As wet weather continues this season, the positions of tile lines are abundantly clear. Crops over those lines are darker, larger and more robust than crops between the lines. This variation illustrates differences between the presence and bioavailablity of nutrients.
A soil test measures the concentration of available nutrients per unit of soil. Many (arguably, too many) years ago, the relationship between crop yield and nutrient concentrations were quantified; these relationships continue to be the basis of modern recommendations.
However, adequate nutrient test levels do not guarantee availability throughout the season. Compaction and dry weather can limit root growth, nutrient solubility or even movement within the plant.
This year we can see how excessive moisture is depriving plants of the oxygen and energy they need to concentrate nutrients in roots. With the exception of well-drained sections over tile lines, plants are pale, stunted and starving.
Presumably these fields received fertilizer at either a straight rate or a variable rate that matched fertility with production goals. Either practice should have ensured enough nutrition for plants across the field. But while nutrients are present in adequate amounts, they are not bioavailable to plants lacking in oxygen and energy.
When soil-based nutrients fail due to excessive or limited moisture, foliar feeding bypasses the soil so that nutrients directly enter the plant. While these few pounds or ounces of nutrition don’t replace the hundreds of pounds of soil-based nutrients consumed over a growing season, they buy time until weather returns to more normal patterns.
While wet fields may not be accessible with foliar fertilizers, this is a great time to reflect on the relative advantages of foliar- and soil-based nutrition.