The Right Time for Soil and Tissue Testing

The right time to take soil samples is in rhythm with the crop rotation. Normally it’s best to sample following back-to-back plantings of the same crop, which creates a consistent basis for comparing fields and picking out trends over time. Most samples are taken in late summer and fall to allow ample time for planning a crop nutrition program based on the 4Rs of Nutrient Stewardship — applying the right nutrient source, at the right rate, time and place. But, in a drought, is fall sampling still a good idea? Yes. And the facts support it.

Severe drought reduces the crop’s uptake and removal of nutrients. The effect on the soil test isn’t likely to be large. Leftover nutrients from a typical corn crop might increase soil test levels by 3 to 5 parts per million (ppm) for phosphorus (P) and potassium (K), assuming the worst-case scenario with nothing harvested from the field. If the drought cuts yields in half, the increase is likely to be smaller. And leftover nutrients that show up in the soil test are likely to be available to the next year’s crops.

Of course, nutrients can also change chemical forms when the soil dries out, but it can be hard to predict whether their availability will increase or decrease. Generally, K increases as a soil dries, so we expect to see higher levels in samples taken during a drought. Most laboratories dry all their samples before testing, but some use a field-moist sample for K analysis. The drought difference is likely to be larger for the latter. In general, however, a recent sample affected by drought is a better basis for next year’s crop nutrition program than a sample older than the typically recommended sampling interval of three or four years.

Are soils sampled often enough? It seems, on average, that most cropland is sampled about as often as agronomists recommend. In the Northeast U.S., including states from the watersheds of the Chesapeake Bay and the Great Lakes, the number of samples represented in the 2010 IPNI Soil Test Summary ( amounted to one for every 45 acres of census cropland. That’s more intensive than generally recommended — one sample from every 25 acres every three years translates into one sample from every 75 acres each year. But included in this average are the more intensive sampling schemes of many practitioners of precision agriculture (with each sample representing as little as an acre). So while most of the cropland gets sampled often enough, some might be under-tested.

What’s more important is what the soil test shows. While the distribution of soil testing below, within and above the optimum range varies by region, most states in the Northeast and most provinces in Eastern Canada still show a substantial portion of soils testing below and above the optimum range for both P and K. Obviously, this is not because the soils aren’t being sampled. Sub-optimal soil test levels persist partly because some growers don’t follow the application recommendations.

Sampling more frequently than once every three years will not change these soil test levels. Following the soil test-based recommendation moves most soils into the optimum ranges for both P and K, and keeps them there. Sampling soils more often than once every three years won’t reduce losses of P in runoff, either.

Soils testing above the optimum range may increase risk of P loss to some degree. Fertilized at rates below crop removal, such soils will decline in P. Sampling once every three years will suffice to track and prevent a decline below the optimum range.

4R Nutrient Stewardship also encourages the tracking of crop nutrient balances. Recommendations from a soil test often relate to crop removal: The basic recommendation is to apply more or less than crop removal for soils testing below or above, respectively, the optimum or maintenance range. Further, uncertainty always exists with both soil testing and with nutrient removal information. Doing both increases confidence, because the uncertainties in each tend to cancel each other out.

Source: Dr. Tom Bruulsema, Northeast Director, International Plant Nutrition Institute

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