back to basics soil fertility part 2: phosphate€¦ · back to basics soil fertility part 2:...

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Back to Basics Soil Fertility Part 2: Phosphate By Thom Weir, Senior Precision Agronomist This is part two of five of Back to Basics: Soil Fertility that deals with soil and fertility. We will discuss Phosphate today as it is the nutrient that I fear is most deficient on farms across the Canadian prairies and the northern Great Plains area of the US.

It’s not the nutrient that unlike nitrogen, when missed, shows deficiency symptoms across the field. Phosphate deficiencies are more subtle and usually go unnoticed. In fact, I have only seen what I would identify as significant phosphate deficiencies four to five times in my career as an Agronomist. Interestingly, three of these fields showed up in the same year.

The year was 2009. If you recall, fertilizer prices spiked in 2008 to the point where many growers simply didn’t apply phosphate, and the sky didn’t fall in. In fact, because some growers didn’t see any issues, they eliminated their phosphate again in 2009. That year, I was called out to investigate fields that showed very slow growth compared to neighbouring fields. After investigation, soil and tissue sampling, I determined the stunted growth was due to phosphate deficiencies.

We have been removing more phosphate from the fields than we have been applying for years. A bit of a history lesion shows that phosphate was the original fertilizer used on the Canadian prairies and the northern Great Plains area of the US. The product 11-48-0 was introduced by Cominco as Elephant Brand Fertilizer. Farmers found that applying this product gave immediate results. Research work determined that 50 lbs of this product or 24 lbs P205 per acre applied with the seed was a preferred rate to apply. It was also easy for farmers to calculate as fertilizer was sold in 50-lb bags in those days – so the rate was a bag an acre. For various reasons, this rate hasn’t changed a lot over the years.

Farmers don’t see big visual differences and with wider row spacings, the safe applied rates in the seed row are limited for crops such as pulses, flax and canola. The result is that, while yields have doubled over the last 40-50 years, phosphate use has changed little since the late ‘70s. This can be seen in the following graph of data compiled by Dr. Rigas Karamanos. In the graph, he charts the usage of nitrogen and phosphate in western Canada and compares the ratio between the two.


The fact is we are removing 400,000 to 500,00 pounds more P205 than we apply.

This should concern you as a producer. You have – for the most part – turned around the mining of organic matter that was common across the Canadian prairies and the northern Great Plains area of the US with the adoption of No-Till. Today, on many farms where No-Till is practices, we have seen an increase in organic matter. Sadly, this is not the case with phosphate. We are mining the phosphate out of the soil!

Here are examples of four typical rotations and a phosphate balance sheet for each.

Brown Soil Zone

Crop Year 1 Crop Year 2 Crop Year 3 Crop Year 4 Total 4 years

Durum Wheat

Canola HRS Wheat Lentils

Phosphate (P205) applied (lbs/acre)

30 25 30 0 85

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0

500

1,000

1,500

2,000

2,500N:P

2 O5 rario

Nut

rient

, ton

nes

(000

)

Year

N P O2 5

N:P2O5 ratio


Crop Yield (Bu/Ac)

45 35 40 30 Total 4 Years

Phosphate (P205) removed by crop (lbs/acre)

25.6 28 22.8 18.5 94.9

Net Phosphate (P205) gain or loss

+4.4 -3 +7.2 -18.5 -9.9

Dark Brown Soil Zone

Crop Year 1 Crop Year 2 Crop Year 3 Crop Year 4 Total 4 years

HRS Wheat Canola Malt Barley Peas


30 25 30 0 85

Crop Yield (Bu/Ac)

55 40 70 45


31.3 32 28 54 145.3


-1.3 -7 +2 -54 -60.3

Black Soil Zone 1 Crop Year 1

Crop Year 2 Crop Year 3 Crop Year 4 Total 4 years

HRS Wheat

Canola HRS Wheat Canola



35 25 35 25 120

Crop Yield (Bu/Ac) 70 60 70 60


40 48 4o 48 176


-5 -23 -5 -23 -56

Black Soil Zone 2 Crop Year 1

Crop Year 2 Crop Year 3 Crop Year 4 Total 4 years

Oats Canola Soybeans HRS Wheat


35 25 0 35 95

Crop Yield (Bu/Ac) 150 60 40 70


42 48 29.2 40 159.2


-7 -23 -29.2 -5 -64.2

We can determine a few things from these charts:

• They all show a negative number regarding P205 usage or in other words these four rotations show a mining of between 10 and 65 lbs of P205 a year!

• The inclusion of a legume usually means little or no phosphate is used. This is often due to the difficulty of adding both a granular inoculant and phosphate into the seed row, legumes showing little crop response to phosphate as well as the fact that peas and lentils can be damaged with seed row phos.


Let’s now look at phosphate from the perspective of the three laws I proposed in the part one of this series.

Liebig’ Law of the Minimum – While Phosphate is often limiting yields, it usually only reduces yields by 2-5 bu/acre. These yields will increase with the addition of nitrogen but often show the same 2-5 bu/acre reduction at various N levels.

The Law of Diminishing Returns – This law can be illustrated with the following data from a malt barley study done by Westco in 1987. The study was carried out at four locations in Alberta. Four rates of phosphate were applied with the Nitrogen rate being held constant at 54 lbs / acre.

The 4 Rs for Managing Phosphate

1. Let’s look at the 4R’s for managing Phosphate. The first R we will discuss is The Right Source.

Monoammonium Phosphate (MAP) The vast majority of phosphate is applied across the Canadian prairies and the northern Great Plains area of the US as Monoammonium Phosphate (MAP) or products like 12-51-0 or 11-52-0. This product is very flexible and easy to use. It has quite a bit of safety for seed-placed applications and can be blended with most other granular products.

Ammonium Polyphosphate (APP) (10-34-0) is the most common liquid product used. While being an effective form of fertilizer, due to its increased nitrogen content, it has been found to cause more seedling damage when placed in the seed row. (Laryssa Grenkow Masters Thesis 2013). However, other studies have shown in corn that it is safer than an equivalent application of MAP. There are a number of other products that show up in the region to varying acres.


Diammonium Phosphate (DAP)

Orthophosphate (6-24-6) is another liquid product. It is promoted as being available to plants for a longer period of time than MAP or APP. While studies have shown this to be true, the difference is only a matter of a couple of days and this occurs before the plant is up taking phos. The application of 6-24-6 is usually at 3.5 – 4 gal / acre which is only providing 8-10 lbs P205 per acre. If this is the only source of phosphate, the soil will soon be depleted.

Rock Phosphate (typically 0-3-0 to 0-5-0) is a granular product also known as soft rock phosphate. The phosphate is not very soluble so 3-5 times the equivalent P205 may be required to get a similar response as compared to MAP. This product has the advantage in that it is able to be used in organic production.

Triple superphosphate (0–46–0) is a granular product that shows up from time to time. It is an excellent product for seed row application. TSP contains only phosphate because 1-31 Plant Nutrients and Plant Nutrient Products it is produced by reacting phosphoric acid with additional high-grade phosphate rock. TSP also is a solid plant nutrient product, but it is hydroscopic or absorbs moisture and therefore cannot be blended with some products such as urea.

Combination Products – There are a number of products that contain other nutrients besides nitrogen in their formula. These include:

MES - MicroEssentials® S15 (13-33-0-15S); SZ 1(2-40-0 10S 1Zn): S10 12-40-0-10S product comprised of monoammonium phosphate (MAP) plus ammonium sulfate (AMS) and elemental S in equal proportions] as an S and P fertilizer source. This product are promoted as an all in one alternative to a MAP / Ammonium Sulphate (AmSul) blend. While safer in the seed row vs such a blend, it is not as seed row safe as MAP alone.

Ammonium Phosphate Sulphate (16-20-0-13) is a homogenous granule containing monoammonium phosphate (MAP) plus ammonium sulfate (AMS). This product has been used for years in high sulphur demanding crops such as canola. While now mainly used in turf, it occasionally shows up in agricultural applications.

40 ROCK™ (12-40-0-6) is a homogenous granule containing Ammonium Phosphate Sulfate and Zinc Sulfate. This product has many of the advantages of products listed above.

NPS + Zn is a homogenous granule containing Ammonium Phosphate, Ammonium Sulphate, Calcium Sulphate and Zinc Oxide. Again, refer above for individual product characteristics.

Crystal Green® (5-20-0-10Mg) (struvite) is a phosphate product derived from the water treatment plants at various cities across North America. The continuous release granule formulation has a low salt index and offers a season-long release of nutrients that minimizes tie-up and reduces the risk of leaching and runoff. However, due to the relative high cost of this


product, it probably fits best as a “top up” product for seed row applications in crops such as canola or in a high value crop like potatoes in high pH soils.

There are two other products that must be mentioned when discussing phosphate. PB-50 (Penicillium bilaji) sold as JumpStart or TagTeam, is a phosphate-solubilizing inoculant. What this organism does is that it releases organic acids that acidifies the area around it. It solubilizes calcium phosphates, for example, and makes that phosphorus more available to the plant. However, as it “Mines” phosphate out of the soil at the rate of approximately 10 lbs annually, you have to remember that this phosphate has to be replaced at some time to not risk creating severe deficiencies.

Many crops form a relationship with arbuscular mycorrhizae (AM), which are the combination of particular fungi with crop roots that enhance the uptake of phosphorus (P) from the soil. These fungi are quite diverse and appear to be quite crop specific. At this time, there are few published trials that indicate commercial AM products are effective in the field.

The performance of either of these biological products will vary with individual soil characteristics in a field and the environment under which they are used.

2. The second R is Right Rate.

The Right Rate is going to depend on three factors. The number on factor being the crop that is being grown. From the chart below, one can see that for many of the crops grown in western Canada and the Northern Great Plains, 50 seems to be the sweet spot for required P205. Most fields in these areas do not have that amount of P205 available so the Right Rate would be the amount required less the amount plant-available in the soil. The best way to determine the amount plant-available in the soil is with a soil analysis. However, with phosphate, the extraction method used by a lab may not relate to plant-available. There are a number of extraction methods used by labs across North America.

These include Mehlich, Olson (bicarbonate), Bray 1 (weak), Bray ll (strong),Morgan, Miller Axley and Modified Kelowna. Each have unique soil parameters including soil pH where they work best. Miller Axley (low soil pH), Olsen (bicarbonate), Kelowna and Modified Kelowna have all been calibrated in Western Canadian field studies. Other tests are not recommended for deriving accurate recommendations in these locations.


Phosphate Usage and Removal by Crop Lbs P205 / Acre

Crop Yield / Ac Grain Straw Total

Wheat 60 32 - 39 12 - 14 44 - 53

Barley 90 30 - 37 15 - 18 45 - 55

Canola 50 28 - 31 19 - 25 47 - 56

Flax 35 14 – 18 6 - 7 20 - 25

Peas 50 24 - 30 7 – 8 31 – 38

Corn 120 47 - 58 21 – 25 68 - 83

Soybeans 40 32 - 35 0 - 5 32 - 40

Reference: Canadian Fertilizer Institute

Phosphate levels are generally below what is considered CRITICAL LEVELS in most fields across western Canada and the Northern Great Plains as can be seen in the map below.


The most recent “Phosphate Correlation” studies done in Alberta in the 1990s found that 81 per cent of wheat sites, 90 per cent of barley sites and 72 per cent of canola sites responded to added phosphate fertilizer and that these responses correlated closely with the soil analysis results. This data allowed for the development of “probability of yield response tables” found here.

Roughly speaking, there is a very high probability of getting a yield response from applying phosphorus if the test shows less than five parts per million, or 10 pounds per acre, of the ingredient in the top six inches of the soil. Anything in the five to 10 p.p.m. range has a 75 percent chance, 10 to 20 p.p.m. has a 50 percent probability and more than 20 p.p.m. has a 25 percent chance of getting a yield response for most crops.

3. The third R is Right Time. From my perspective, there are very few alternatives. The best time for most situations is at seeding. This is best applied either as seed placed or as a side-dressed placement. I would strongly recommend seed-placing 10 – 15 lbs P205i. The remainder can ideally be placed as a side band or midrow band with nitrogen and other nutrients or as a separate banding operation in the spring near seeding. The second timing is in the fall as a mixture or dual band with nitrogen. Again, I would strongly recommend seed-placing 10 – 15 lbs P205. However, research has shown dual bands of N & P in the fall has performed quite well. 4. The fourth R is Right Place. I think you can see from above that there are again only 2 “Right Places”. They are – in the seed-row and/or in a band adjacent to the seed row or placed in a band with nitrogen. There is quite a bit of research that shows a broadcast and incorporated application is only 25% to 30% as affective as banded applications in soils that are moderately or highly deficient.

back to basics soil fertility part 2: phosphate€¦ · back to basics soil fertility part 2:...

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