By by Madeleine Baerg
Conducting regular soil tests is one of the simplest, fastest and least expensive ways to optimize one’s fertilizer program and maximize crop yield. Yet many producers still underuse this vital tool.
Some farmers believe their experience and knowledge of their fields offset the need for the test; others are unclear about how to read and analyze results. And for others, the subject may simply not be glamorous enough to draw their attention. In an industry jumping leaps and bounds forward in technology, soil testing remains simple, humble and far from sexy. The reality, however, is that soil testing – be it composite, zone or grid sampling – should be considered a vital tool for every producer.
“It can be a challenge to get producers to think about soil testing,” says Jake Munroe, soil fertility specialist for field crops with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).
“It’s not a subject that people usually get really excited about, but it’s a really important one. Soil test results can guide decision making and make it more likely that a farmer will achieve greater returns, so for those not soil testing regularly, there is definitely a missed opportunity.”
Most farmers still opt for a single composite test, in which samples taken from a minimum of 20 cores from the field are combined and analyzed as one. Depending on one’s field, field knowledge, and preference, composite samples can be conducted entirely randomly or in a more directed manner, prioritizing average producing areas of the field.
A composite sample provides a single snapshot of the average nutrient profile of the field. Though it is far better to base fertilizer rate decisions on composite results than estimation alone, in-field variation will skew the results. Fertilizer applied based on composite results will improve but not optimize in-field fertility.
Producers looking for more in-depth information and a way to better optimize a fertilizer management program should consider zone sampling or grid sampling. Each of these more intensive sampling techniques are intended to assess the specific nutrient needs of different parts of the field.
In zone sampling, growers divide fields into management zones that have similar yield potential and then separately analyze samples from each area. Zones are typically identified by two or more pieces of data or history (topography, slope position, soil type and/or chemistry, microclimate, multi-year historical yield, soil conductivity, etc.). Though zone sampling sounds relatively simple, the reality is that identifying management zones can prove challenging.
Some producers attempt to create zone maps based simply on their memory of historical crop growth variation and visual field variations such as slope or moisture. For better accuracy, zone maps should be drawn with the help of a precision agriculture computer tool designed to accurately layer multiple pieces of data. Currently, OMAFRA and the Grain Farmers of Ontario are collaborating on an in-depth precision agriculture research project that will include the development of a free zone mapping tool. Zones tend to be relatively stable and permanent. As such, investing in developing accurate zone maps can pay dividends in the long run, so long as a producer has the technology and the desire to manage those zones separately.
Producers interested in going a big step further towards full variable rate fertilizing should grid sample, or separately analyze individual soil samples from a large number of carefully plotted locations in a field. Whereas breaking a field into management zones allows a producer to separately manage multiple areas of a field, grid sampling provides one level of data for the framework required to build a prescription map. Once a precision map is finalized, fertilizer application can be fine-tuned to every minute change in a field. Which soil sampling technique is right for an individual producer depends on how they farm and how much they want to spend, Munroe says.
“There can certainly be value in taking things to the next level by sampling more intensively. But, if you’re treating the whole field as a composite and applying fertilizer at the same rate across the field, don’t put the money into grid sampling if you’re not going to act on the results.”
Ontario fields should be sampled at least every three years. Growers of alfalfa, silage corn and other crops that are particularly nutrient-depleting should test annually, especially in sandy soils.
Ideally, growers should sample just after thaw in the spring, since significant nutrient losses through leaching and runoff can occur over the winter. That said, most growers find it more practical to find time for soil sampling in fall or late summer. Regardless of when you choose to sample, be consistent about your timing from year to year in order to ensure results are comparable.
At the very least, have your samples analyzed for soil pH and macronutrients (phosphorus, calcium, magnesium, potassium). Though Ontario soils tend to be relatively high in magnesium and calcium, there has been a trend of dropping levels for both phosphorus and potassium in recent years. Consider also testing for micronutrients (copper, manganese, zinc and boron), especially zinc and manganese. While soil tests are not as well calibrated for micronutrients as they are for macronutrients, micronutrient levels can play an important role in cash crop yields and so should not be ignored. Sample for organic matter too, in order to get a good feel for the biological activity, structure and overall health of the soil.
Completing sampling and sending soil away for analysis are necessary preliminaries. The real work – and the real reward – occurs once analysis reports are returned to the grower.
“Some growers are very competent in interpreting the analysis reports, and are very able to create detailed plans based on that interpretation. Others are less comfortable in making sense of the numbers. Fortunately, there are a variety of resources that can help,” Munroe says.
“Agronomists and certified crop advisors also play a very important role in interpreting soil test results and making fertility recommendations.”
OMAFRA offers a variety of resources online including the Soil Fertility Handbook and the Agronomy Guide for Field Crops to assist growers in interpreting soil test results.
“A great starting point for Ontario producers is to look up the specific crop they plan to grow in our Agronomy Guide. Then, look at how their soil test results match up against our fertilizer guideline tables. That is an excellent first step to figuring out your nutrition plan for the coming year,” Munroe says.
The bottom line is clear: get out there and soil test. The more you know, the better you’ll grow.