Hoard's Dairyman article: Four million soil samples reveal fertility shifts

Four million soil samples reveal fertility shifts

by Tom Bruulsema
The author is the northeast director for the International Plant Nutrition Institute.

Soil fertility rises and falls in response to crop removal rates. At the same time, nutrient surpluses raise soil test levels, while deficits draw them down. It’s not always easy to predict how much, or what the consequences will be, so it’s important to monitor both soil fertility and crop nutrient balances as closely as possible.

The International Plant Nutrition Institute recently completed a survey of the public and private soil test laboratories of North America, similar to surveys done every four to five years for the past several decades by the Potash & Phosphate Institute. There are numerous challenges to conducting such surveys, since soil test methods and interpretations vary among states and provinces and change over time, as well. Nevertheless, important and consequential trends are showing up.

One million more samples
The 2010 survey included more samples than any previous survey. An estimated 4.4 million soil samples were submitted across North America for this survey compared to about 3.4 million five years earlier. The reason for the significant jump in soil sampling by producers is likely twofold:

There have been higher and more rapidly fluctuating prices for fertilizers and crop commodities in recent years.

There’s a great increase in nutrient management plans for livestock producers who apply manure on fields.

In eastern Canada and the northeastern United States, the soil fertility shifts varied. In many areas, soil test levels for K (potassium) have moved downward since the last soil summary. For example, in Ontario, the proportion of soils testing 80 ppm or less in K grew from 15 percent in 2005 to 20 percent in 2010. Soils testing in this range are likely to produce K deficiencies in almost any crop in the absence of fertilization. This trend is not surprising, considering that the amount of K applied to Ontario cropland in the form of fertilizer and manure was only about half that removed by crops in 2009.

However, elsewhere the shifts varied in size and direction. In Pennsylvania, the distribution of soil test K hardly changed at all, while in New York and Virginia, it appears to have shifted upwards.

Soil test P (phosphorus) levels often fall into two groups. A substantial proportion are in the responsive range, but another large proportion are at levels far above the critical level for crop response. The very high levels result from many years of historical nutrient surpluses. Such soils need to be managed in ways that eliminate the surplus, maximize utilization of the P fertility for the benefit of crop production, and minimize surface runoff and erosion to protect water quality. The frequency of very high soil P tests continued to decline in Ontario but rose in New York, New England, and Pennsylvania.

The soils of the region remain quite variable in fertility. Even in states and provinces with overall nutrient surpluses, many soils needing nutrient additions can be found. On the other hand, many soils have built up fertility to the point where inputs of P and K amounting to less than crop removal of the nutrient can continue for years. Of course, in such situations, it would be important to monitor the decline with regular soil testing.

So, nutrient decisions need to be supported not only by crop nutrient balances, and not only by soil tests, but by both. Using the two tools, you can manage nutrients sustainably.

More detailed information on these changing nutrient balances and soil test levels can be found at this site: http://nane.ipni.net/articles/NANE0047-EN.

This is a complete reproduction of the article in the February 10, 2011 issue of Hoard's Dairyman, on page 95.