University of Illinois Extension

Organic Matter and Soil Fertility

Organic matter plays an import role in our gardens, said Richard Hentschel, Extension horticulture specialist.

"This is true in terms of the available nutrient for plant growth and quality vegetable and fruit growth - healthy vegetables with lots of fruit or a shrub that has good bloom, berries or seed pods and fall color," said Hentschel. "Most of the nutrients required by green plants are in the soil in abundant quantities. The rest come from the air, soil moisture and rain water.

"The elements that should be of the most concern to use are those used by the plants in the greatest quantity; nitrogen, phosphorus and potassium. These are often noted as macro nutrients in books and literature."

When nitrogen is limited, the plants growth is stunted, off color and can have smaller leaves than normal.

"Even though organic matter releases nitrogen continuously during the season, additional nitrogen can be given to the plants during their period of maximum vegetative or reproductive stages, after flowers have formed" he said. "This can be accomplished by the addition of composted manure, dried blood, and fish emulsion or cottonseed meal. These additional materials need to be applied at planting time to guarantee the release of nutrients at the proper time. This is the one nutrient that may need to be applied using a commercial inorganic fertilizer source to keep up with the nutritional demands of the plant for a short period of time."

Phosphorus may be lacking in sufficient quantity for quick growing vegetables. Soils with pH levels of 5.5 or lower tend to tie up the phosphorus in a form unavailable to the plants roots.

"Organic materials such as steamed bone meal, finely ground rock phosphate and fresh manure can be added," he said. "Warm season vegetables are often impacted during cool wet weather that can be seen by the red or purplish color in the leaves."

Potassium is usually in sufficient supply if the garden or flower beds have been receiving annual applications of composted organic matter or manure or have had commercial inorganic fertilizers applied. Gardener's can apply fresh manure or wood ash to the soil.

"Organic matter is Mother Natures' slow release fertilizer," Hentschel noted.

"But more than that organic matter provides the soil with the right components to build soil structure, tilth and friability of the soil, something that inorganic fertilizers really cannot do. Organic matter also provides those other lesser used nutrients called micro nutrients, think one a day vitamins for plants."

Organic matter also will help sandy soils hold more water and nutrients and will aid the ability of a heavy clay soil to drain excessive soil moisture by adding porosity. To be effective in supplying all the nutrients a plant will need, applications of organic matter need to be done annually.

"There are concerns today for our environment that gardeners do not over fertilize, increasing the chance of ground water contamination or causing algae blooms in our retention ponds, streams and creeks," he said. "Gardeners should have a soil test done to determine which nutrients if any need to be built up in our soils. Most often, if the pH or our soils is between 6.0- 7.0, then the plants will have plenty of food to grow.

"Those exceptions may be at transplant time for vegetables or when transplanting or planting young trees or shrubs. Additional nutrients are typically added then in a transplant solution for quick absorption by the plants roots and only applied to the planting area itself."

Gardeners should consider composting as a way to provide that organic matter to their beds in the home landscape, he noted.

"Gardeners need to understand that it will take several growing seasons of applying composts and organic matter before the beds become nutritionally self-sufficient and that making applications annually is the best way to maintain those nutrient levels in the soil" Hentschel said.