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A New Soil Test For N?

When soil testing began in Western Canada in the mid 1960s the standard soil test for N was to measure the amount of nitrate N that was present in a soil to a depth of two feet. The original science came from Bob Soper, professor of soil science, University of Manitoba. His correlations with field N response became the standard.

The nitrate test has served Western Canada very well for many years. But it was developed in a time when standard agronomy was fallow/crop/crop with low N fertilizer rates and no legumes in annual crop rotations. Nitrate is a snapshot of a moving picture, and the snapshot worked as long as not much change was happening.

The big chunk of N in any soil is in the organic matter and how much is released from the organic matter to the nitrate pool has been the subject of much research. Various methods have been used to determine the fraction of organic matter that will spit out nitrate when we need it, but none have worked well enough.

LETTING NATURE WORK

I have had an idea for several years — why not let Mother Nature tell us how she is going to spit out nitrate? From the first days of soil testing we told everyone to take the soil samples and immediately rush off and lay the sample out to dry. After all, we do not want those pesky microbes to spit out more nitrate and affect the test.

There is now evidence that our new farming system has created new soils that are mineralizing much more N during the growing season and a new soil test is required.

My very simplistic idea is this: Why not just leave the moist soil sample in the sample bag for a few weeks at room temperature and then test for nitrate. If the bugs do not kick out nitrate from a moist, warm soil in three to four

Table 1. Nitrate N for soil samples dried immediately and for three week incubation in sample bag at room temperature. Grey wooded soil, low organic matter

10 years alfalfa, then 80 bu./ac. barley crop

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weeks, then it is unlikely that the soil in the field will mineralize significant N in time for the current year’s crop.

My first try at the idea was with my Grey Wooded, low organic matter (OM) soil at Spiritwood. My good neighbour Roger Semegen of Spiritwood refers to our soils as being “brain dead.” Table 1 shows what happens with a soil like this with normal cereal cropping and one where alfalfa was grown for 10 years. The data from the alfalfa history was after an 80 bu./acre barley crop had been harvested just before samples were taken (See Table 1).

Now, it is common to give a credit where alfalfa history is known, but this would appear to put a more accurate number on the mineralization.

The date in Table 1 was gathered many years ago; in the meantime I have been splitting my samples on Dundurn farm and doing incubation for three weeks for several years. Dundurn is a Dark Brown soil with medium organic matter. The dried sample was typically about 15 to 30 lbs. N/ac., and, incubated, about 25 to 40 lbs. N/ac. to the one foot depth. The increase from incubation was seven to 14 lbs. N/ac. No big deal.

But in 2010 I split up the quarter into three zones: rroded knolls; a small area of new breaking; and, the remainder of the quarter

The samples were split and half was incubated for four weeks at room temperature — actually in our basement where we also make 90 bottles of chokecherry wine each year.

It is easy to see in Figure 1 that there are huge differences over the field (nothing new) but that the incubation kicks out a lot more N on the soil with a large active fraction of OM.

AT HOME TECHNIQUE

The technique is simple: Just leave the sample in the bag, loosely tied, for about four weeks. It is important that the sample be at field capacity moisture when taken, which is usually the case for a one foot soil sample in fall, but not always. It is important to note that this idea is a “fall only” test — there is no time in the spring to wait around a few weeks. But in fall, we have all winter to get the analysis done and make plans for spring.

There you have it — an idea that I think merits some proper research to see if it will work in practice and to determine the best way to make it happen. There is an increasing body of evidence that suggests that, for some soils at least, N rates could be greatly reduced and a good crop still happen.

J.L.(Les)Henryisaformerprofessorand extensionspecialistattheUniversityof Saskatchewan.HefarmsatDundurn,Sask. Herecentlyfinishedasecondprintingof Henry’sHandbookofSoilandWater”,abook thatmixesthebasicsandpracticalaspects ofsoil,fertilizerandfarming.Leswillcover theshippingandGSTforGrainewsreaders. Simplysendachequefor$50toHenry Perspectives,143TuckerCres,Saskatoon, SK,S7H3H7,andhewilldispatchasigned bookposte-haste

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Normal Annual cropping

Dried immediately lb N/ac. one foot sample Three week incubation lb. N/ac. one foot sample

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About the author

Columnist

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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