The 4R’s of Fertilizing in the Fall

Sep 8, 2017

For the Canadian Prairies and Northern Plains

We’ve previously reviewed the 4Rs of nutrient management as they relate to Fall Nitrogen. This article will focus on “the others” or PK and S.

With regards to “Right Time”, Phosphate (P), Potash (K) and Sulphur (S) are quite a bit less complicated than N.  All three products can be applied in the fall.

Phosphate can be effectively applied in the fall but should be applied in a band.  More about that in “Right Place”.  Fall is an excellent opportunity to get more phosphate into a crop rotation.  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 like 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

Potassium can also be effectively applied in the fall. Like Phosphate, Potassium is not subjected to loss mechanisms that Nitrogen is. In soils that have recommendations for Potash, it can effectively be broadcasted in the fall. Often it is blended with a sulphur product or a stabilized nitrogen product and broadcasted.  Alternatively, it can be banded with Urea and/or Phosphate. Sulphur can also be effectively applied in the fall. In fact, elemental S is most effective when broadcasted in the fall.  Ammonium Sulphate is very versatile and can be broadcasted or banded alone or blended with other products.

Lens 2 is “Right Place”.  As discussed above, Phosphate is recommended to be banded. While it can be broadcast applied, research indicates it is only 25 – 35% as effective broadcasted vs. banded. This is due to a tie up of the nutrient as compared to a loss. Eventually, this product will be used by the crop but we are looking at 10 – 20 years to get the benefit from a broadcast application. Dual banding N and P 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 lbs./acre N is applied with the phosphate. The centre of the bands have too high ammonium concentrations to allow for root survival. If applying phosphate in the fall, you may want to also apply 10 – 15 lbs. 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 sulphur 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 lbs K20 / acre. This response is most dramatic in barley, occurring at an economic level approximately 1 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 K with your cereals.

Sulphur is also very flexible. Ammonium Sulphate (20 or 21-0-0-24) can be broadcasted or banded.  Elemental S (0-0-95 or 0-0-90) or products such as Bio-Sul (0-0-70) should be broadcasted and left on the soil surface.

 

Now let’s look at the “Right Source” of product.  For much of the Canadian Prairies and the Northern Plains, MAP is the only source of Phosphate although Ammonium phosphate -sulphate (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 “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 Sulphur based products available including Ammonium Sulphate (20 or 21-0-0-24), Elemental S (0-0-95 or 0-0-90) or products such as Bio-Sul (0-0-70) or co-formulated prodcuts such as ammonium phosphate – sulphate (16-20-0-15); “Tiger 50” (12-0-0-50 ) or MES (13-30-0-15) .  These products can easily divided into two product groups. Group 1 are products containing more than 50% of the sulphur in the Ammonium sulphate form. These products will work equally as well banded, blended or broadcasted. Group 2 are products that contain 50% or more of the sulphur as elemental S.  Elemental S is totally unavailable to plants. Plants simply cannot absorb So through the root system. Elemental S is inert and water insoluble. However, when farmers add So to soil, it’s an entirely different matter. In the soil, So converts (oxidizes) to the plant-available SO42- form, and the rate at which this conversion takes place is the determining factor regarding the effectiveness of So as a fertilizer source of S.  The oxidation process takes place when the So is in very fine particles. Most products will break down more quickly if broadcasted in the fall and allowed to “weather” on the soil surface during the winter.

Greater 50% SO42- form 50% or more S in Elemental Form
Amonium Sulphate (20 or 21-0-0-24) Elemental S (0-0-95 or 0-0-90)
Ammonium phosphate – sulphate (16-20-0-15) Bio-Sul (0-0-70)
Ammonium thiosulphate (15-0-0-20) “Tiger 50” (12-0-0-50 )
 MES (13-30-0-15)

 

The fourth and final lens that we can peer through when looking at our nutrient strategies is “Right Rate”.  Several pieces of information are required to determine the correct nitrogen rate.  These include:

  1. What is the crop that will be grown? Different crops have different nutrient requirements.  However, you may not want to apply excessive rates for a crop on all your acres if you may wish to change your cropping plans. Moderate rates can be topped up in the spring.

 

  1. What are your yield goals? Of course, higher yield goals require fertilizer rates. Realistic goals will ensure optimum nitrogen fertilizer utilization. An accurate yield goal can be set by using such things as available soil moisture, soil texture and long term weather forecasts. However, we all know Mother Nature has the last say when it comes to yields!

 

  1. How much of each of our nutrients is in the soil? This can only be determined by a soil test. Soil samples properly taken from a field, when analyzed will give an accurate prediction of how much plant available P, K or S remains. A more accurate picture comes from using production zones to guide testing.  This takes into consideration the difference in productivity in your fields.  For P, 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 applications is a very good method of increasing application rates on crops where seed placed P is restricted.

Another novel approach to taking a soil test is to use what is called a Virtual Soil Test.  This system uses an algorithm to predict P, K and S levels in a zone by using previous soil test results, physical and chemical soil characteristics, previous fertility applications and crop removal. There is a very high correlation between these results and actual soil test results provided accurate local weather data is used.

 

Using the four steps above, an accurate fertility prescription can be determined – hopefully not too little, not too much but just right.