#13 The Physics of: Nutrient Optimization in precision agriculture
The critical macronutrient
Nitrogen is the critical macronutrient required for plant growth. Precision agriculture as the name implies aims to provide the economically optimum amount of inputs (fertilizers, water, pesticides) to the crop while aiming to maximize crop yield (tonnes/ha). In this post we look at how we can determine a crop’s Nitrogen requirements for a target yield in real time with earth observation.
Micro-organisms in the soil are constantly cycling Nitrogen from the atmosphere (N2) into Ammonia (NH3) and Nitrates (NO3-) which plants can absorb. Due to these processes the quantum of Nitrogen in the soil is dynamic, constantly varying throughout the season. If we didn’t know the magnitude of this natural contribution of nutrients to plants by microorganisms we might follow the default prescription rate for nitrogen (via synthetic fertilizer), which at times will be excessive and other times inadequate.
A logical formula that forms the basis for determining eN requirements therefore is
where Ncrop - Nsoil (N deficit) is the difference between what the soil is providing and what the crop needs at any given point in time during the season. While Ncrop can be calculated via modeling, in agricultural models (e.g. APSIM), Nsoil is much harder to model or measure directly and economically. Thus the N deficit itself cannot easily be known. efert is the efficiency with which a particular crop takes up Nitrogen during its growth. Lower efficiency of uptake therefore implies greater N requirement for the crop in the form of applied fertilizer.
The N rich strip protocol
Instead of attempting to individually calculate these terms, scientists based in the mid-western U.S. [1] have, long before earth observation became commonplace developed the N rich strip protocol. The protocol requires application of a larger quantum of N to a long strip within the field. This is typically done early in the season, while the rest of the field receives the default prescribed application of N.
In a few weeks, farmers typically observe one of two scenarios, 1) there is no difference in greenness between the N rich strip and the rest of the field, indicating that N isn’t a limiting factor for plant growth, 2) there is a noteable difference in greenness indicating that the field could benefit from more N application. Greenness here is a proxy for photosynthetic activity which drives plant growth. The farmer then applies more N (a flat rate) to the entire field (before flowering and grain growth) which allows the crop to recover from early season N deficit. This is a crude method, with a simple “Apply more/Apply the same” binary recommendation.
Since the early days however this protocol has been improved and streamlined for real time agricultural intervention, aided by Earth Observation. More than 1 million acres of farms in the Great Plains in the U.S.A now implement this practice. [4] Prominently, OneSoil, an earth observation focused ag tech company has an end to end application that is designed for variable rate application (VRA). Variable rate is distinct and more precise than flat rate because it accounts for spatial variation in soil N content across a field. OneSoil uses crop maps & historical NDVI to map out zones within each field that are low, medium and high productivity (a proxy for soil N content) and allows farmers to create N rich control strips within each zone to observe the effects of variable rate application.
Benefits of the protocol
One Soil counts 4.5% of the arable land globally as registered on it’s platform. It reports an average of $40/ha as net profits from following it’s protocols. Farmers across the Great Plains too similarly report increased net profits from following this practice. [4]
In India too, studies have shown that based on the desired end goal, N rich strip guided application of fertilizers can either result in monetary savings with reduced application of N while achieving the same yield [2] or increasing the yield itself with a higher application of N [3]. This reflects the reality that in different parts of India, fertilizer application is not precision guided. In some regions, N is not a limiting factor and yield is already being maximized , whereas in other regions, N is the limiting factor and further increasing N application could increase yields further.
Both these papers [2], [3] employ the N rich strip protocol with the handheld GreenSeeker for observing the NDVI reflectance of the N rich strip. However Indian companies such as Satyukt Analytics are now implementing similar logic to NDVI from earth observation imagery to take precision nutrient management to larger scales.
The non monetary benefits from these protocols must not be ignored. They manifest as reduced non point source pollution that arises primarily from excessive fertilizer use which then runs off into surface water.
Challenges in implementation of the N rich strip
Successful implementation of this protocol requires at a minimum good knowledge of the physiological processes of crop growth, to determine the optimum timings and quantum of application of fertilizers. The relationship between projected yield YP0 and potential yield YPn must be carefully calibrated for calculating the Economically Optimum Nitrogen Rate (EONR). Even if this is taken care of by ag-tech earth observation companies, increasing awareness regarding the benefits of these protocols is important before widespread adoption can be feasible.
Region specific challenges also exist. The resolution of smallholder farms in India as expected proves to be a challenge to precisely placing a N control strip large enough to be observed reliably with earth observation imagery. Precise implementation is also tricky without hardware that automates the precise application rate recommendation. Both the cost of trained manual labour or mechanization might offset the monetary benefits to be gained from precise N application.
References
[1] “Nitrogen rich strips, a reminder” Brian Arnall (2023),
[2] “In-season estimation of yield and nitrogen management in irrigated wheat using a hand-held optical sensor in the Indo-Gangetic plains of South Asia”, Bijay-Singh et al.
[3] “Optical sensor-based nitrogen management: an environmentally friendly and cost-effective approach for sustainable wheat (Triticum aestivum L.) production on Eastern plains of India“, Biplab Mitra et al
[4] “VirtualAg Expert Series - Crop nitrogen monitoring and optimization with remote sensing” Regrow Ag, 2020