How Satellite Imagery helps us in understanding VRA and Productivity Zones
What is VRA?
Variable Rate Application or VRA refers to the precise application of nutrients and fertilizers to a farm. It allows you to apply different rates of fertilizer in each part of the farm on the basis of within-field variabilities such as variation in soil fertility, topography, irrigation water depth, etc. This simply translates to what is often defined as the 4 R strategy: the right thing, at the right place, at the right time, and in the right manner.
Why do we need VRA?
All farms are heterogeneous. Soil fertility and yield across a single farm are always different. If a constant fertilizer rate is applied in a farm like this, it could result in excess nutrients in some parts and a deficit in other parts. As a result, the fertilizer’s efficiency will be low in some parts of the same farm. VRA helps in applying the right amount of resources like fertilizers, chemicals, and seeds — all of which help to optimize one’s crop production. Growers can increase their yield by using an optimal level of resources in the areas where the input is going to add the most value while lowering the input in places where they have less impact on returns and yield.
Let's understand productivity zones
The productivity of land means the capacity of the land to produce a crop. The average productivity of land means the output obtained from the land divided by the area of land usually represented in the form of yield per acre or yield per hectare of land. Productivity zones are areas in a farm with different yield histories. The area with the highest output for several seasons is considered a high productivity zone and so on. Productivity Trend or Health Trend of a plot are the areas of the farm which has shown better productivity over the past few years.
Benefits of productivity zones based VRA for fertigation
- Nutrient balance: All areas of the farm receive the optimum amount of fertilizer hence avoiding over usage of fertilizers in areas where productivity is low.
- Potential increase in yield: More efficient use of fertilizer will increase the overall yield of the farm.
- Identification of problem areas: Productivity zones could further help in the identification of low yield areas that requires more attention.
Productivity zone maps using satellite imagery
Currently, NDVI is used widely as a crop health assessment parameter because of its scientifically proven formula validated over the past decades. Thus, to analyze the productivity of a farm, historical NDVI data, obtained through satellite images of a farm, could be used to generate productivity zone maps using pixel-wise NDVI analyses. The variation in NDVI could be then categorized as High, Moderate, and Low Productivity Zones.
Similarly analyzing NDVI variation over a time period, productivity trends or health trends for each zone could be mapped (see below figure).
How to use productivity zone maps for variable-rate fertilizer application
A field experiment conducted by OneSoil on productivity zone-based variable-rate nitrogen application showed the following results:
- Low productivity zones did not respond to a heavy increased nitrogen fertilizer application in either the first or second rounds of fertigation.
- Moderate productivity zones responded to higher nitrogen fertilizer rates, but not by much.
- High productivity zones responded well to increased fertilizer application rates.
Therefore, the fertilizer application rate for each zone needs to be calculated separately. If historical data is available then based on nutrient removal rate from each zone, fertilizer application rate could be determined more accurately. Otherwise as a generic approach, one can follow the below process:
- For high productivity zones, increase fertilizer application rate by 25% to 30%.
- For moderate zones increase the rate by 5% to 10%.
- For low productivity zones, decrease the rate by 30% to 35%.
Additionally, soil testing of the low productivity zones will help in identifying underlying causes for low productivity and the right measures could be taken to further improve it.
In conclusion, productivity zone maps can help in optimizing resources and improve farm productivity. The productivity trends can also help in keeping an eye on long-term farm productivity change.
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