Fertilization Q and A

cornUniversity of Georgia Extension soil scientist, Glen Harris, answers some common questions about corn fertility.

Q. How much fertilizer does it take to get 300 bushels?

A. This is a question I get often. And the simple answer is “a lot!” Also, quite a few people think, “If I fertilize for 300 bushels, I’ll make 300 bushels,” but there are many things that can go wrong. In other words, there can be many other limiting factors.

For the University of Georgia recommendation, we use a base yield for irrigated corn of 150 bushels. After that, for every 10 bushels yield goal above 150, you add 12 pounds of nitrogen, 6 pounds of phosphate and 10 pounds of potash per acre.

In my research trials, in a five-year study, when we shot for 150 bushels, we usually made it or a little more, but when we shot for 200, we made above that two out of five years and below it one year. When we shot for 250, we make it two out of five years. Finally, when we tried for 300, we topped out around 280 two years, bottomed out below 200 two years and one year we were the middle.

If your land has the potential to make 250 bushels, but you fertilize for 150, then that’s all you’ll get and you’re leaving money on the table. If you have a field that’s never made more than 100 bushels, it doesn’t make sense to try for 300. If you have a field that’s made 200, then, yes, it makes sense to try for 250.

Fertilizer recommendations depend on the soil fertility level as determined by soil tests and a realistic yield goal.

Q. Is a liquid fertilizer more available to the plant than solid product?

A. Some people think this is the case — that a liquid is more readily taken up and available to the plant because it is already in a liquid form, but that is not always the case. Most solids fertilizers dissolve quickly and get into the corn plant just as well as a liquid product. There can be some differences in availability between forms of nitrogen, like urea versus ammonium versus nitrate, but usually these are not enough to make a big difference.

Q. Where should starter fertilizer be placed?

A. Placement is one of the 4Rs of nutrient management, along with the right rate, the right source and the right timing. I think about all of these factors because I can look at rate or I can look at rate and timing or rate and placement. All of these interact. And it makes a difference if you are talking about starter fertilizer, banding P and K or sidedressing nitrogen.

As for placement of a starter fertilizer, I don’t like anything in the furrow. I prefer that it is placed two by two: two inches to the side and two inches below the seed. Banding a starter fertilizer two inches to the side and two inches below the seed increases the chances of roots penetrating the fertilizer band and taking up needed nitrogen and phosphorus.

There are products you can put in the furrow, but if you really look at those products, you either have to lower the nutrient content so it won’t burn the seedling and then you lose your starter effect, or it’s a special formulation that won’t burn, but is more expensive.

When using starter fertilizer, deduct the amount of nitrogen and phosphorus used from the total nitrogen and phosphorus needed for the season. Sometimes, if your soil test P levels are high enough, the total phosphate requirements of the corn crop can be supplied in the starter fertilizer.

Q. If I apply fertilizer by banding it next to the row, can I use less fertilizer?

A. I do know that some farmers have been told that if you band your P and K next to the row, you don’t need as much. But that’s not really true. Crops need a certain amount of fertilizer. They need what they need to make a realistic desired yield, and it doesn’t matter if it is banded or broadcast. If your soil test shows phosphorus and potassium to be medium or high, there’s really no advantage to banding your P and K fertilizer next to the row.

Fertilization programs not based on soil tests may result in excessive and/or not enough nutrients being applied. Take soil samples each fall to monitor the current fertility level. Use the yield goal to determine the quantity of nitrogen, phosphate and potash to be used. At high yield levels, the balance of nutrients in relation to one another also is important.

Soil test information and fertilizer recommendations based on yield goals can be found at http://aesl.ces.uga.edu/.

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