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What is your approach to soil sampling? How often you should sample?

Many times, through the years, I have explained the history of soil testing in agriculture and its evolution as an approved farming practice. I usually begin by asking the question, “When did soil testing as we know it today, first begin?” Followed by, “In the beginning, what did farmers generally test for?” And finally, “What do we typically test for today?”

History of Soil Testing
Well, let’s start out with a bit of a history lesson. Soil testing can be traced back to early 1920s and, to some extent, earlier. However, advance of the practice and use of soil testing information to make decisions really began to pick up speed in the mid-1940s. At this time in history, World War II was ending, and American GIs were coming home, many of them returning to farms, having acquired advanced technological knowledge during their service.

Land grant universities began developing information to guide the growing number of farmers across the country. This was necessary because the vast number farmers made one-on-one guidance impractical. At this time, each county generally had one “county agent,” who could not possibly remain in contact with each individual farmer. Therefore, agents compiled information, so farmers could help themselves as they desired. Much of the information was very general in nature and was meant to provide overall guidance rather than exact hard-and-fast rules.

At the time, the popular advice was to test soil once every four years. Four was never a magical number; it simply made sense logistically and economically. At the time, corn was becoming a major crop, and it was generally grown once every four years in the crop rotation. Testing every four years allowed farmers to maximize corn yields. Because there were very few testing facilities at the time, more frequent sampling would not have been practical. As a result, testing soil every four years became a minimum standard, and it continues to be the practice today.

Data Collected
In the beginning, soils were tested for phosphorus, potassium, pH and organic matter. Testing methods have been improved over time, but often, these are still the only four variables that are measured. However, it’s important to ask whether valuable information can be gained by taking advantage of some of the technological advances that soil testing has seen over the past several decades. The better you understand the current nutrient levels and other indicators of soil health, the better equipped you are to improve it.

Soil Testing Frequency
How do you decide how often you should collect soil data? That depends on how precisely you wish to manage your fertility inputs and eliminate guesswork. The fresher and more consistent your soil data collection, the better informed your decision making will be.

Today, with tighter margins across agriculture and the affordability of soil testing technology, frequent soil sampling is an investment you can’t afford not to make. Consider sampling once every four years as the minimum standard and annual sampling as the maximum. Annually collected information will enable more precise decision making, while less frequent data collection injects a degree of guesswork into the process. How much guessing can you afford?