Fertilization of gardens and landscapes typically begins in April after soils have warmed. With fertilization, one size (or fertilizer) does not fit all. A soil test, along with knowing plant needs, is the best way to determine nutrients needed.
Plants require 17 essential nutrients which includes carbon, hydrogen and oxygen provided by air and water. Of these, nitrogen, phosphorous and potassium (NPK) are needed in larger amounts and most fertilizers contain these macronutrients. Others, referred to as micronutrients, are needed in smaller amounts and usually available from healthy soils.
Using too much fertilizer is a waste of money and can damage plants or harm the environment, especially water quality. For example, too much nitrogen can burn plant roots or leaves, lead to excess leafy growth at the expense of fruiting in fruits and vegetables, and increase a plants susceptibility to insects.
Excess phosphorous or potassium can reduce a plants ability to take up micronutrients. Plants then become chlorotic and have pale green to yellow leaves with darker green veins. Phosphorous can end up in surface water via soil erosion and water run-off into streams or storm drains. One result of excess P in water is toxic blue-green algae.
The following will help provide a better understanding of macronutrients and fertilizer sources for when the fertilization season begins.
Nitrogen is mobile and moves readily through soil in water. If not taken up by plants, it moves below root zones becoming unavailable to plants. Since it is leached from soil, it is the nutrient most often applied at fertilization. Being mobile, it is best applied when plants are ready to use it such as after soils warm and roots are actively functioning, or at key times during the growing season.
Just before or just after planting annual plants (flowers and vegetables), or just as perennial plants (turfgrass and flowers) are greening up, is a key time to apply nitrogen. In sandier soils, more frequent applications of nitrogen at lower rates should be considered.
Phosphorous (P) and potassium (K) are not mobile in soil and need to be in close contact to roots to be taken up by plants. Because they are not mobile they are not readily leached from soil. If applied when there are already adequate levels, P and K toxicity can occur.
Many soil tests show adequate to high levels of P and K in landscapes. This is likely due to most fertilizers containing NPK and being applied without a soil test. And because we are not harvesting landscape plants and removing nutrients. When needed in vegetable gardens, they’re best applied just prior to planting and incorporated into soil where they will be in contact with plant roots.
Synthetic or organic fertilizers are sources of plant nutrients. Plants cannot tell whether nutrients come from a synthetic or organic source. Each has pros and cons.
Synthetic fertilizers are commercially made. They can provide quick delivery of nutrients (liquid) or release nutrients over a longer period (sulfur coated granules). They tend to be cheaper than organic fertilizer per pound; and determining application rates is easier since nutrient amounts are known. There is a higher risk of burning plants and greater potential for loss of nutrients to leaching or runoff.
Organic fertilizers like manure, compost, or fish emulsion are slow-release. Soil microorganisms need to transform them into nutrients before plants can take them up. It is more difficult to determine their nutrient content and application rates. They have lower burn potential and are less likely to be lost to the environment. They can contain weed seeds or human pathogens like E. coli (i.e. uncomposted manure).