Willness: Building VRA Fertility Zones on Plant Biomass Alone is Short-Sighted

Willness: Building VRA Fertility Zones on Plant Biomass Alone is Short-Sighted

I posed a question in a twitter poll in order to raise awareness of how measurements of biomass from remote sensing have major limitations in a soil applied variable-rate fertilizer program. 324 voters participated.


The correct answer? D. There is not enough information collected to provide the correct answer for this.

This is why I conducted the poll. 90% of respondents answered A, B or C. It is my opinion, as this poll demonstrates, that most people do not understand that looking at biomass alone is NOT enough information to decide if an area needs low rates or high rates of fertilizer.

Why? Let’s explore the two main issues:

  1. Biomass alone without soil testing (checking the fridge) does not work. There are companies that do this. They sample no zones, or maybe just the field average zone, or maybe do all zones every 3 years. If you don’t know what is in the fridge, you don’t know if the zone will need groceries. For mobile nutrients like nitrogen and sulfur, they can vary a lot from year to year. Furthermore, taking biomass alone and turning it into zones doesn’t work. Why? Because a biomass area is not consistent with fertilizer response as discussed below.

2. Biomass alone as a predictor of fertilizer response does not work.

Willness: “On this BAY MAPS image (left) you can clearly see it puts the eroded, thin topsoil, dry hills as the same biomass as the wet, saline depressions. BAY is “correlated imagery” in this example. On the right is SWAT, which is our patented soil, water and topography modeling done by field soil surveys.” (Maps Submitted by Cory Willness) 

Scenario 1: Low biomass (skinny): yields will be lower…something is wrong…what is the cause?

Fridge full — The areas could have very high fertility levels but still be skinny due to disease, insects, salinity, weed issues, too wet, or have a single nutrient deficiency. Low biomass areas may not have spatial consistency, in fact they could be opposites. Hills may have high biomass in wet years and low biomass in dry years. In most cases, the reason for low biomass is something other than fertility. This is why all low biomass areas should not be sampled as a zone – they could be low for many different reasons. If a skinny person went to a doctor how likely is it that they are skinny because their fridge is empty? Most likely it is because of a different health problem.

Fridge empty — Areas may just need a better overall fertility program. A low biomass area may benefit from more fertility if it is an eroded hill that is lower in organic matter and thin topsoil. In conditions with ample moisture, they could be some of the highest responders to fertilizer in the field. If the skinny person has the right amount of water, has no health issue, and their fridge is empty then they need groceries.

Takeaway: In the low biomass areas the fridge can be full or empty.

Scenario 2: Very high biomass (obese): excess vegetation…lodging…disease…yields may suffer.

Fridge full — These areas could be an old yard site, new breaking, old pastures, areas collecting eroded soil such as depressions or old fence lines. These are examples where you would expect high mineralization potential and very low fertilizer response. They will have very high biomass due to high organic levels and high soil test levels of nutrients. If you are obese and your fridge is full you do not need groceries.

Fridge empty — Good examples of very high biomass areas with overall low fertility levels are not as common. Peat soils typically have high biomass due to high organic levels and excess N mineralization but often show up deficient in potassium and copper. In this case you need high rates of specific fertilizers only, but they are high biomass areas that require a lot of it to correct the issue.

Scenario 3: High biomass (muscular): everything is just right in the consistently high producing areas.

Fridge full — A well drained depression that is very high in organic matter and 2 feet thick topsoil can mineralize a lot of nitrogen and contain a lot of nutrients. It would turn obese if you put a lot of fertilizer there. It does not require extremely high rates of fertilizer to attain excellent yields.

Fridge empty — It is common that the highest biomass areas could be midslope and field average soils producing consistently high yields every year – never too wet or too dry. They can often have the lowest soil test levels of nutrients in the field because of this consistently high productivity. These areas may require the highest fertilizer rates in the field, being mined of nutrients in the past. However, hills in wet years with thin topsoil, low organic matter, and do not contain a lot of nutrients will require a lot of fertilizer to produce the high end yields they are capable of. In wet years these hills can produce excellent crops if fertilized aggressively. In the wet years of 2010 and 2011 hills were the high biomass areas in most of our fields. Depressions could be the exact opposite. As mentioned already, spatial consistency from dry to wet years can be inconsistent.

Takeaway: In the top performing areas the fridge can be full or empty.

Author Summary:

Are low biomass areas a fertility zone?

No. They can have poor fertility or high fertility.

Should all low biomass areas be treated the same?

No. The areas of low biomass are typically not low for the same reasons, some may require no fertilizer and some may require high rates of fertilizer. This exact question can be posed for all sizes of biomass and you will get the same answer. You cannot deliver groceries to homes and drop them off based on the body type that answers the door. Each body type can have a full or empty fridge.

The dominant zone mapping process in the variable-rate industry is what? Biomass. Using biomass to make zones, soil sampling the biomass zones, recommending fertilizer rates for prescriptions based on biomass zones.

Is biomass a good foundation to make zones?

No. As my examples suggest, biomass does not have a good relationship with fertility requirements. So why is it the dominant zone process then? It is quick, easy, and scalable. There are probably 100 software platforms you can purchase to use biomass to build zones, so that is what people do if they don’t know any different.

But is it the foundation of a solid variable-rate zone building, soil sampling, and prescription application for application of soil applied fertilizer?

Not for me.

RELATED CONTENT: January 2018 PrecisionAg® Professional profile on Cory Willness and his CropPro Consulting outfit. 

Leave a Reply