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Fertilize for Optimum Economic Yields

Posted Nov. 1, 2009

With the variability of fertilizer prices, it is essential that fertilization schedules be updated and in harmony with optimal economic yield. As a rule, the profitability of a cropping system is dependent on fertilizer applications that are based on sound science and knowledge of the various production risks relative to the desired yield responses. Does this mean that we should increase the amount of fertilizer as long as it gives a little higher yield? No, increased fertilizer applications are not always the best way to increase profits.

Profitability usually results from establishing a realistic yield goal and using appropriate tools and technologies to achieve that goal. The ability to estimate a realistic yield goal, therefore, is critical to farm profitability and sustainability. Yields vary from field to field and year to year, making the establishment of a farm goal difficult. By understanding the variables that affect potential yield, we can begin to make better predictions and establish achievable goals.

Crop yield is dependent on the physical and chemical properties of soil, soil moisture, genetics (varieties), climate, crop management, and weed and pest control. Setting a yield goal based on all these factors is more realistic than relying on previous data. Fertilizer decisions made without a good understanding of these interactions may result in excessive or inadequate applications. Overly optimistic yield goals may result in excessive levels of fertilizer, contamination of surface and underground water, and unwarranted costs. Conversely, inadequate rates can result in low yields, increased costs of production and decreased profitability.

Physical properties such as soil texture, structure, particle density, bulk density, pore space and organic matter interact with fertilizer application and affect yield potential. Soil texture refers to the relative proportion of sand, silt and clay in the soil. It directly affects soil water-holding capacity and cation exchange capacity. Fine-textured soils are characterized by high percentages of clay, which enable a higher water-holding capacity as compared to sandy, coarse-textured soils. Medium-textured soils such as loams, sandy loams and silt loams are ideal for most crops.

Good air and water movement through the soil is critical to root growth. Soil structure refers to the presence of aggregates of soil particles that are bound together to form distinct shapes. This affects root growth, and water and air movement. The better the soil structure, the higher the productivity of soil.

Granular structure (rounded, spherical and lacking sharp cleavage planes), particularly in fine-textured soils, is ideal for air and water movement. Blocky structures, where the height and width of the structures are equal, have good air and water movement. Prismatic structures (height greater than width) result in poor air and water movement, because they swell when wet and shrink when dry. Platy structures (width greater than height) restrict air, water movement and root penetration because particles are tightly packed in the soil. Factors that will change soil structure are root growth, soil organisms, wetting and drying, freezing and thawing, and cultivation.

Particle density determines the dry soil weight and is measured by the volume of solids in soil. Bulk density is a measure of dry soil mass as compared to the soil volume. The main difference is that bulk density takes into consideration the volume of pores between soil particles. Particle density is a constant, while bulk density can be altered. For example, reduction of organic matter or destruction of soil structure will increase bulk density. Crop productivity decreases with an increase in bulk density, resulting from a reduction in pore volume and nutrient flow.

As you can see, many factors beyond fertilizer application have a significant effect on crop performance and the ability to predict yield.