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How Soil N Is Like Money

I recently read an article in a farm magazine that made the point that nitrogen (N) is the biggest and most important of our farm inputs — and it is never far from the mind of a modern farmer. Let us try to explain the intricate workings of Mother Nature’s N cycle in comparison to our personal or corporate finances. I have done this before — but each time it comes out a little different — so here goes.


Each January 1 many of us sit down and prepare a net worth statement. It lists all our assets (A) and their value, all our liabilities (B) and their value. When we subtract A from B we have a positive number (we hope) and that is our net worth.

In our soil system the current account available for “spending” is the soil test nitrate. Ammonium is also in the current account but in normal circumstances it is fairly constant at a low amount. That may not be true when liquid hog manure is part of the equation, as about 70 per cent of N is in ammonium form.

The bulk of the net worth of nitrogen is tied up in the soil organic matter in the topsoil (A horizon).


The bulk of our soil N supply is tied up in capital investments. Some parts of it are not very liquid — just as our land itself is not liquid. We can’t take a piece of it and spend it. Other parts of it are more liquid and are more easily converted to cash (nitrate and/or ammonium). Many famous scientists have spent a lifetime figuring out the various “pools” of N in organic matter and how easy it is converted to cash in some of that capital.


An important part of cash flow is making sure there is cash in the bank when we have to write out the big cheques. As farmers we need bags of cash in spring to get the crop in the ground.

For most of our annual crops the bulk of cash flow (N) is required from mid June to mid July when most of the N is taken up by crops. The exact dates vary with crop, year, soil zone and so on, but during the high uptake period there must be an adequate supply of cash (nitrate/ ammonium) to meet the needs of the crop.

To provide “cash flow” at the correct time the cash can come from 3 sources:

1) The nitrate N content of the soil at time of seeding. We determine that by soil testing late fall or early spring 2) Interest or dividends that are paid out at the time of great need. In soil terms this is the mineralization of organic N to ammonium and nitrate. This is an important source of “cash flow” because it occurs at the same time as need so efficiency is greater. 3) The difference between crop uptake (cash flow) needs and the sum of (1) and (2) above is made up by adding N fertilizer.


In soil testing for N, make sure your advisor uses a system that is reproducible. Table 2 shows the results of duplicate sampling over many years on my Dundurn farm. Each number in Table 2 comes from a composite sample of 30 random cores, taken in select parts of the quarter section that leaves out eroded knolls, recent breaking, slightly saline areas and sloughs with very thick topsoil. In that system, N results are quite reproducible but P and S are sometimes a problem. The bold numbers highlight the unacceptable duplication.


The way we convert N from organic matter (capital) to the chequing account (nitrate/ammonium) is by mineralization. As you look at the size of the N capital it is easy to see that Black and Thick Black soils have the big net worth and any way we can manage our soils/crops to increase the conversion of capital to the chequing account will have a big effect on our need for our cash to take care of the “cash flow” — nitrate/ammonium needs of crops.

When the early soil test N correlation work was done in the 1960s and 1970s, the N level down to two feet was a good first approximation of how much N to use on crops for lower organic matter Brown, Dark Brown and Grey soils. But for Black and Thick Black the correlations were not as good, And, as we are well along to converting our land to a very different ecosystem it should not be a surprise to find that the nitrate test has limitations, and that a measure of mineralization would be a useful addition to managing our N “cash flow” and being more specific about the size of the N fertilizer bill.

J. L.(Les) Henry is a former professor and extension specialist at the University of Saskatchewan. He farms at Dundurn, Sask.

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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