Building organic matter is easy, right? Just zero till and leave the residue on the surface. Instant organic matter. It is not quite that easy. In order to keep nutrient cycles moving and keep the diversity of soil microbiology, one has to think about what kinds of matter are to be added to the soil and what amounts based on the seeding technology.
One of the quickest ways of increasing organic matter is forage production, especially if grazed or harvested for seed. This is true zero till for the duration of the stand life. All of the material is returned to the soil, and assuming the forage stand is managed for good biomass production, will gain organic matter.
On our lowest quality land, we rotated grazing corn through it, grazing for three years then rotating back into grain production for three years. In fifteen years, the soil organic matter levels went from under 2% to over 6% on a 15 centimeter (six inch) sample depth. The grain rotation included canola which has been replaced by cover crops now, winter triticale, and soybeans. The corn crops have averaged around 15 wet tonnes per acre or just over four dry tonnes per acre.
On the rest of the farm, there has been a large amount of oats and winter triticale grown on the land. These are high biomass crops with relatively low quality organic matter. By adding soybeans into the rotation, it has introduced a low biomass crop with high quality organic matter. Another strategy implemented on our farm is to include a short term alfalfa stand into our rotation. A fast regrowing variety is picked so the first cut is taken off for hay at the bud stage the end of June, then mow the second cut down as a green manure, leaving the regrowth stand through the winter. The bales give us a cash return, the green manured cut feeds the bacteria, and the lignified mature plants will feed our fungi.
In a conversation with Dr. Diane Knight, she said not to worry about targeting a certain microbe in particular when setting a strategy for soil improvement, just add organic matter and it will correct itself.
Increasing organic matter in the soil also decreases the bulk density of the soil which means the soil will weigh less per volume measure. Having a lower density means roots will be able to move through the soil easier, there will be more air spaces, and more places to sustain microbes. Seedbeds will be improved, with less lumps, and moisture will be more consistent in the rooting zone, making growth more predictable.
Increasing organic matter by 1% in the soil will add about 1,000 pounds of nitrogen per acre of which 2% is released to plants each year or about 20 pounds per acre. This will allow an extra 100,000 liters (26,000 US gallons) of water storage per acre, add an extra 4.5 pounds of P2O5 per acre, and improve root efficiency. To sustain the extra organic matter, the soil needs to have an extra 100 pounds of nitrogen in it to cycle.
Building organic matter by adding high quality residues from various crop types will help build bacterial and fungal populations in the soil, thus building soil health. Managing the buildup can be challenging due to trash management with your current seeding technology. That is where crop rotation really kicks in, especially by adding a perennial forage into rotation.