Nutrient Stratification in Long-term No-till Fields in Ontario
Tony J. Vyn and Xinhua Yin, Plant Agriculture Department, University of Guelph

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The adoption of no tillage has increased rapidly in Ontario during the past decade. Currently, approximately 20 per cent of Ontario’s field crop area is produced using no-till. However, there is concern that surface application and/or shallow band application of relatively immobile nutrients such as potassium (K) and phosphorous (P) may lead to a substantial accumulation of these nutrients in the surface layer and, potentially, a depletion of nutrients in the deeper layers of soil. Stratification of these essential nutrients may result in yield reductions, especially in dry years. In 1996, 54 fields which were in continuous no-till production for between 5 to 20 years were surveyed for their P and K fertility levels. Cooperating farmers were from Essex County in the southwest to the Ottawa Valleyin the east; the fields averaged 9 years in continuous no-till. The objective of this study was to determine the extent of nutrient stratification in long-term no-till fields under current management practices.

Potassium and Phosphorous Stratification
Very obvious potassium and phosphorous stratification had occurred in long-term no-till fields over the predominant soil types in Ontario. Highest levels of exchangeable K and available P existed in the top 0-5 cm (i.e. 0 to 2”) layer of soil and concentrations decreased sharply with depth. The concentration of exchangeable K in the 0-5 cm layer of soil was 1.35 times higher than that in the 0-20 cm (i.e. 0-8”) and 1.75 times higher than in the 10-20 cm layer of soil. Available P level in the 0-5 cm layer was, on the average, 1.36 and 1.70 times higher than that in 0-20 and 10-20 cm layers of soil respectively.

Length of time in No-till
The degree of stratification of both K and P increased with the number of years in continuous no-till. Potassium concentration in the top 0-5 cm layer of soil was 32 per cent higher than in the 0-20 cm layer in the fields with only 5-7 years of no tillage. For fields with 8-9 years and >9 years of no tillage, K level in the surface layers was as large as 42 per cent and 47 per cent higher than in the 0-20 cm layer. A similar pattern was observed for P stratification. The P content in the 0-5 cm layer was 31%, 35% and 42% greater over that in the 0-20 cm layer in the fields with 5-7, 8-9 and >9 years of no tillage respectively.

Most long-term no-till fields had K and P concentrations in the medium range (Table 1). For corn production, about half of these fields had a low to medium content of exchangeable K, and required K fertilization. The majority of the no-till fields had low to medium levels of available P. The rates of suggested K and P fertilizer for each field were determined by the levels of soil available K and P, respectively.

Table 1	Potassium and phosphorous fertility levels (0-15 cm depth) in long-term no-till fields*
Fertility Level	Low	Medium	High	Very High	Excessive
K Criterion (ppm)	<61	61-120	121-150	151-250	>250
K Required (kg K20/ha)	110-170	30-80	0	0	0
Fields Sampled (%)	2	44	41	11	2
P Criterion (ppm)	<10	10-20	21-30	31-60	>60
P Required (kg P205/ha)	70-110	20-50	20	0	0
Fields Sampled (%)	19	39	20	18	4
* Criterion and fertilizer requirement for K and P were based on the recommendation for corn (OMAFRA, 1997)

Management Factors
We observed that the intensity of K stratification increased with the rate of surface applied K20 or (and) the more often that soybeans were in the rotation, the more significant K stratification.

The survey was conducted on soil types ranging from sandy loams to clays. The stratification pattern occurred on all of the soil types examined, but since different soil types couldn’t be compared side-by-side it wouldn’t be fair to draw any conclusions about whether soil texture influenced the degree of stratification. Normally, you would expect faster downward movement of surface-applied K on sands than on clays. Downward movement is also affected by precipitation levels.

The most common application method for P and K in no-till systems was by broadcast application on the soil surface plus band application of starter fertilizer (usually high in P) for corn. For soybean and wheat crops, however, both P and K fertilizers were typically broadcast if applied at all. Most no-till drills in Ontario don’t have the capability of applying any fertilizer. In some cases, K was only applied before corn, even though soybeans usually remove more K from soil than grain corn. For instance, a 45 bu/acre soybean crop removes more than twice as much K from the field as a 120 bu/ac grain corn crop.

Broadcast application as the only system to supply essential nutrients in no-till is usually inefficient since only small proportions of surface applied K and P fertilizers may be available to crop for uptake. Both K and P readily bind to clay particles and, as a consequence, are relatively immobile (particularly on clay-textured soils).

The stratification of K and P in these no-till fields could be attributed to (a) no incorporation of surface-applied K and P fertilizers, (b) uneven extraction of soil K and P by crop roots and accumulation in above-ground plant parts, (c) crop residues that decompose at the soil surface, and (d) relative immobility of K and P in soil.

Recommendations
Some agronomists advise growers to build up soil K and P levels to very high levels before shifting to long-term no-till; others suggest plowing once every few years to uniformly distribute K and P through the plow layer. However, we believe that the best way to solve the problem lies in the improvement of nutrient management. Ideally, both P and K levels would be in at least the medium range when continuous no-till is first started in a particular field. However, once farmers have converted to no-till, we would be reluctant to recommend periodic tillage simply to incorporate K. Alternative fertilizer management strategies that we are currently investigating for K placement include deep placement in the row zone (with coulters or zone tillage tools), side banding alone, broadcast K application plus side-banding, and altering the depth of fertilizer placement occasionally. In the short-term, farmers should be prepared to apply more K20 fertilizer before soybeans, especially if the overall soil test K is below 120 ppm. Potassium fertility management becomes even more critical if soybeans make up more than 50 per cent of the rotation sequence.

Acknowledgements:
We thank all the cooperating farmers, the technicians involved in field sampling, and the OCPA for providing financial support.

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