How do we decide how much fertilizer to use on our fields?

I see Internet articles floating around from time to time about farmers "dousing" their soil and crops with chemicals and synthetic fertilizers.  The part these so called "expose" articles, (often written by someone who has never actually stepped foot on a farm or smelled fresh soil) do not account for is the cost of fertilizers and chemicals we use on the farm.  They are not cheap!  Farmers don't want to waste any of our hard earned money!  We analyze the soil for its nutrient content before adding more fertilizer.  There is a point of declining economic benefit from applying fertilizer to our crops.  We want to be as efficient as possible when purchasing and applying any crop input on our land.

Today's post is about the process we call "soil sampling."  We sample the soil periodically to learn whats going on in a field, a snap shot if you will, of the nutrients and their availability in the soil.  It all starts with this little brown bag.        

We use a soil probe like the one pictured below to get a 6 inch profile of the soil.  This process is repeated all across the field.  Often times in  2.5 acre squares we call "grids."  

We put the core samples in a small bucket to crumble the soil up evenly and put it inside the brown bag.  This process is repeated all over the field.  

We then ship the bags from the field to a lab for analysis.  There are private labs and most Land Grant University in each state has a soil lab as well.  These samples are analyzed for nutrient content such as Phosphorous, Potash and Nitrogen.  The pH of the soil is measured.  The pH of the soil tells us if the soils are basic or acidic.  We want our soils to have a pH around 6 - 6.5.  

Other nutrient holding capacities are measured as well such as Cation Exchange Capacity.  If you remember chemistry class every element has a charge positive or negative.  The nutrients are charged as well.  The soil can only hold on to a certain amount of a nutrient based on this charge.  We have to consider this before applying any fertilizer as well.  It's a little more complicated than just "dousing" our fields with fertilizer!  

This example field had 43 samples taken from it.  This tells us what is going on in the field in 43 different areas.  Each area will have a unique recommendation for nutrients and lime.  I would call this a very informed management decision tool for farmers.  This matrix is used to develop a prescription for the field for potash, phosphate fertilizers and lime.  We aren't just making a wild guess when it comes to how much fertilizer to apply to our fields!

                                 

A yield goal is established for the crop that will be planted in the field and computer software is used to correlate the GPS coordinate of each sample to generate a nutrient and lime prescription for the field to supply the needs of the future crop.  Then it's time to upload the recommendation as a prescription into the lime or fertilizer truck to spread the field.

The areas in red have a lower pH or nutrient holding capacity.  The red areas will receive more lime.  The yellow and green areas we have a smaller rate applied.  The areas in grey will actually be skipped over and no lime applied.  This process is repeated for Potash and Phosphate fertilizer applications.  It makes sense to apply the fertilizers and lime where they are needed in an appropriate amount instead of putting a flat rate across a field.  This process helps us manage our fertility in each area of a field or soil type instead of assuming all parts of a field should be treated the same.  The spreader truck uses GPS to make the precision application of lime to the field based on the prescription map and it's GPS coordinates for each management zone.

We hired Crop Production Services to spread the lime in this field in Robertson County, TN.  The spreader truck was operated by our neighbor and friend Justin Scharcklett an employee of CPS.  So the next time you see a fertilizer truck in a farmer's field, remember there were quite a few steps involved in deciding what nutrients and amounts to apply!