The soil analysis requires the sample to be transported to a laboratory. This means work and costs. In the laboratory the process steps take place: Soil preparation, analysis and documentation of the results. It usually takes between 2 to 4 (6) weeks, depending on the season, until the soil nutrient content is available.
An analysis on the field means that the result is available immediately.
But an analysis in the field does not correspond 100% with the laboratory standard. Differences between the result of the field analysis and the laboratory analysis are to be expected.
Your question has answer too. Thats a fact , and this is the reason , why are we so reluctant to use field tested data with crop performance. Most of the soil test data in field are by and large indicative than interpretative.....
Stefan Hinck Dear Stefan,
Yes, absolutely it makes sense to do soil nutrient analysis directly on the field. I have been working on this topic since 1992 for 18 years. Specifically, my colleagues and I were researching how to measure Soil Nitrate, On-the-go in real time, using a Tractor-mounted, Fully Automated Soil Nitrate Measurement and Control System. My colleagues and I were pioneers in this field of research within what is now known as Precision Agriculture (also known as Precision Farming)—as the dates of our early publications will attest.
In 1994 we had a fully functioning prototype that could control the application rate of a fertilizer spreader attached to the back of the tractor in response to soil nitrate measurements being taken near the front of the tractor. GPS Positional data was also collected for each sampling location. However, agronomy data to combine with the system's nitrate data to create crop-specific algorithms for determining what fertilizer application rates to use for optimizing growth and yield were not available at the time.
So, we continued or research using the system only as a soil nitrate monitoring and mapping tool. Our several research publications answer all parts of your question, including my Ph.D. thesis published by Wageningen UR, the Netherlands in 2008.
Specific to the relationship between in-field nitrate analysis and laboratory nitrate analysis, see our published article using the URL link below.
Article Field-scale validation of an automated soil nitrate extracti...
The soil nitrate mapping system (SNMS) (Fig. 1) can obtain the data to analyze soil nitrate variation both spatially and temporally. The SNMS consists of six sub-units (as numbered in Fig. 1): (1) soil sampler, (2) soil metering and conveying, (3) nitrate extraction and measurement, (4) auto-calibration, (5) control and (6) global positioning system (GPS). The system automatically takes a soil sample (0–15-cm depth), mixes it with water, and analyzes it electrochemically for nitrate concentration in real-time (6 s) using a commercially available standard NO3- ion-selective electrode used in many labs. Geo-referenced data are recorded simultaneously by GPS at each sampling location to enable a nitrate map to be created for the field.
The system can be used to analyze soil samples automatically while on-the-go, or manually while stationary by placing samples by hand into the nitrate extraction and measurement sub-unit (NEMS). It was envisioned that two configurations of the system would eventually be used in practice—a tractor-mounted version (as shown in Fig. 1) and a portable 12Vdc powered ‘suitcase’ version (not shown). Unfortunately, neither of those visions progressed past the prototype stage in the innovating and commercializing processes for various reasons I will not get into with my answer to your question.
Specific to your comment that analysis in the field does not correspond 100% with the laboratory standard, you are quite correct as nothing in Science ever agrees 100%. That, then, moves one's thoughts to how close is 'close enough for all intents and purposes', does it not?
Engineers are faced with that question continuously. So, it was natural for me to take an approach that would obtain robust measurement results that were as we say in engineering "good enough for engineering purposes". As an engineer, I also attempt to design systems that are simple, elegant, reliable, low maintenance, cost effective to operate, and easy-to use solutions to the design challenges I face. Being an agricultural machinery engineer specialist who also had a high level of knowledge and experience in agronomy and soil science, I was able to take into account the varied requirements and nuances of these subjects too throughout the research and development process. I hold professional designations as both a professional engineer and a professional agrologist here in Nova Scotia, Canada.
In the end, we were able to obtain real-time, in-field, soil nitrate measurements with lab-grade accuracy at a calculated 1/10th of the cost of lab analyses. Using paired samples, the level of agreement between the Nitrate Extraction and Measurement Subsystem (NEMS) soil NO3–N levels and standard laboratory method soil NO3–N levels for the field conditions and cropping systems tested was excellent.
The results also suggest that the NEMS is robust enough to be used in the crops, soil groups, fertility types, and tillage conditions tested, as well as at any time throughout the season.
Regression analyses showed Lab NO3-N = 0.031 + 1.24 NEMS RN NO3-N (R2 = 0.910, n=380) Eqn. (5); Fig. 3(b) without the need for soil sample moisture content correction.
What...wait a minute...without the need for soil moisture content correction? How is that possible you and others reading this reply may be wondering? And it is all-ways a good thing to wonder, is it not? Well, I have three personal aphorism that I live by (I have created many more):
1. "Success is the result of dedicated perseverance." – (CC BY-NC-ND 4.0) 1984-2020 Kevin J. Sibley. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Public License. For uses beyond this license, you may ask me directly.
2. “Nothing is impossible. We just haven’t figured it out yet.” – (CC BY-NC-ND 4.0) 1986-2020 Kevin J. Sibley. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Public License. For uses beyond this license, you may ask me directly.
3. “Anything that can be imagined can eventually be created. Imagining of the thing is its virtual creation…then innovating leads to its real creation.” – (CC BY-NC-ND 4.0) 2001-2020 Kevin J. Sibley. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Public License. For uses beyond this license, you may ask me directly.
More complete details of how the soil nitrate measurement system works are contained in this book chapter published in 2010:
Chapter In-Field Measurement of Soil Nitrate Using an Ion-Selective Electrode
And of course, the other related publication found in my ResearchGate Research Contributions round out the remaining information about the entire system.
I hope my reply to your question has be useful to you and others.
Kind regards,
Kevin
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