Did the soil change, or did the laboratory?

A lower phosphorus number on this year’s soil test can send a farmer to buy fertilizer the field does not need. In an example, a farmer with 800 acres of cotton and soybeans in West Tennessee sees phosphorus fall from 40 to 30 parts per million (ppm). The farmer spreads 20 pounds of phosphate fertilizer, measured as P2O5, on every acre. That is 16,000 pounds at $0.65 per pound, or $10,400. Copy each report onto one page per field and draw a line wherever the laboratory, test, unit, depth or month changed, before anyone sets a rate. All values in this example are invented.

The soil did not change. Last year’s 40 ppm came from the Mehlich-3 test and this year’s 30 ppm from Bray P1, two ways a laboratory pulls phosphorus out of a soil sample. In an Ohio State study of 2,323 samples from Ohio and Indiana, Mehlich-3 extracted 35 percent more phosphorus than Bray P1 in the agronomic range under 50 ppm. Thirty ppm on Bray P1 times 1.35 is about 40 ppm on Mehlich-3, the number the farm had last year.

Which fields have a current test, and how old is it?

You cannot answer until you list every field with the date of its last sampling. The reports are filed, but the list rarely exists, and asking from memory misses a field. In the example, the 120 rented acres that joined the farm two seasons ago were never sampled, because the tenant thought the owner had a report and the owner thought the tenant would collect. A field split in two when the crop changed is still sampled as one.

No single interval fits every field. University of Massachusetts Extension advises sampling every 2 to 3 years and keeping a record of the results for each field, under a permanent name or number for the field. The interval for your farm is a choice you make with your agronomist and write at the top of the list, where every date can be read against it.

The interval also sets how long a change in fertility goes unseen. A change can start on any day between two tests, so it stays unseen, on average, for half the interval.

The same count, in days, appears in how long it takes you to find out that something went wrong. Write the list today: one line per field, with its name, the date of its last test and the years since.

Why does the same soil give two numbers?

A soil test is two things at once: the solution that pulls the nutrient out of the sample, called the extractant, and the way the laboratory then measures what came out. Iowa State agronomists write that both define the test, so a laboratory should say clearly which method it used.

Ohio State Extension notes that most laboratories use Mehlich-3 as the default extractant and tells farmers to multiply a Bray P1 phosphorus value by 1.35 to put it on the Mehlich-3 scale. That 1.35 is the 35 percent gap from the opening: the same soil gives 30 ppm one way and about 40 ppm the other.

Do not apply the 1.35 yourself. It was fitted for Mehlich-3 read on an ICP instrument (an inductively coupled plasma spectrometer) and Bray read by color, and the relationship changes if either one is measured another way. Some laboratories report Mehlich-3 without saying which of the two they used. Others measure it by ICP and report it as the color-based value with a factor of their own. For potassium, no simple factor turns a result on dried soil into one on field-moist soil.

Your part is to copy the name of the test beside each result. When a report leaves it out, Ohio State’s advice is to contact the laboratory to understand the values in its report. Which test to use, and what the number then means, is the work of the agronomist who signs the recommendation.

Do I copy the number or the rating (low, medium, high)?

Copy the number. The rating is the laboratory’s own scale. University of Minnesota Extension writes that a laboratory can use any range of values it chooses, and that the difference between laboratories is one source of confusion when results from more than one are compared. The same guide adds that the number is an index value, not the amount of the nutrient available or present in the soil.

The Minnesota table shows how far two scales can sit apart. For phosphorus, 12 ppm is medium on the scale for the Bray and Mehlich III tests and high on the scale for the Olsen test.

A report from another laboratory can print a different rating for the same number. Copied with its unit and its test, the number can still be compared next year. The rating cannot.

What else must match from one year to the next?

The unit, the depth, the month and the laboratory. Laboratories report in two units, and Minnesota Extension gives the conversion: ppm times 2 gives pounds per acre. Leave the unit off and one sample reads 30 on one report and 60 on the other. Copy the unit as printed, in the same row as its number, and do not convert it to the one you know.

The depth is part of the number. The U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS) asks for soil health testing that soil be sampled at the same depth, 0 to 6 inches (0 to 15 cm), at each collection. UMass advises sampling to the tillage depth, the upper 6 to 8 inches for most crops and only 4 inches for perennial pasture. A pasture page beside a cropped-field page, without the depth on both, compares two layers of soil.

The NRCS asks for the same time of year at each collection and for the same laboratory over time, because laboratories often have different instruments or protocols that affect the results. Copy the sampling date with its month, not only the year, so a difference between two years can be traced to the soil or to the calendar.

What do I copy first, and who signs the page?

Open the most recent report for the field that someone asked about last, and copy the six items below. Each answers a question that someone will ask later, and a blank item leaves that question open.

When the field has earlier reports, copy each below the last as a new block with its date, and draw a line where the laboratory, test, unit, depth or month changed. In the example, the line falls between 2025 and 2026 because the laboratory and the test both changed, and the two phosphorus numbers sit on opposite sides of it.

The person who read the report signs the foot of the page with the date and notes which recommendation the report backed. Writing the conditions beside the measurement is the same habit as a dated compaction check. A page like this for every field is a record someone other than you can find and read, which is what running a farm as a business rests on. The production section of this library holds the rest of the production records.

• Open the most recent report for one field and write the field’s name at the top of a blank page
• Copy the sampling date and the depth from the top of the report, not from memory
• Copy the laboratory’s name as it is written, with the branch if there is one
• Copy the results row by row, each number with its unit exactly as printed, and leave out the low, medium or high rating beside it
• Copy the extractant or method printed for each result into the same row as that result
• If this field has earlier reports, copy each below as a new block of rows with its date, and draw a line between two blocks where the laboratory, extractant, unit, depth or month changed
• Leave blank any item the report does not carry, and write beside each blank who will call the laboratory and on what date
• Sign the foot of the page with your name and today’s date, note which recommendation this report backed, and file it with the field’s documents

Leave a blank as a blank. A number written from memory looks like an answer and cannot be checked, while a blank with a name and a date beside it is a call someone has to make.