Fine-tuning your fertilizer budget
We’re in the midst of harvest after another challenging year. Am I off base thinking it seems like our weather has been more erratic the last few seasons? Let’s hope it cooperates so we can wrap up harvest and get some fall fieldwork done before a brutal winter (according to the Old Farmer’s Almanac).
While a fair amount of soil sampling is done and some fall fertilizer spreading has started, most dry fertilizer applications are waiting for harvest to end. As growers look at yields, low prices and tight budgets, we are getting a lot of questions about trimming back fertilizer and lime costs for 2016.
Here are some common questions we’re getting. My answers contain highlights from ISU’s PM-1688, “A General Guide for Crop Nutrient and Limestone Recommendations in Iowa” and PM-0287 “Take a Good Soil Sample to Help Make Good Decisions.” The publications are at .
How often should you sample soil and have it tested for P, K and pH? Sample fields every two to four years for most crops, or once in a crop rotation. Fields should be in the same crop each time when sampled to reduce variability of results. Once you have a comprehensive soil fertility map for a field, resample the entire field following the same pattern, or sample only representative portions of each field and problem areas every two to four years as a check on the fertilizer program.
My personal bias is to keep sampling intervals closer to two years for soils that test toward the lower end of the “optimum” ranges or below. Once levels are in the middle of the optimum ranges or higher, then I feel comfortable stretching out sampling intervals to three to four years.
What time of year is best to sample? The best time is either after harvest and before fall fertilization, or before spring fertilization. We prefer not to sample shortly after a lime, fertilizer or manure application, or when soil is excessively wet. Sampling at other times such as in winter or in a growing crop is discouraged for determining pH, P and K because interpreting results are difficult. Field research calibrations for these soil tests are based on samples collected in fall or spring.
How deep should you pull samples? For most of my career, nearly all soil samples were pulled by agronomists like me at retail locations or by independent crop consultants. Today, more growers like to take some or all of their samples. They have the GPS equipment, ATVs and computer skills to do the job and write their own prescriptions. Six inches is the sample depth we use for all tillage systems in sampling for P, K, Zn, organic matter, pH and buffer pH. For no-till acres, we like growers to monitor surface pH by taking 2- to 3-inch samples in areas across fields.
Something to watch for in most soils, and particularly in no-till systems where soil tilth tends to be exceptional, is “soft” soils that compact as they go in the probe. Take a look at the 6 inches of soil in your probe and then drop a dowel in the sample hole to compare depth. Often we end up going too deep; adjust accordingly.
We always test for P, K and pH. What about testing soil for micronutrients in Iowa? ISU provides micronutrient recommendations only for zinc based on soil testing. Soil test procedures for the other micronutrients haven’t been calibrated because of either lack of or inconsistent occurrence of deficiencies. With research and soil analysis technology continually advancing, and yields (and corresponding micronutrient use) increasing, we could see this changing in the future.
Occasionally, we hear about some sort of cutting-edge testing technology for micronutrients. For now, given the tight crop budgets, I’d have a hard time being convinced I should try some untested new sampling technique for micronutrients no matter how compelling the sales pitch.
What should you look for on the soil test report you get back from the lab? This is where ISU publication PM 1688 really shines; it walks readers through what the results mean and what steps to take in understandable terms so I won’t dive into this question much deeper. Your agronomist is a great resource to sit down with you and go through soil test results.
Is starter fertilizer part of your program? If it is, be sure to take the P and K credit from that into account.
Do you have the farm year to year, or on a longer basis? I could spend paragraphs on this, but the bottom line is: Take a good look at the soil samples, crop yields and how long you’ll be farming that land. If you’re confident you’re still in the optimum ranges for 2016 and won’t be farming that ground for long, skipping P or K could be a chance to trim costs safely.
If you have this land for the long term, the strategy may shift slightly. Trimming fertilizer application rates rather than skipping is something to consider. If you skip P or K, limit risks next fall by soil sampling to ensure you stay in the optimum categories for P and K.
While you build on your understanding of soil testing and fertility, it’s also a great opportunity for your agronomist to learn more about your management and decision-making style. I’m a proponent of building trust and capacity between growers and advisers; creating a team mentality benefits both operations.
This article published in the October, 2015 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2015.