Crops
by Dr. Paul Tracy

Have you tracked your crop nutrients lately?

Economics have shifted your fertility; it’s time make a new plan.

As you read this, we’ve moved into February. It is the month before field season starts, a time I call the pre-season jitters. Hopefully, you have already decided upon the major crop production input purchases planned for the season. Since no other cropping input has fluctuated as dramatically as plant foods, I’ll discuss my major crop fertility concern for 2009.

I’ve spent an appreciable amount of time over the past few years trying to promote the benefits of solid fertilization programs to our row crop and forage producers. I will continue to beat that drum. It has been a message we’ve stressed for decades, maybe to the point of apathy. However, it is an issue we cannot afford to be apathetic about.

For several reasons previously discussed in Today’s Farmer, many of our farms and ranches have reduced essential crop nutrient use. Agronomically, environmentally and economically, that in itself can be a very good thing, especially if the reduction was based upon resource inventory (soil sampling) crop needs and crop removal tracking.

However, we often get ourselves in trouble when input reduction is based upon short-term economics, land ownership/tenure, emotion, misinformation or some other related factor. I will reiterate: Agronomics and economics do not follow parallel pathways. They never have and maybe never will. As an agronomist, this has been one of the toughest issues I face. As crop nutrient purchasers, I realize your personal agronomic/economic dilemma is ten-fold compared to mine.

Ask yourself, “What has been my fertilization program over the past few years? Have I cut back on easily inventoried soil nutrients like phosphorus and potassium? Have I maintained soil pH through a strategic liming program? Have my yields increased above past targeted input levels used? Have I increased hay or silage production on fields traditionally managed as pastures, and have I accounted for the increased amount of nutrients leaving those fields? Have I sat down with my landowner/tenant to discuss the long-term soil health of the fields we cooperatively farm?”

It is relatively easy to estimate balances of soil-supplied crop nutrients. You need to know the soil build/soil removal factors, optimum soil test levels (defined as to when appreciable yield reductions start) and crop removal rates for the grain and forage crops produced. Let’s do an example of an estimated agronomic nutrient budget given the following:

1. Based upon short-term economic restrictions, a crop producer has not applied potassium fertilizer to a corn/soybean rotation field for the last two seasons.
2. This field yielded 60 bushels per acre soybeans in 2007 and 180 bushels per acre corn in 2008.
3. On average, corn removes 0.25 pounds of potash per bushel and soybeans remove 1.5 pounds of potash per bushel.
4. The most commonly used soil test build/removal factor for potassium is 3, meaning it takes approximately 3 pounds of potash fertilizer to raise the soil test level by one pound per acre of potassium. Inversely, for every 3 pounds of potash removed as grain or forage, the soil test level decreases by approximately 1 pound.
5. The optimum potassium soil test level for the dominant soil in this field is 325 pounds per acre, and the last actual soil test measurement for the field was 350 pounds per acre.

First, we need to figure total potassium removed since the last fertilization. Sixty bushels of soybeans x 1.5 pounds potassium removed per bushel = 90 pounds of potash. One hundred and eighty bushel of corn x 0.25 pounds potassium removed per bushel = 45 pounds of potash. Therefore, total removal since last fertilization is 135 pounds of potash.

To estimate soil depletion, we take the potash removed divided by the depletion factor (135 ÷ 3) to arrive at an estimated 45 pounds per acre drop in soil test level.

Using this example, the approximate current value for soil test potassium would be 350 (old soil test) – 45 (depletion since last soil test) = 305 pounds per acre. You can see how easy it is to go from an adequate to a deficient soil potassium supplying capacity. Please keep nutrient balancing in mind when planning crop production inputs. MFA currently uses three progressively intense methods of nutrient balancing. One is planned soil sampling of all fields every 4 years. Second would be developing a whole farm nutrient management plan similar to what is required with NRCS EQIP programs. Third would be to use the site-specific approach of tracking removal through yield mapping followed by applying varying rates of nutrients based upon that removal using MFA’s NUTRI-TRACK Program. If you have any questions concerning nutrient tracking, please contact your local MFA Certified Crop Adviser.

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