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Nutrient mobilizer and nutrient stabilizer? What do they mean and what do they pertain to?

They're the differing traits and effects of more than a dozen cover crops that Dan Olson has been tracking in test plots on his family's farm near Lena in Oconto County. In addition to carrying out plot studies on the dairy and crop farm for the past six years, he is a product support specialist for Byron Seeds.

Cover crop comparisons

In a presentation at the 2016 annual meeting of the Outagamie County Forage Council, Olson outlined the differences that were observed in the 2015 corn crop that was grown where cover crops were planted in August 2014 with an infusion of an equivalent of 50 pounds of nitrogen per acre. At planting, the corn also received 20 pounds of nitrogen as a starter fertilizer.

The cover crops included oats, sudangrass, winter peas, buckwheat, annual rye, vetch and such nitrogen suppliers as Mammoth clover, Nitro radish and turnips, Olson said. Those latter four also rated well for available potassium and calcium for the subsequent corn crop.

Among the other possibilities for cover crop choices are warm season species such teff grass, millet, grain and forage sorghum and short season or grazing corn along with cool season rye grasses, small grains and both warm and cool season broadleaf species, he said.

Olson is convinced that there is an overall link to the activity of soil bacteria that differs according to what cover crop species are grown. He compares the phenomenon to how the rumen bacteria perform in ruminant animals, depending on what feedstuffs they are provided.

Nutrient differentiations

In the terminology that Olson employed, the 'nutrient stabilizer' cover crops proved to be oats, rye grasses, triticale, and sorghum while the 'nutrient mobilizer' group included radish, rapeseed, turnips, alfalfa and peas. Grouped another way, he said they could viewed as having significant fiber in the foliage of the stabilizers and low fiber in the mobilizers.

As a crop management approach, Olson suggests growing the mobilizer type species in soils with high organic content, at times of high surface residue such as from crop debris or bed pack manure, at times of cash flow challenge on the farm or when higher yields are sought. He described the mobilizer effects as short term.

The stabilizer types are most suitable in cases of low soil organic matter, minimal crop residue and droughty or sandy soil, Olson indicated. He explained that the stabilizers have long term effects such as adding fiber to strengthen the soil and to hold more water. In the long term, fiber and carbon become synonymous.

While the mobilizers tend to bring quick effects on production of following crops, expect to 'take a short-term hit' with the stabilizers similar to what often happens during the early years of conversion to no-till, Olson advised.

As an overall strategy, Olson suggests catering one's species choices to the soil types, the previous and subsequent crops, and, because of the combination of agronomic benefits, growing a cocktail mix of species rather than restricting the choices to those can be assigned to one of the two general trait groups.

Success with cover crops can be judged by the effects on organic matter and carbon in the soil, the soil tilth, water-holding capacity, soil compaction and higher crop yields. He noted that because grasses will add organic matter to the soil while alfalfa will not, farmers should consider growing them together.

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