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“Spending” phosphorus fertilizer from the soil

We’ve greatly increased the spending from our capital accounts of phosphorus

In my previous column, we established that phosphorus fertilization comes down to simple arithmetic: If we haul more phosphorus off to the elevator than we put on in fertilizer or manure, the phosphorus (P) soil test will go down and with it the crop yield potential.

If we add more P than we haul away, over time, the soil test P will go up along with the crop yield potential.

Now we will look at the effect of our spending rate (crop removal) on the capital account (total soil P).

“Spending” P from the soil account

For crop removal (what we haul to the elevator) I will use the following rates:

  • Wheat: 0.6 lbs. P2O5/bu.
  • Canola: 1.0 lbs. P2O5/bu.
  • Peas: 0.7 lbs. P2O5/bu.
  • Soybeans: 0.85 lbs. P2O5/bu.
  • Fababeans 1.1 lbs. P2O5/bu.

I am using a 12-year period to accommodate three cycles of a four-year rotation.

Numbers will be rounded — these are approximate numbers, and I don’t want to leave an impression of precision that is not there.

In the good old days, when the first phosphorus fertilizer recommendations were made, wheat was the crop, half summerfallow was common and yields were low. A 30-bushel fallow wheat crop was considered great back on Brunswick farm at Milden, Sask., in the 1950s.

Using 0.6 lbs. P2O5/bu., a 30-bushel wheat crop would remove 18 bushels of P2O5 per acre. With a wheat/summerfallow rotation, over 12 years there would be six crops, for a total of about 108 lbs. P2O5 per acre over the 12-year period, or about 10 lbs./acre/year.

But, fast forward to now. As an example, let’s take a four-year rotation: wheat, peas, wheat, canola.

In 12 years we will have six wheat crops, three pea crops and three canola crops.

For yields we will use: wheat 50 bu./ac.; peas 50 bu./ac.; canola 40 bu./acre.

Many of you beat those yields often, so the number we come up with will be low for some areas and farms.

This scenario would remove about 405 lbs. of P2O5/acre or about 35 lbs. P2O5/acre/year.

Plug in higher yields that many now achieve and the “spending” can go higher.

In the early days I can remember being asked about the total P2O5 in soils and we gave a very general answer. “It is 2,500 lbs./ac. in six inches, or more, and when we are taking off only a few pounds per acre per year it is not an issue.” Not to be worried about for a few hundred years!

The capital account: what’s the total

When we started to realize the much larger amounts of P being hauled off to the elevator I went looking for total soil P data. We have recently learned that in the long run fertilizer P use efficiency is high and in dry areas nitrogen can be more than 100 per cent.

At Scott, Sask., in the dark brown soil zone, Agriculture and Agri-Food Canada has a wheat field that has been in continuous wheat production for over 100 years with no input of manure or fertilizer. The field is still producing 15 or so bu./ac. of wheat each year. Now, we know that some nitrogen comes from rain and lightning and a bit of fixation by soil organisms all by themselves. That explains the nitrogen. But what about P?

In organic farming, nitrogen can be handled by including legumes in the rotation and some green manure crops. Green manuring is also a phosphorus fertilizer, but the P still all comes from the soil. Most organic farms rely on livestock and manure to maintain P in the long term. So the question arises as to how long the soil will continue to supply P. At the low yields of continuous wheat the removal is low but with legumes supplying nitrogen and better yields how long will the P last?

In conventional farming large nitrogen rates and other good agronomy has lead to much larger nutrient removal.

In looking for the answer about total soil phosphorus we soon realized: we really do not have a good database of total P data for soils of Western Canada.

When soil fertility work began it was soon learned that the total amount of a nutrient in a soil was not a predictor of the need for that nutrient in any given year. Some early soil survey reports had a table of data including total P for a good cross section of soils. It turns out that total P is a messy and time-consuming lab procedure so no total P data is in any modern soil survey report. Alberta soil survey reports have total soil P data up to the late 1950s, Saskatchewan to the 1950s. Early Manitoba soil reports did not have total P data.

But, the library is a great place and many journals are now available with a few mouse clicks so I have assembled my estimate of total P data for Canadian Prairie soils.

phosphorous application chart
x photo: Les Henry

It is still all too common to see P and P2O5 mixed up in various publications. Almost all field work on residual P gives the application rates in kilograms of P per hectare. Many studies had rates up to 400 kg P/ha applied, which is equivalent to 1,568 lbs. of 11-52-0/acre. That is 356 lbs. P/acre, which equals 815 lbs. P2O5/acre. This is equivalent to 1,568 lbs. of 11-52-0/acre.

If the above numbers are divided by 10 — to go back to the “good old days” it is easy to see why we paid no attention to total P in the soil. But, if we divide the numbers by 30 or 40 some of the “weaker” soils would be a cause for concern.

In soil development the P ends up being moved by plants from the subsoil to the topsoil, so topsoil usually has higher levels. The upper subsoil is somewhat depleted. In thick black soils like those at Melfort, Sask., there is almost as much P in the six- to 12-inch depth as in the zero- to six-inch depth, so supply is large there. But in “weaker” soils the upper subsoil can be much less than the topsoil.

We will be pushing for somebody at Scott to get a measure of the total P in the soil of that 100+ year wheat field.

I hope this little piece stirs up enough interest to encourage some work to get a better set of numbers than that provided here.

Stumbling around in the scientific literature from before even this old fossil was born is a bit sobering. There is a lot of U.S. literature from the 1940s and earlier that shows clearly that large P fertilizer rates lead to significant residual effects. In Eastern Seaboard states, a 1945 paper shows the balance between P fertilizer and crop removal by potatoes. Even then they were porking on as much three times crop removal — and it showed up in the soil tests of the day.

Now that so much of the older literature is so easily available there is no reason not to know about it.

Parting Note: Manure

In feedlot alley in Alberta there are quarter sections with 2,200 lbs. P2O5/ac. of available soil test P in the top six inches of soil. Who knows what the total is, but the soil test P is higher on those heavily manured soils than the total in similar soils without manure. Once again I say: we need to find ways to make use of that excess P to the benefit of much larger acreages. In 100 years folks will look back and wonder why such a valuable resource was not better utilized.

Mind your Ps and Ks

P x 2.29 = P2O5
K x 1.21 = K2O

It is becoming all too common to see slippage in the correct use of P versus P2O5 and K versus K2O. I have seen too many presentations lately stating that a farmer used 20 lbs./ac. of P, when I suspect s/he actually used 20 lbs. P2O5 per acre.

If 20 lbs. P/ac. are used that is:

  • 20 x 2.29 = 45.8 lbs. P2O5/ acre — a very large error.

Most soil test labs report P as ppm and they mean P. To convert that to lbs./acre P it is multiplied by two; to convert to lbs./ac. P2O5 it is again multiplied by 2.29.

With K: the factor to con- vert K to K2O is only 1.21 so the error is not as big.

Be sure to know the units when calculating P fertilizer use.

If you want the complete explanation see page 32 of Henry’s Handbook of Soil and Water. It is a dumb confusing system that should have been changed decades ago.

About the author


Les Henry

J.L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask. He recently finished a second printing of “Henry’s Handbook of Soil and Water,” a book that mixes the basics and practical aspects of soil, fertilizer and farming. Les will cover the shipping and GST for “Grainews” readers. Simply send a cheque for $50 to Henry Perspectives, 143 Tucker Cres., Saskatoon, Sask., S7H 3H7, and he will dispatch a signed book.



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