Then and Now is the Theme of the Conference – and the challenge in explaining the changes in nutrient supply and demand under zero till. Research has armed us well to explain "THEN" or the nutrient changes in start-up zero-till years. But NOW there is largely speculation of what is happening in mature ZT fields.

Numerous studies have established the following fertility related changes with developing ZT systems and are summarized in greater detail elsewhere (Johnston, 2002, Schoenau, 1995, Silgram and Shepherd, 1999):

Numerous studies indicate that organic matter content increases under ZT conditions. One of the components of soil organic matter is nitrogen, so organic nitrogen levels increase correspondingly. Typical study results are found in Table 1.

In the initial transition period into ZT, plant available N is often actually lower in no-till as the soil microorganisms adjust (immobilization) to the surface placement of crop residues, rather than soil mixing. Failure to adjust N rates in such soils has shown production of lower yielding and lower protein crops (McConkey et al, 1986).

A world-wide review of N management in ZT indicates that some 20 lb N/ac additional N may be required in the early ZT years to account for this increased immobilization (Silgram and Shepherd, 1999).

One strategy to minimize this apparent N loss is the band placement of nutrients below the residue-covered surface.

Much of the original research evaluating nutrient dynamics under no-till was conducted in the US cornbelt. In such systems surface broadcast applications of NPK were traditional. This led to identification of 2 main phenomenon – the development of an acidic surface layer, due to transformations of N fertilizer, and stratification or layering of immobile nutrients (P and K) in the surface layer. Surface stratification can develop through 2 processes under ZT – firstly surface broadcast fertilizer applications and secondly through nutrient cycling with crop residues remaining on the soil surface.

Acidification of surface soil in long-terms ZT just has not been documented in western Canada (excepting one study in the grey wooded soils of the Peace River region). In general, our soils are higher in pH initially and many are well buffered with free carbonates, especially in Manitoba. Additionally, our N application rates are considerably less than the corn growers.

Research in the Black soil zone has looked at nutrient stratification. After 4 years of ZT at Indian Head SK there was no difference in P and K levels in the surface soil (0-6") even when dissected in 2" increments. Alberta research found similar results. At Brandon MB, researchers sampled 4 year old ZT at 1" increments, and detected an increase in P concentration at the 4" depth, the depth at which P was being banded with N. Broadcast potash at Brandon tended to accumulate in the surface 1" of silty clay soil, but was distributed through the surface soil on a sandy loam soil.

It has been speculated that surface stratified nutrients may be unavailable in a dry year. However this has not generally been observed, since the same accumulation of residues near the surface maintains higher soil moisture, which will keep roots active and in a position to take up these accumulated nutrients.

Recently Zero tillers feel they have entered a phase where N previously immobilized in the surface layers are being released at a faster rate than the microorganisms can immobilize it again. This is the anxiously anticipated "new equilibrium" in ZT. Many researchers are hesitant to speculate on the time required for this maturation. German researchers reported that 15 years are required before the new steady state between immobilization and mineralization was obtained after changing to a ZT system (Silgram and Shepherd, 1999)).

I’m told by Zero-tillers that they are now observing:

• More crop lodging
• Protein is too high for malting barley
• Fall soil nitrate levels are higher than normal
• Lower N rates produce equal yields

These same growers are now challenging soil scientists in research, extension and industry on 2 fronts:

• Develop "made for ZT" nitrogen recommendations
• Develop a testing method that will allow us to predict the N release from these soils

When I consulted with several of these soil scientists, they highlight other factors that are also playing a role in increased N release to crops:

1. Recent weather that has been moist in the summer promotes mineralization of organic N – in both ZT and CT systems. The larger difference is probably between continuous cropped vs fallowed systems (where easily mineralizable N has already been cropped out).
2. We are growing more crops in the rotation with better residue quality (lower C: N ratio) lending itself to breakdown and release without immobilization (ie field peas)
3. Some inadvertent "tillage" is occurring in ZT fields – fall banding NH₃, higher disturbance openers with side banding attachments, etc. Long term ZT fields are known to have greater amounts of potentially mineralizable N – and given the opportunity (breakage of stable soil aggregates, more oxidation, etc) a flush can be expected.
4. Movement to more "efficient" N use than traditional surface broadcast ammonium nitrate. Subsurface banded N, such as seedplacing, sidebanding or midrow banding may be up to 20% more efficient, largely due to less immobilization in surface residue. Moving to such placement methods without reducing rates mean more N is supplied to the crop
5. If fertilizer recommendations are being requested for yield goals that are not attained, protein will be higher and soil N will accumulate.

I will grant that soil test recommendations should be updated. Much of the Manitoba studies used in developing N recommendations were conducted 20-40 years ago. Cropping systems were different then – fallowing was done, pulse crops and manure were less widespread, yield potentials were lower (hybrids were corn varieties) – and the soils were all tilled conventionally. Cumulatively these factors are screaming for attention – but soil test calibration is not a priority for funding agencies and researchers are unable to move forward with this work. This may change as environmental agencies move to embrace our "recommendations" as the required standard.

Work has been progressing in the effort to predict N release for the crop – through measurements or estimates of that organic N that is likely to mineralize during the growing season (Green et al, 2002). These tests are:

1. phosphate borate test – measures ammonium-N and amino sugar-N.
2. hot KCl extraction of ammonium-N
3. Illinois N test – measures amino sugar-N
4. Plant Root Simulator- 24 hour extraction of nitrate and ammonium-N on exchange resins

These are meant to estimate the relative amount of easily mineralizable N that will be released to a crop over the growing season. The shortcoming of such tests is that though they can identify the relative magnitude of potentially mineralizable N, they cannot predict the driving forces – growing season moisture, heat, oxygen and microbial activity - that will meter out this in-season nitrogen.

In preliminary evaluations, the addition of the "potentially mineralizable N test" to the conventional nitrate soil test markedly improved the correlation with actual N uptake by the crop (Flaten, 2001).

But our nitrate soil test will continue to be the foundation for estimating N availability to crops. The more frantic search for mineralizable N measurements is in those areas where the preplant nitrate soil test does not work well -the humid climates such as eastern Canada and the US corn belt.

Some growers are pondering whether they should change fertilization strategies by:

1. backing off their N rates based on use of these mineralizable N tests or
2. delaying their N until they can estimate how much mineralization is occurring.

With our present knowledge and experience the answer would be "No" on both accounts. Here’s why:

• They depend on second-guessing growing season weather and microbial activity
• Timing of N release may not meet the early season requirement for cool season, fast growing crops like cereals, canola and flax

• Long season crops with modest early season N requirements, such as corn, may lend themselves to late June N assessments (chlorophyll content or soil nitrate levels) and side-dressing the supplemental N. This is just too late for cereals, canola and flax crops that set yield potential much earlier.

Growers wishing to try these strategies should do so on test strips, and start detailed tracking of crop N budgets, considering N additions, crop removal and soil test residuals.

To end with the "age analogy" of ZT, I am compelled to mention euthanasia. Some researchers and others are now evaluating what is covertly referred to as "Strategic Tillage" of ZT fields for any number of reasons (not discussed here). Reversion to a tilled system is surely one way to unlock that potentially mineralizable N that has been building in ZT over the years.

Flaten, D. 2001. The nitrate soil test: Is it reliable? Manitoba Agronomist Conference proceedings. pp 95-101.

Grant, C.A. and G.P. lafond. 1994. The effects of tillage system s and crop rotations on soil chemical properties of a Black Chernozemic soil. Can. J. Soil Sci. 74:307-314

Green, B., D. Keyes and J. Lee. 2002. Soil testing laboratories – the future of soil test recommendations. Manitoba Society of Soil Science Proceedings. http://www.gov.mb.ca/agriculture/news/msss/index.html

Johnston, A. 2002. Fertility issues and long-term no-till. Alberta Reduced Tillage Initiative Workshop 2002.

McConkey, B., S. Brandt, C. Campbell, D. Curtin, J. Schoenau and F. Selles. 1998. Reduced Tillage = Reduced Protein?.Wheat protein production and marketing. Proceedings of 1998 Wheat Protein Symposium. Pp331-336. Univ. of Saskatchewan, Saskatoon SK1998.

Schoenau, J.J. 1995. Moving to no-till: Nutrient cycling implications. Western Canada Agronomy Workshop. Proceedings Vol II. Red Deer, 1995. Published by Potash and Phosphate Institute of Canada.

Silgram, M. and M. Shepherd. 1999. The effects of cultivation on soil nitrogen mineralization. Advances in Agronomy. Vol.65 pp 267-311.