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

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.

1