The author is a soil scientist with the USDA Agricultural Research Service in Raleigh, N.C.

Is it safe to assume that we all look forward to a bit of spring green up? While writing this story at the beginning of winter, it is difficult to know if it will be a typical winter with freezing overnight temperatures or if one of those unusually warm periods occupies some or all of January and February again this year.

In either case, spring green up gives me an opportunity to describe some peculiarities between soil organic matter and nitrogen. Soils with low soil organic matter concentration can limit the supply of nitrogen to plants. If soil is low in organic matter, then external nitrogen inputs would likely be needed from either synthetic nitrogen (like urea, urea ammonium nitrate, or ammonium sulfate) or organic nitrogen sources such as composted yard waste, poultry litter, or dairy effluent.

These external nitrogen sources would be necessary to meet the high demands and ability of forages to grow rapidly when exposed to abundant sunshine and precipitation in the spring. Without sufficient nitrogen, forages may grow slowly and not accumulate much biomass. They may also lack sufficient green color, which can be indicative of protein deficiency.

In general, soils with high soil organic matter concentration can supply a robust quantity of nitrogen to forages over the course of a growing season or even on an annual basis. This statement assumes some conditions are met, though.

Decomposition dependent

For nitrogen to be released from soil organic matter, soil microorganisms must be actively decomposing organic substrates. These tiny organisms that are classified as innumerous species of bacteria, fungi, archea, and actinomycetes require a microscope to be seen. They can be found in soil, on plants, and in animals all around the world, yet they are most abundant wherever organic resources are present. Although we may consider some of these organisms pests, parasites, or disease-causing (as some are), the vast majority of soil microorganisms are beneficial, and many even directly assist plants in their development.

The conditions that favor soil microbial activity are warm temperatures and soil pores with a desirable balance of water and air. When overnight freezing temperatures give way to mid-morning warmth after some winter snow or rains have occurred the week prior, cool-season forages are activated, and so are soil microorganisms. But what if newly emerging grasses or abundant residual forage mass cover the ground completely and the warmth of the sun doesn’t penetrate into the soil? Or what if the days are sunny but the soil doesn’t break free of its frozen condition?

These early spring growing conditions can limit the activity of soil microorganisms feeding on the abundant resources of soil organic matter present in a well-managed pasture, which may give rise to the notion that all pastures need to be fertilized in the spring to make sufficient growth for grazing livestock or making hay.

Sufficient time is necessary for soil microorganisms to consume soil organic matter and excrete enough nitrogen to feed a robust stand of forage. Therefore, if time is needed, then patience will also be needed. Even more time may be needed to accumulate sufficient biomass for grazing livestock on highly enriched surface soil that is full of organic nitrogen developed from a long history of good grazing management. If you were to wait another week or two, the growing forage will almost assuredly show you that nitrogen was being released from organic matter into the plant-available inorganic forms of ammonium and nitrate that are rapidly absorbed by the actively growing roots of the forage sward.

Reassess the application

When you don’t have the patience to wait for nature to keep its balance, applying readily available inorganic nitrogen to an emerging forage sward can jumpstart the stand and allow for an earlier stocking time or hay cutting. Indeed, nitrogen is needed by growing forages. The issue becomes when and how much nitrogen is supplied either by nature or by you, the land manager.

To illustrate the point of nitrogen availability from soil organic matter, consider the results from a trial on a farm in Haywood County, N.C. Surface soil samples collected in September indicated about 50 pounds of inorganic nitrogen per acre and 300 pounds of potentially mineralizable nitrogen per acre with a high level of biological activity shown on the soil test.

Fall stockpiled tall fescue mixed with other cool-season forages did not produce enough additional forage with urea application at the end of summer to justify its application compared to forage with no nitrogen fertilizer. With that said, plots with urea applied did produce forage with a statistically greater crude protein (CP) concentration, which was 14.7% CP versus 13.5% CP without amendment.

When plots were harvested again six weeks later, there was an additional 1,400 pounds of forage per acre. With the original fall nitrogen application, forage yield in the second harvest was, again, not statistically different from forage yield without nitrogen applied. Crude protein was still greater with urea (21.9%) than without nitrogen applied (20.6%).

Was external nitrogen needed? If CP was the indicator, then the answer might have been yes. If forage dry matter yield was the indicator, then the answer should have been no. Relying on the supply of nitrogen from organic matter mineralization is possible. However, it might be important to know how much nitrogen can be released from soil organic matter over time and if that is within your comfort zone. •