Over the past few years, we have seen an increase in crop deficiencies for the big three (macro) nutrients-nitrogen, phosphorus and potassium. Reasons for those deficiencies have been a combination of high replacement costs and excellent crop yields that have increased nutrient removal. We have been pulling nutrients from the soil fertility bank account that took decades to build. That account is now depleted to the point that crop yields are suffering. Now that prices have stabilized, we are poised to reinvest in the temporarily stressed soil bank account.

As we shore-up traditional fertilizer programs, it is time to pay attention to the full spectrum of crop nutritional needs.

We generally refer to magnesium, calcium and sulfur as the secondary nutrients. They are described as secondary because they are needed in similar amounts as the macro nutrients, but their supplementation has been required less often because of adequate amounts supplied via the soil, atmosphere, other fertilizers, manures and agriculture limestone.

I have yet to see a calcium deficiency on any row or forage crop in our trade territory. This is because many of our soils are naturally high in calcium, and all fields involved in a liming program receive many times more calcium through limestone application than their crops require.

Similar to calcium, magnesium is prevalent across most of the soils in our region. Dolomitic limestone contains a fair amount of magnesium. However, some regions have predominately calcium carbonate limestone combined with low native soil levels of magnesium.

These areas are represented as the 14 percent testing below optimum in the table below. Although this is not a large percentage of our soils, those that fall into this category require additional magnesium to maximize crop yield potential and improve the quality of certain crops (especially forages).

Of the secondary nutrients in our region, sulfur is by far the most commonly needed as a supplemental fertilizer. Ninety-three percent of all soil sulfate samples we ran this year contained less than 15 parts per million sulfate-sulfur, a number we use as a target for current crop adequacy (see table). Although sulfate is mobile in the environment and is an indirect measurement of season-long crop-available sulfur, these numbers indicate that our soils are trending low in potential sulfur supply. We have verified this through extensive visual sulfur deficiencies, and we see low plant-tissue sulfur levels occurring throughout our region. I attribute this to a combination of declining soil organic matter, increasing crop yields, less atmospheric sulfur deposition and the move to higher quality phosphorus fertilizers that contain low levels of sulfur impurities.

Essential crop micronutrients most commonly discussed are boron, chloride, copper, iron, manganese and zinc. Recently, cobalt, selenium, silicon, sodium and vanadium have been added to the list. These are described as micro nutrients because they are needed by crops at very low levels. They are considered essential, because they are required at some physiological level for crops to properly function.

To put micronutrients in perspective, consider the actual micronutrient requirement for corn as listed in the University of Nebraska Extension Guide G1830. They estimate that 2.4 pounds of iron, 0.4 pounds each of manganese and zinc, 0.2 pounds of boron, 0.1 pounds of copper and 0.01 pounds of molybdenum are required to produce 200 bushels of corn. These numbers appear very small, but they represent total uptake need values, which can be quite different than application rates or soil test levels of these nutrients.

Many of the micros are present in nature at low levels. Also, the metal nutrients are insoluble and not readily plant available. Most form insoluble mineral precipitates. Others are chelated and held in organic forms so tightly that the plant cannot access them.

Soil pH plays a critical role in the solubility and availability of micro nutrients. This is one of the reasons we place so much importance in soil pH management. Generally, as soil pH exceeds 7.0, the availability of soil micronutrients declines and the need for adding them as crop fertilizers increases.

High yields are demanding that more micronutrients be available to our crops. In many situations, we have reached the point of consistent responses to the addition of several of these nutrients.

Since micronutrients are needed in low amounts, and many can become "fixed" by the soil, their management is different than the primary and secondary crop nutrients. However, they can fit well into annual applications with standard soil testing based fertilizer