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67 cents an acre

Eric Young for Progressive Forage Published on 30 October 2020
New growth

When it comes to efficiency, it’s tough to beat soil testing. What else can you do for less than a dollar an acre that has such a large potential impact on crop yield, quality and farm profitability?

Properly managed forage crops are the basis of high-quality feed, and soil testing is an integral part of optimizing both forage yield and quality.

Since soils naturally vary in drainage, texture, pH, biological activity and other important properties affecting crop growth, soil testing helps reveal how these interact with cropping history and soil management to influence soil fertility.

Other crop input costs (fertilizer, fuel, labor, pesticides, seed and labor) dwarf the cost of soil testing. A basic agronomic soil test in the U.S. costs from $10 to $20 per sample depending on the laboratory and desired analysis. Soil samples taken once every three years at $20 per sample amounts to an average cost of 67 cents per acre per year.

The best way to appreciate the power of soil testing is by example. Consider fields receiving annual manure applications (usually ones closer to the barn with easy spreader access). Without soil testing, it could be tempting to apply fertilizer to these fields at the same rate lower-fertility fields on the farm receive. However, fields with a substantial manure history rarely require phosphorus (P) or potassium (K) and also have lower nitrogen (N) needs than fields getting fertilizer only.

Let’s assume the hypothetical field close to the barn gets 200 pounds per acre of 20-20-20 before a corn crop and another 100 pounds per acre of 10-10-10 as a starter at planting. At $400 per ton, that’s around $600 of fertilizer that may not have been necessary.

What’s the take-home? Don’t guess; soil test. Guessing is too expensive with yield penalties for insufficient nutrient application and economic losses where applied in excess. The law of diminishing returns says there’s a sweet spot where profitability is optimized for both crop and livestock nutrition; nutrients applied beyond this is money wasted and an environmental risk.

Taking samples correctly is important. Land-grant universities typically recommend that individual soil samples represent no more than 10 to 20 acres of field area. This makes sense because as fields get larger, so does the soil variability reflected in the sample collected. Improper sampling introduces considerable error and can result in meaningless fertilizer recommendations.

For conventional tillage, a standard agronomic soil sample should consist of 10 to 15 cores taken to an 8-inch depth (collected randomly while avoiding known outlier areas, i.e., wet spots, drainage ways or areas not treated the same as the rest of the field in the past). Farms using variable-rate fertilizer application take samples randomly within systematic grids, generally ranging from 1 to 10 acres depending on objectives.

Crop nutrient recommendations derived from soil testing should be based on replicated research relating specific nutrient levels to measured crop yield in differing soils, so the likelihood of a crop response from added nutrients can be gauged appropriately (termed “soil calibration”). Soil calibration research is the basis for all land-grant university recommendations.

Effective testing pays whether a crop needs fertilizer or not. For example, if a field has a low pH (less than 6), P availability is generally much lower (unless the field has a long manure or fertilizer application history above crop needs). Without knowing of the low pH and continuing to purchase and apply fertilizer P, much of the P is rendered unavailable, while fertilizer bills remain high.

Worse yet, soil test P can remain low despite multiple P applications until the pH is raised by lime addition. In this situation, not testing is very costly since P still limits yield while the applied P is ineffective at increasing soil P levels. Had the pH of the same field been closer to neutral (pH = 7.0) prior to P application, much less P would have been rendered unavailable with correspondingly higher soil test P concentrations (largely due to less P removal from the soil solution by aluminum, iron and manganese oxides).

In fact, soil test P and K concentrations are often well above optimum in fields with a significant manure history, so that no purchased P or K is needed for optimum yield. Not knowing these situations is just as costly as being unaware of fields in need of nutrients, lime or both.

Legumes need a pH close to neutral for optimum health and yield. Whereas corn and grasses can tolerate lower pH levels, as pH continues to drop, other inefficiencies develop including suboptimal microbial functioning (including nitrogen fixers and bacteria converting organic N to plant-available N). The importance of maintaining optimum pH for the specific crops grown should not be underestimated and is money well spent.

While confined animal feeding operation (CAFO) farms are required by law to soil test fields every three years, smaller farms may or may not soil test at all. Hopefully, this article will help convince those who have not soil tested or only intermittently tested to give it another try. There is not much to lose and a lot to gain, particularly in today’s challenging economic conditions. The bottom line is: Soil testing is time and money well spent regardless of farm size or type. Where else can you spend so little on crop production and profitability and have such a large potential impact?  end mark

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Eric Young
  • Eric Young

  • Research Soil Scientist
  • USDA-ARS, Dairy Forage Research Center
  • Email Eric Young