Do fertility agronomics change when one moves to a direct seeding program? The idea that fertilizer practices may require fine tuning when one changes from a conventional seeding system to a low disturbance seeding system was addressed at the Farm Technology Expo in Red Deer (Feb. 6 - 8/97). Tom Jensen and Jeff Shoenau presented some of the latest insights into this issue.
The simple answer to the fertility agronomics question is probably not. However less mobile nutrients may require some special attention. In low disturbance seeding, the goals are to retain the maximum amount of surface litter and to keep the entrenched standing stubble in place (the soil erosion eliminator). This means much of the plant decomposition will take place on the soil surface. For mobile nutrients like nitrogen, sulfur, boron and chloride, a reduction in tillage will have little effect on availability to the crop. For crop residues such as wheat and canola which have a high carbon to nitrogen ratio, maintaining the residues on the surface may actually increase the availability of nitrogen to the subsequent crop. This is particularly true if the nitrogen source is placed below the thatch layer (at seeding time) and not spread on the surface. If the nitrogen source is spread on the surface, a percentage of the nitrogen will be temporarily tied up in microbial decomposition of the plant material. This is similar to the nitrogen that would be tied up if the residues were mechanically plowed into the soil. However for plant residues like pulse crops, the carbon to nitrogen ratio is much tighter, which results in faster microbial breakdown. Also it may supply 10 to 20 lbs. of nitrogen to the subsequent crop.
For less mobile plant nutrients like phosphorous, potassium, copper and zinc, direct seeding can lead to the stratification of these nutrients in the soil profile. The few inches of surface material tend to be relatively higher in these less mobile nutrients than the subsequent 4 or 5 inches in depth, as compared to the plowed layer of a cultivated soil. In conventionally-tilled soil, all plant nutrients tend to be homogeneous throughout the plow layer because the plant residues are mixed uniformly within this layer. Research has found this stratification in direct seeding, but it is not known whether or not the concentration of less mobile nutrients near the surface is less available to crops. It appears the greatest disadvantage will occur under dry conditions, as the less mobile nutrients will remain stranded in the dry surface layer. However with direct seeding, good spring soil moisture coupled with shallow seeding may make these nutrients more available than anticipated. Where the stratification is deemed to be a concern, occasional tillage as part of a rotation may be beneficial to redistribute these less mobile nutrients from the enriched surface layer throughout the plow layer. The type of direct seeding opener one is using will likely be an integral part of any stratification problem which may exist. Future research will need to address types of openers when looking at nutrient stratification as there may be differences between the opener designs (ie. disc vs hoe opener).