AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Fall 2001, No. 5
Phosphorus is especially critical early in the season and during grain filling. The young crop’s limited root system may not be able to supply enough phosphorus unless an adequate supply is available because of high soil fertility or as a result of specific placement, such as a starter band or strip fertilization. During the rapid growth stages, there may also be short-term phosphorus deficiencies when demand exceeds the ability of the soil to supply phosphorus. If these occur during grain filling, yield potential is reduced. Maintaining a high soil phosphorus test level is the best defense against such deficiencies.
Potassium is responsible for maintaining cell turgor and for cell expansion. It regulates movement of water into and through the plant. Shortage of potassium is especially critical in times of moisture stress, because the stomates that regulate water loss from leaves and carbon dioxide movement into the leaves become leaky if potassium is inadequate. Maintaining high soil potassium test is the best way to ensure an adequate supply to the crop throughout the growing season. Over the past 10 years, use of potassium fertilizer has not kept up with removal by major crops. Deficiency symptoms have become more common in the field. Especially in reduced tillage systems, potassium from lower in the soil profile is depleted as it is taken up by plant roots and then deposited on the surface as the plant dies. Potassium moves very slowly in the soil, so the lower profile areas are not easily replenished. Soil test levels at the surface increase, so the need for potassium fertilizer may not be detected by traditional soil sampling. In cotton, the development of late-season potassium deficiencies in the upper leaves has been shown to result from depletion of lower root zone potassium supplies. A similar problem is developing in some areas of the Midwest in corn and soybean crops where potassium fertilization has not kept up with removal from lower in the root zone. In severe cases, deficiency symptoms are visible, but the impact on yield comes long before visible symptoms occur.
Nitrogen cannot be readily maintained in most soils, so it must be applied each season for responsive crops. The interactions among nitrogen, phosphorus and potassium, are critical to optimum crop growth and yield. Efficiency in nitrogen use is improved by maintaining high phosphorus and potassium soil test levels. Any condition that reduces water availability also adds to nitrogen deficiency because nitrogen is transported to and into the plant along with water movement.
Other stress situations may make the crop more susceptible to stress later in the season. Compaction, for example, provides physical restrictions to root growth and water infiltration. Both set up a potential for drought stress later in the season. They also limit potential efficiency in nutrient uptake by roots. Field activities when the soil is too wet are a major cause of compaction. Appropriate tillage is the best way to remove it. In no-till systems, compaction is probably unavoidable, but effects can be reduced over time as channels form in the soil due to root growth, natural cracking, and biological activity such as earthworms.
Nutrients are important to minimizing impact of environmental stresses. A crop with well-balanced nutrition will be much more resistant to other stresses. Crops with nutrient deficiencies will be more susceptible to drought, disease, insect injury, and other stress factors. Stresses from insects and disease cannot be controlled by fertilizer, but the impact of these stresses on yield can be greatly reduced if the crop has a non-limiting supply of nutrients, so that its resistance is high and its ability to recover from stress is enhanced.