As noted in colleague Chad Kruger’s informative posts about soil carbon sequestration (1
), it takes a long time to reap the benefits of building up soil organic matter. There is, however, a quick way to improve the function of your soil; keep it covered with crop residues.
Crop residues are those parts of a crop left in the field after harvest. Where much of the crop is harvested, as with hay, residue cover will be light, but high residue crops like corn and wheat leave a mulch that covers the soil completely. For much of agriculture’s history, it was common to look at this residue as something to get rid of, either by removing it from the field, by burning it, or by tilling it into the soil so it would decompose more rapidly. However, left covering the soil surface, this residue can be the source of many benefits, even within a single season.
First, residue reduces evaporation by shading the soil and limiting airflow to the soil surface. Evaporation can be 20-30% of the water used by an irrigated corn crop, and whether that means a water savings or a yield increase, the value is significant. Under irrigated conditions, residue cover allows crops to utilize irrigation water more efficiently, using less water per unit of yield, producing more crop per drop. Where water supply is a limiting factor, such as in Eastern Washington, or for non-irrigated summer crops in Western Washington, the saved water can be used by the crop to increase yield.
Maintaining residue cover will usually require reduced tillage, with the highest residue levels maintained with direct seeding or no-till planting. Because tillage tends to dry out the soil, reducing tillage passes saves additional soil water. Research in California, Nebraska, and Kansas has shown that the combination of residue cover and direct seeding can save 3.3-6.5” of water per season under irrigated conditions. Depending on the cost of water and power for pumping, this can result in savings of $9-40 per acre.
In addition, residue on the soil surface prevents soil crusting and so increases water infiltration rates. This, in turn, reduces runoff and associated water erosion. Other benefits are decreased wind erosion, and better conditions for soil microbes and earthworms. Although these benefits are all available as soon as a residue mulch is established, optimal use of crop residues is not without challenges.
Planting through all that residue requires careful management of the residue – it should be spread as uniformly as possible. Planting equipment will either have to be modified, as in the case of most planters, or purchased, as in the case of no-till drills.
There are also trade-offs to consider. Wetter soil in the spring may not be a problem in the arid Columbia Basin, but can prevent access to fields in wetter areas. Wet soils also warm more slowly, which can slow crop growth in the spring, but be a benefit later when it gets hot.
Where does this fit in moving towards a more sustainable agriculture? While high residue farming systems (those that maintain a mulch over the soil) are highly developed in some regions, they are relatively new in the irrigated, vegetable-producing regions of the Western US. In these regions, continuous no-till is impossible due to crops like potatoes and onions where harvest is a major tillage operation. High Plains no-till guru Dwayne Beck argues that we need to figure out how to produce these crops without tillage. This may or may not be possible. If not possible, the required tillage may preclude many of the long-term benefits from building up soil organic matter. However, maintaining residue cover on the soil in crops between these vegetables could still provide many benefits to farmers and the soil whenever it is practical to do so.