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.Maintain Soil Fertility Now to Avoid Intensive Intervention Later
According to Michigan State, 200 bushels of corn removes 70 pounds P2O5and 54 pounds K2O. 60 bushels of soybean removes 53 pounds P2O5and 84 pounds K2O. How will this reduce soil test levels?
200 bushels of corn reduces the soil P2O5test level by about 8 pounds and the K2O test level by over 13 pounds. 60 bushels of soybeans reduces soil P2O5almost 6 pounds and soil K2O over 9 pounds. These numbers are based on the old rule that adding/removing 9 pounds of P2O5will increase/decrease the soil P2O5test level by 1 lb, and adding/removing 4 pounds of K2O will increase/decrease the soil K2O test level by 1 lb.
To replace P and K removed by corn, we need 134 pounds MAP (11-52-0) and 90 pounds potash (0-0-60). Over 100 pounds MAP and 140 pounds potash is needed to replace P and K removed by soybeans.
Actually, these ratios change with soil test levels. According to the University of Kentucky, the 9:1 ratio of fertilizer P2O5to soil P2O5test level is accurate around soil test levels of 30 lb/ac. It takes over 14 pounds P2O5to build the soil P2O5by 1 pound when the soil test level recedes below 15.
In other words, it takes 50 percent more P2O5to build a soil test level by 1 pound P2O5in infertile soil than in fertile soil. This greater ratio also applies to nutrient removal: soil test levels will decrease 50 percent faster with P2O5removal in infertile soils.
Similarly, the 4:1 ratio of fertilizer K2O to soil test K2O is most appropriate to soil tests levels around 275 pounds/ac K2O. Soils testing around 175 lb/ac K2O require 5 pounds/ac K2O fertilizer — 20 percent more — to raise the soil test level 1 pound. Test levels in infertile soils also decrease 20 percent faster with K2O removal.
Also note that potassium gets fixed, or trapped, in clay soils, so as potassium levels decrease, more sites become vacant to trap new potassium. Therefore, K2O requirements increase even faster in these soils as test levels decline.
These results have important implications for soil fertility management: as soil test levels of P and K decrease, it will take more fertilizer per pound soil test value to repair them.The Importance of Plant Tissue Testing
Why invest in tissue sampling when soil sampling is cheaper, easy to perform along a grid and the basis of variable rate applications?
Soil samples tell you the concentration of nutrients in the soil. However, nutrient “concentration” is not the equivalent of nutrient “availability.” What is in the soil may not be available to your plant. Why not?
Compact, cold or wet soils may limit root growth. Dry soils or chemical interactions may reduce nutrient solubility. Insect damage to roots and leaves can limit uptake and transport. Negatively charged anions in the soil with the exception of phosphorous can move out of the root zone especially in soybeans with excessive rain event. Critical anions in the soil include sulfur and boron. Boron can be addressed easily with foliar materials. We can adjust sulfur somewhat with foliar but most focus should be on soil applications.
Agronomists also like to look at specific ratios of selected nutrients in plants. Specifically, we like to compare levels of nitrogen to sulfur as the corn plant uses N+S at a specific ratio of 10:1 at the cellular level and we like to see the N:K ratio at a 1.4/1.6:1 level.
Agronomists use term “bioavailability” to describe the actual nutrient availability to living plants. The only way to quantify this is to measure the plant itself through tissue sampling.
Sunrise Cooperative uses local laboratories to provide real time results from tissue samples. Results are often implemented in time to combine foliar feeding with post herbicide and /or fungicide applications. Results are presented in a customized report that describes the availability and significance of each nutrient and if/how our basic foliar products should be augmented with additional nutrients.