Inspection Services Blog

2020 Nutrient Management Conference Q&As – Session 4: Soil and Crop Nutrient Management

The California Department of Food and Agriculture (CDFA) Fertilizer Research and Education Program (FREP) and Western Plant Health Association (WPHA) presented their annual Nutrient Management Conference as a webinar October 28-29. This series of five blog posts links to each conference session and highlights the most common questions asked by the conference audience and presenters’ responses during the Q&A sessions.

Presentation: Evaluation of Certified Organic Fertilizers for Long-term Nutrient Planning

Presenter: Cole Smith, Staff Research Associate, University of California Cooperative Extension, Santa Clara and Monterey Counties (Presentation link)

The factors influencing long-term nutrient availability from organic fertilizers is not well understood. Cole presented data from an ongoing project combining meta-analysis, laboratory incubations and field trials to evaluate factors such as temperature and soil type on N mineralization from organic fertilizers. Results show that N availability is generally predictable using amendment characteristics, but environmental and soil effects interact to influence N availability in organic fertilizers. This information can be incorporated into guidelines to help growers adjust applications of organic fertilizers according to seasonal and site-specific considerations.


Q: How do you measure soil moisture to determine whether decomposition is happening? Should soil moisture probes be used for that?

A: We’re measuring soil water content as water holding capacity (WHC) percentage and using 60% WHC as our target. Different soil moisture measurement techniques can be used including soil moisture sensors. Depending on the soil type, target soil moisture content should be more than 50% WHC. You can still do it by feel, squeezing the soil and see if water is dripping out, then that’s roughly more than 50% WHC. Lower WHC slows down the decomposition process as well as fully flooded/saturated soil.

Q: How might low or high levels of soil organic matter impact availability of N from fertilizers?

A: It depends on the C and N content of the soil. If the soil is low in N and C then it can suppress the release of N from fertilizers, but if soil has high C content with limited N then adding N fertilizer would speed up the decomposition rate.

Presentation: Soil Nitrate Quick Tests and Nitrogen Management in Strawberry Production

Presenter: Dr. Gerry Spinelli, Agricultural Technical Specialist, Santa Cruz Resource Conservation District (Presentation link)

This presentation included the activities of a FREP-funded soil nitrate quick test project: The project focused on one-on-one trainings in Spanish for irrigators and ranch managers on using soil nitrate quick tests in strawberry and vegetables. Trainings on how to calculate a nitrogen (N) balance in strawberry and vegetables was provided, including accounting for the N provided by the irrigation water, soil mineralization and the residue of the previous crop. Outreach activities for the project included group trainings and YouTube videos in Spanish.


Q: Have you found a different accuracy or a bias in test results when users do not use calcium chloride?

A: The calcium chloride doesn’t take part in reaction so you do not need it and can still use water, but the challenge is you need to make sure the water does not include any nitrate. So, make sure you test the extraction water nitrate concentration with a strip, but an easier alternative is to use de-ionized (DI) water.

Q: Is there an interest among growers to adopt use of nitrate quick tests?

A: There is a lot of interest in using this technique because growers find it to be an easy method. However, some growers find the calculation and conversion parts (for example how to calculate pounds of N to be applied per acre) confusing. The methods for using test results at the field level can also be difficult to understand.

Q: Did you find that the irrigators were over-applying water and/or fertilizer in strawberries?

A: Although they might be over-irrigating in a few cases, strawberry growers usually avoid over-application because they want to prevent wet soils to allow workers and pickers easily walk through the rows and work conveniently. However, in lettuce, where sprinklers are used at the beginning of the season, over-irrigation occurs because they put the aluminum pipes in the field and the grower tries to fill the soil profile. Then there is a window of about two weeks that irrigation is not occurring due to field traffic for pipe removal, thinning, cultivating etc. Growers want to make sure there is enough water in the soil profile during that window of time.

Presentation: Hemp for Essential Oils: Water and Nutrient Management Considerations

Presenter: Dr. Bob Hutmacher, Cooperative Extension Specialist and Center Director, University of California West Side Research and Education Center (Presentation link)

This presentation provided an overview of the current irrigation and nutrient management research being carried out by the University of California on industrial hemp. Specifically, Bob covered details on an irrigation trial that he is conducting at the Westside Research and Extension Center in cooperation with Oregon State. The trial is investigating the impact of deficit irrigation on yield and CBD oil production in both photoperiod sensitive and auto-flowering cultivars.


Q: Does cannabis have the potential to become a significant weed problem in cultivated cropland?

A: Depending on how many types of hemp are grown in the region there is a high possibility to see volunteer plants. We had production of feminized plants in a research trial and despite complete termination of crop at the end of season we still observed volunteer plants. Also, pollen is very light and can move significant distances from other fields miles away producing a crop that you did not expect.

Q: Has there been any work on the salinity tolerance of hemp? Can hemp tolerate high levels of salt in the soil?

A: As far as I know no salinity evaluations have been done on hemp, but we’ve raised the issue with seed companies to conduct some research trials to evaluate salinity in the west side station. Some auto flowering types that have smaller above ground biomass and smaller roots, compared to the full season types, may likely be impacted by high salt concentrations under surface or subsurface drip systems. However, full season types (110-140 maturity) may be tolerant to drought and salt because of their extensive tap root system, which grows aggressively in the soil profile down to 6 to 8 feet.

Q: What are the requirements to become a commercial hemp seed producer? 

A: Legal requirements should be met for any production purposes including fiber and grain. You must be registered and pay the registration fee to the CDFA through the Ag commissioners in each county. You must show proof that you are producing seeds that meet the THC concentration limitations. You can find some approved varieties from the California Cannabis Industry Association (CCIA) and CDFA websites to choose the approved varieties.

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For more than 25 years, FREP has presented its pioneering fertilizer research at annual conferences. Since 2007, FREP has collaborated with the Western Plant Health Association (WPHA) to create a conference that balances technical research with discussion on practical application. Presenters from academia, industry, and agricultural consulting provide general and technical information, current research and data, and practical applications addressing statewide and regional nutrient management issues. Visit the Annual FREP/WPHA Conference webpage for links to all the 2020 conference sessions and the 2020 Conference Proceedings booklet.

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