In most native soils in our area, organic matter accounts for less than 1 percent of the total weight.
Organic material exists in several states in the soil. There are the organisms in the soil — bacteria, fungi, worms, insects, arthropods, mollusks and the like. There is also undecomposed plant debris: roots, buried stems and leaves. And there is the fully decomposed organic matter, humus.
When you do a standard soil test, amounts of organic matter are not measured. This is an additional test you can request and usually costs an extra $5 or so.
What does organic matter do for your soil?
Increases nutrient supply: Organic matter is a reservoir of nutrients that can be released to the soil. Each percent of organic matter in the soil releases 20 to 30 pounds of nitrogen, 4.5 to 6.6 pounds of P2O5, and 2 to 3 pounds of sulfur per year.
Increases water-holding capacity: Organic matter behaves somewhat like a sponge, with the ability to absorb and hold up to 90 percent of its weight in water. A great advantage of the water-holding capacity of organic matter is that the matter will release most of the water that it absorbs to plants.
Improves soil structure aggregation: Organic matter causes soil to clump and form soil aggregates, which improves soil structure. With better soil structure, permeability (infiltration of water through the soil) improves, in turn improving the soil's ability to take up and hold water.
Prevents erosion: Data indicates that increasing soil organic matter from 1 to 3 percent can reduce erosion 20 to 33 percent because of increased water infiltration and stable soil aggregate formation.
Increases soil organisms: The organic matter in soil is food for the majority of organisms in the soil. Increasing the food levels leads to an increase in soil-inhabiting organism populations.
Can improve productivity: There are even studies that show that for each 1 percent increase in organic matter, there can be as much as a 12 percent increase in productivity.
Many publications on soil organic matter and organic gardening recommend achieving a soil organic matter level of 5 percent. To achieve this, organic amendments must be added to the soil.
These include fresh manures, composted manures, plant-based compost and "green manure." For safety reasons, fresh manure is not recommended. Composted manures and plant-based composts are the easiest to use and have virtually no odor. "Green manure," or plants grown specifically to be tilled back into the soil, also make great amendments — especially legumes. Just remember, to achieve the benefits of green manure, the entire plant needs to be tilled into the soil.
When organic material is added to the soil, about 90 percent will break down and/or be utilized by your plants during the year. In other words, if you add 100 pounds of organic matter, only 10 pounds will remain in the soil at the end of the year.
Maintaining soil organic matter is an ongoing process that requires regular replenishment. General recommendations for increasing soil organic matter levels is to add 2-4 lbs. per square foot annually.
Once desired levels are attained, adding 1-2 pounds of organic matter per square foot annually is the recommendation in order to maintain the soil organic matter percentage.
It is a good idea to develop the habit of adding organic matter regularly between crops or when mulching. Increasing soil organic matter is a practice that will never go unrewarded.
My fig has spots on the leaves and it is beginning to lose its leaves. What should I do? — Jeremy
About this time of year every year, figs in our area become infected with a rust fungus (Cerotelium fici). There are no fungicides registered to treat this disease, but it is generally a late-season problem that is primarily cosmetic and will not kill or reduce your fig production. Just rake up and dispose of the infected leaves as they fall to reduce the reinfection and lessen the disease impact in subsequent years.