Inspection Services Blog

2020 Nutrient Management Conference Q&As – Session 1: Sampling Techniques and Analyses

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: Assessment of Harvested and Sequestered Nitrogen Content in California Crops

Presenter: Dr. John Dickey, Technical Program Manager, South San Joaquin Valley Management Practices Evaluation Program (Presentation link)

Balancing the rate of fertilizer nitrogen (N) application with amounts needed by crops serves to ensure adequacy of N supply and to minimize environmental losses. One of many pieces of information needed to craft efficient fertilization programs is therefore the quantity of N removed in harvested materials or sequestered into perennial plant parts. This is an update on work done by Central Valley grower coalitions formed for the Irrigated Lands Regulatory Program (ILRP), working with commodities groups, industries, FREP, the U.S. Department of Agriculture (USDA) Natural Resources Conservation Services (NRCS), and University of California (UC) Cooperative Extension, to refine these quantities in support of N management planning and assessment.

Q&As

Q: The Geissler Report and the subsequent studies (phase 1 and 2) show N removed in harvested crop; however, this number can be very different than the total N used by the crop. How can these numbers be useful to growers or crop consultants when they are not the actual amount of N the crop needs to grow?

A: There are many other places that N can go, which growers need to consider, such as N in the perennial parts, N losses, etc. However, the N that is present in the harvested portion of the plants and ultimately removed from the field is also an important factor and growers need to know this value as well. This study was intended to just focus on that component of a crop’s N requirements. There are also field specific factors that need to be accounted for and we know they exist, but this study just focuses on the N in the harvested portion.

Q: The 20% coefficient of variation (CV) that you reported seems to be quite high; with all that variability, is it realistic to have a single coefficient if you look at the genetic differences, soil types and other factors that contribute to that high CV?

A: A single coefficient will always have shortcomings; different crops inherit characteristics and crops are grown in different climates under different management practices. It is normal to see variations and a single coefficient would not reflect these variations. It is important to account for those factors contributing to variations, though, in calculating fertilizer applications. For example, harvest time, whether N will be diluted in heavier crops, etc. Most commodities with the ILRP are keeping it simple by using single coefficients because they are looking at the broad landscape level N balance and it works for them.

Q: How do you anticipate these removal values be used for regulatory purposes in the future? Do you think these number will be binding?

A: No! There is no history of them being used that way or even to be used that way in the future. The main purpose is to provide information to the coalitions so they can identify outliers and reach out to them and identify whether it’s a reporting problem or management issue or caused by other reasons. 


Presentation: Assessing the Accuracy and Precision of Soil Chemical Analyses Performed by Eight Agricultural Laboratories

Presenter: Andre Biscaro, Irrigation and Water Resources Advisor, University of California Cooperative Extension, Ventura County (Presentation link)

An accurate soil chemical analysis is the cornerstone of an efficient nutrient management program. Without a reliable soil test result, significant mistakes in fertilization programs can occur, resulting in inappropriate (under or over) fertilizer recommendations and potentially yield losses due to nutrient deficiency or over-fertilization. This can dramatically affect profitability and can potentially have negative environmental consequences. This project assessed the accuracy and precision of eight commonly used ag laboratories for soil chemical analysis in California.

Q&As

Q: Did the laboratories that underperformed have a rational for why they varied so much from the median values? Were all the participating labs given the results in how they compared to their peers? Was advice given on how to improve their analysis?

A: We have contacted labs and asked them whether they’re interested in the results and we share the results with those who are interested. There needs to be conversations, but I believe that it is lab users’ right to get decent and accurate results for what they pay. There should be conversations between the labs and lab users to have an agreement and that a proficiency program will be beneficial for everyone.

Q: Why was nitrate so difficult to analyze? And if the results across labs were so inconsistent, how can growers and consultants be more confident in making fertilizer recommendations and decisions.

A: It is easy for the labs to analyze the nitrate, yet the numbers vary across the labs. Since the samples used for nitrate analysis were very dry it is less likely that inconsistencies are caused by ongoing nitrification. Therefore, it is not clear why the there is such a variation in reported results.

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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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