Fertilizing in the fall; part 2

Last week we looked at the 4Rs of nutrient management as they relate to fall nitrogen. This article will focus on “the others,” or phosphorus, potassium and sulfur.

With regards to “right time,” phosphate, potash and sulfur are quite a bit less complicated than nitrogen. The good news is all three products can be applied in the fall.

Phosphate can be effectively applied after harvest, but should be applied in a band.

Generally, we have been removing more phosphate than we have been applying for years. We are constrained with the amount we can seed-place with crops such as canola, so a fall banding operation is an excellent opportunity to increase the amounts applied. Often a blend of urea and MAP or DAP are applied in a dual band.

The right place can be more flexible when it comes to some of these macros.

Potassium can also be effectively applied in the fall. Like phosphate, potassium is not subjected to loss mechanisms the way nitrogen is. In soils that have recommendations for potash, it can effectively be broadcasted in the fall. Often it is blended with a sulfur product or a stabilized nitrogen and broadcasted onto the field.

Alternatively, it can be banded with urea and/or phosphate. Sulfur can also be effectively applied in the fall. In fact, elemental sulfur is most effective when broadcasted in the fall.

Ammonium sulfate is very versatile and can be broadcasted or banded alone or blended with other products.

It is recommended that phosphate be banded. While it can be broadcast applied, research indicates it is only 25 to 35 percent as effective broadcasted versus banded. This is due to a tie-up of the nutrient as compared to a loss.

The crop will eventually use this product, but we are looking at 10 to 20 years to get the benefit from a broadcast application. Dual banding nitrogen and phosphorus can be highly effective.

Some studies have shown that this improves phosphate uptake as the roots concentrate in the nitrogen rich environment and there is an increased probability of root–phosphate interaction.

One caution is that hot-bands may occur when more than 75 pounds per acre of nitrogen are applied with the phosphate. The centre of the bands has ammonium concentrations too high to allow for root survival. If you are applying phosphate in the fall, you might want to also apply 10 to 15 lb. with the seed to ensure your crop gets off to a quick start.

Potassium offers a variety of application options. Often it is blended with a sulfur product or a stabilized nitrogen product and broadcasted.

Alternatively, it can be banded with urea and/or phosphate. In some soils, in some years, even when soil tests indicate adequate potassium levels, there have been shown to be a yield response to seed placing 25 lb. potassium oxide per acre.

This response is most dramatic in barley, occurring at an economic level approximately one in 5 years. A less frequent response has been seen in wheat. If you see this response in your fields, continue with seed-placing some potassium with your cereals.

Sulfur is also very flexible. Ammonium sulfate, 20 or 21-0-0-24, can be broadcasted or banded. Elemental sulfur, 0-0-95 or 0-0-90, or products such as Bio-Sul, 0-0-70, should be broadcast and left on the soil’s surface.

Product source

For much of the Canadian Prairies and the northern Great Plains, MAP is the only source of phosphate, although ammonium phosphate sulfate,16-20-0-15, is available in some markets.

Both these products work well in a fall banding program. Ammonium poly-phosphate, 10-34-0, liquid can also be banded in the fall.

DAP may be available in southern part of the northern plains markets. It too works well in a fall banding operation.

With potash, we are looking at murate of potash, 0-0-60 or 0-0-62. These products work equally well as a fall applied product.

As indicated above, we have a number of sulfur based products available including ammonium sulfate, 20 or 21-0-0-24, elemental sulfur, 0-0-95 or 0-0-90, or products such as Bio-Sul, 0-0-70, or co-formulated products such as ammonium phosphate sulfate, 16-20-0-15, Tiger 50, 12-0-0-50, or MES, 13-30-0-15.

These products can be divided into two product groups:

  • Group 1 comprises products containing more than 50 percent of the sulfur in the ammonium sulfate form. These products will work equally as well banded, blended or broadcasted.
  • Group 2 products contain 50 percent or more of the sulfur as elemental sulfur. Elemental sulfur is totally unavailable to plants. Plants simply cannot absorb sulfur through the root system. Elemental sulfur is inert and water insoluble. However, when farmers add it to the soil, it’s an entirely different matter. In the soil, sulfur converts to the plant-available, oxidized form and the rate at which this conversion takes place is the determining factor regarding the effectiveness of sulfur as a fertilizer source.

The oxidation process takes place when the sulfur is in very fine particles. Most products will break down more quickly if broadcasted in the fall and allowed to weather on the soil’s surface during the winter.

When determining the right rate, a more accurate picture comes from using production zones to guide soil testing. This takes into consideration the difference in productivity in your fields. For phosphorous, we have been seeing a steady reduction in phosphate levels in our soils.

This is due to many years of crop removal being above the application rates. Fall application is a very good method of increasing application rates on crops where seed placed phosphorous is restricted.

Thom Weir PAg is an agrologist with Farmer’s Edge. He can be reached by emailing thom.weir@farmersedge.ca.

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