Why does my lawn need applications of fertilizer?
Researchers who study the science of lawns generally agree that overall nutrient losses from turf are small when clippings are retained. Can grass clippings be used to set up a sustainable biological system? If nutrient loss is low when clippings are retained, does turf require annual fertilizer application in order to maintain plant stamina and growth?
Most plant nutrients, especially nitrogen and sulfur, are incorporated into the organic matter that is a component of soil. Microorganisms in the soil, which make nutrients more available to plants through their activity, require warm temperatures. The timing of this microbial activity is a poor match for the needs of turf grass. Most of our turf grass varieties are cool-season plants that develop root systems and absorb the majority of needed nutrients during the cool seasons of the year. In early spring, when turf grasses are rapidly growing and greening, soils are cold and the nutrient supply rate from organic matter is slow. That's why grass frequently shows hunger signs in the spring; soil temperatures are still too low to support vigorous microbial activity and nutrient release.
Later in the spring, when soils warm and microbial activity releases nutrients, the air temperatures often become so hot that leaf photorespiration drains off much of the plant's photosynthetic productivity. This reduced photosynthesis depletes roots of energy (especially if shoots are growing rapidly), and retards their growth and ability to absorb nutrients. The effect is a less efficient root system, poorer nutrient recovery and a general slowing of turf growth.
It is common for cool season turf grasses to lose more than half of their roots during the stressed summer months. When this is added to damage from insects feeding on turf roots and drought injury, it becomes easy to see why turf grasses are not very effective in absorbing nutrients during the summer, despite their increased availability.
Turf roots resume growth in the fall when air temperature cools and more energy is available through photosynthesis to support root growth. However, low temperatures in the fall season also cool the soil, causing a slowing in microbial activity and a reduced rate of nutrient release from soil's organic matter.
The root growth of the turf grasses that thrive in our cooler climate is not often synchronized with nutrient availability from the soil. This explains why turf can exhibit symptoms of nitrogen deficiency when growing in a soil high in organic matter. In addition to the clippings you leave on the lawn after mowing, an annual or semi-annual feeding with lawn fertilizer will ensure you have a thick lawn that looks healthy and provides a safe, cushioning surface for family activities.
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