with expensive off-farm inputs. But what if you were able to credit each and every source of nutrients in your farm system? What if there was a better way to calculate the differences between crop needs and nutrient credits already on-site?

There are at least three lessons learned from smart nutrient management planning:

  • Accounting for every source contributing nutrients to the soil save money and time.
  • Knowing your soil’s nutrients can protect natural resources from leaching and runoff.
  • There is no point in making a plan unless you monitor and adapt as needed.

Step 1

Often, nitrogen is the main factor in making crop nutrient needs decisions. This is as simple as knowing your needs, your credits (what’s already available) and determining the difference between the two.

Streamlining your nutrient management plan by creating “nutrient need groups” removes the burden of knowing exact nutrient levels for each individual crop.

  • Budget for crops by grouping needs: low, medium and high
    When planning for amendments it helps to know how other crop decisions can directly impact the nutrient needs of a given area on your operation. For instance, light feeders might not impact nutrient availability for the following season. Rotations can be tweaked to relieve tired soil without requiring significant added organic matter. And intercropping nitrogen fixers – cover crops or cash crops like beans – might allow you to extend your nutrient plan even further by reducing overall inputs. Groupings allow for quick decisions that keep you within your budget.
  • Test your soil @ the end of the season (and before and in between)
    While testing is a vital tool for every farmer to know the ins and outs of soil composition, too often we test at the beginning of the season in order to estimate fertility needs. Creating a plan to test during and at the end of the season allows you to understand if your budgeting was over, under or just right around on target. Monitoring as-you-go will also make sure that you remain on track throughout a given season and address issues as they occur.
  • Don’t forget about phosphorous
    While nitrogen is critical for success and using amendments like manure help deliver the amounts needed for good growing conditions, phosphorous can accumulate in the soil in unsafe or excess amounts. Be sure to pay close attention to these levels since leaching can occur and high amounts will cause runoff into other water sources. There is such a thing as too much organic manure and compost; your plants will only uptake what is needed and the rest becomes a potential issue for water quality and overall soil health.


Organic amendments provide a large quantity of nutrients to crops in the first several weeks and year of application. But additional plant-available nitrogen (PAN) – and this is true for other nutrients as well – is released into the soil over time. This directly impacts and lessens the amounts needed in subsequent years.

Accounting for an amendment’s nutrient credit over time saves money on fertilization and stops pollution from soil-leaching nutrients. In addition, repeated applications of some common amendments often lead to undesirable amounts of phosphorous and ought to be closely monitored. Check out our infographic to see how to continue to plan by accounting for an amendment applied years earlier:

Nutrient Management

General rule of (green) thumb:
According to the Nutrient Management in Organic Systems guide, if a field received substantial organic inputs (i.e. cover crops and manure or compost) for at least three consecutive years, you can estimate a credit of approximately 50 lbs. of nitrogen per acre.


Carefully planning and accounting for nitrogen contributions from cover crops can help further financial savings and create a more accurate nutrient budget. But this depends on the species or mix, crop biomass and growth stage when a cover crop is terminated.

The most accurate way to determine cover crops nutrient credits is through lab analysis. You can also cut and weigh your cover crops for dry matter; tips for calculations are outlined in Estimating Plant-Available Nitrogen Release from Cover Crops.

General rule of (green) thumb:
Use height and groundcover estimates of your cover crops to get simple and quick valuations of dry matter to predict the nutrient credits for a given cover crop species or mix.

Mini Case Study:

Cover Crop Calculations Example, Part I
Estimate dry matter (DM) per acre by using cover crop height and groundcover percentage. The resource, Managing Cover Crops Profitably by Sustainable Agriculture Research and Education (SARE) offers useful guidance. Let’s look at our example using their advice:

  • For every legume, 6 inches in height = 2,000 lbs. DM / acre
  • For every additional (1) inch in height = 150 lbs. DM / acre
  • Multiply total pounds DM per acre with groundcover percentage

Mini case study calculation:
Total dry matter of 3,800* lbs. DM / acre x 75% groundcover = 2,280 lbs. DM / acre

Cover Crop Calculations Example, Part II
Next, estimate nitrogen availability based on cover crop type and termination timing. Annual legumes typically have between 3.5 and 4 percent nitrogen in their aboveground parts prior to flowering (for young material, use the higher end of the range), and 3 to 3.5 percent at flowering.

Mini case study calculation:
Total nitrogen in the example would then be 2,280 lbs. x 3.5/100 = 79.8 lbs. PAN

Cover Crop Calculations Example, Part III
Lastly, estimate what will be available to your crop this year.

  • Divide by 2, if the green manure will be conventionally tilled
  • Divide by 4, if it will be left on the surface in a no-till system in Northern climates
  • Divide by 2, if it will be left on the surface in a no-till system in Southern climates

Mini case study calculation:
79.8 lbs. PAN / 2 = TOTAL of 39.9 lbs. PAN / acre this yea

For more, be sure to see pages 22-23 in Managing Cover Crops Profitably.

Additional resources

Creating a nutrient management plan takes time and a bit of homework. But once up and running, you can just maintain your baselines with careful monitoring. The dividends for your farm in terms of savings, yields and conservation can be immense. Check out some other great resources:

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