Annual soil sampling: Make it a habit

Written by Cassandra Sondershausen on Aug 07 2019

Category: Grow Grain

The power is in the savings

Fertilizer is arguably your most important input. So it’s imperative to get soil fertility right. The only way to know if you’re doing the right thing is to take soil samples and have them tested. Otherwise, even as you see the results (or lack thereof) in your crop, you’re still guessing.

Guess no more. Instead, plan now to get soil samples in the fall and be ready to fertilize to each crop’s needs.

If you’re still hesitating, let’s address the potential obstacles. I realize that for the past several years, fall conditions have made it extra challenging to schedule soil sampling. Don’t wait to fit it in. The best way to deal with an uncertain fall is to book sampling ahead of time to avoid trying to squeeze it in under a time crunch.

We recommend fall sampling – or at least sampling at the same time each year – to get a true understanding of what nutrients you’re working with. Spring presents the further complication of mucky fields that are too wet to enter up against the deadline of seeding. There’s no need to take soil samples in-season because plant tissue tests will tell you what you need to know for a potential foliar nutrient application.

If the expense of soil sampling is your concern, let’s look at how that might play out. I advise farmers to test at least half their farm each year to stay on top of fertility planning. If you’re looking at a 5,000 acre farm, at $1 per acre that’s a $2,500 investment.

We’ve been seeing plenty of residual nitrogen left in the soil over the past several years, and that could change the amount of additional N required to feed subsequent crops. Understanding your residual N, organic matter, and other nutrient levels could help you realize a significant savings.

Here’s a hypothetical example:

Soil samples taken between 0 and 24 inches deep show 60lb/ac nitrate and 4% organic matter. The farmer is targeting 80 bushel/ac wheat, which requires 184 lb N uptake per acre. With 60 lb already in the soil, and a contribution of 20 extra lbs from the organic matter. 

With approximately 80 lb residual N, the farmer only needs to apply about 104 lb N to target the same 80 bushels. If the farmer is uncomfortable reducing N application that far, 120 lb N applied can still represent significant savings that can more than take up for the initial investment in soil sampling and testing. 

As a 4R certified agronomist, I know that 4R practices (right fertilizer source, right rate, right time, right place) like soil sampling help you understand what amount of any given nutrient is lacking in a field. This allows you to tailor your fertilizer application to the needs of the next crop, focusing on uptake (right source, right rate) and not depleting the soil. Applying the same blend across the farm just doesn’t make sense, economically or environmentally. Instead, apply a custom blend per field or at least per crop.

One final note about fertilizer cost: history has shown fertilizer is cheaper in the fall, or during mid-late summer. In fall 2018 urea was selling at approximately $420/tonne, but this March it was selling at $550.

If you have questions about a fertilizer plan for your farm or you’d like to book soil sampling for this fall, call your local Cargill agronomist.

Tags: Soil, Soil sampling, Fertilizer, micronutrients, 4R

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