Any farmer will tell you that if the same crop is grown on a field year after year there will be a decline in yield. They know that as they harvest each crop, the soil loses some essential elements and the plants do not grow so well.
A fallow period can help to replenish soil nutrient stocks but usually an application of costly fertilizer is required. A big challenge is that only half the nutrients applied to a crop as fertilizer make it into the plant. Some of the nutrient sticks to the minerals in the soil, some is lost to the atmosphere (through a process called volatilization) and some leaks off the field through soil and surface water drainage. Many times farmers must apply at least two units of chemical fertilizer for every one unit that actually makes it into the plant.
One natural solution is to increase the amount of carbon in the soil. This is because carbon helps soil to hold on to nutrients and moisture, allowing plants to take up nutrients when they need them most.
Carbon in soil is also food for soil organisms like earthworms, mites, springtails and multitudes of smaller creatures. Many are beneficial as they mix and aerate the soil as well as help to break down the dead plant material. The addition of manure, retention of crop residues, and the use of mulch all add carbon to the soil and can slow down or even halt the declines in yield.
However, according to agricultural wisdom, adding carbon to soil from crop residues also encourages pathogens and pests, organisms that can damage future crops. So, in a fickle business where margins can be tight, farmers prefer to minimize risk. Most follow the standard practice and remove crop residues through fire or grazing by stock. It is hard for carbon to accumulate under such management.
So when the next crop needs nutrients the only way to supply them is through fertilizers. Yet it is cheaper, natural and more sustainable to let carbon help do this job. Because soil high in carbon also stays moist for longer after rain, has better structure for the growth of plant roots and exchanges nutrients more efficiently that the same soil with less carbon.
We all tend to avoid risk and our choices are based on what we perceive to be risky. Unfortunately our knowledge is incomplete and our perceptions. In this example we have opted out of using soil carbon to help maintain yields because of perceived disease risk.
There are two very good reasons why farmers will soon reassess this decision. One is that as climate conditions change so, in many places, the effects of moisture will become as important to crop yield as nutrients. The second is that soil is a very good place to store carbon that we no longer want in the atmosphere – we may even pay the farmer to help us store it. For the farmer adding carbon to the soil may soon help reduce risk to his crop and to his wallet.