Soil Structure - Tillage, no-till, and that elusive "tilth"

Well, as you might be able to tell from the title, this is an article on soils, not manure. What's up with that? Well, the discussion of soil structure in relation to tillage and no-till will focus on what it means for us in terms of our manure management and utilization decisions, and how it can impact nutrient export from our fields. This week we are focusing on basics, so hopefully in the future we can chat about them in a bit more detail and what we can do to build our soil's tilth and manure's role in that.

Soils can retain water for substantial periods of time. Despite the incessant pull of gravity, water entering the soil surface by rainfall or irrigation stays in the upper zone long enough for plant roots to extract what they need to survive. Water is held in such a manner that under normal conditions gases can also move through air spaces, allowing oxygen to reach plant roots and maintaining aerobic conditions. While doing all this it can hold and eventually release some crop nutrients, holds our plants in place, and helps in the breakdown of old organic residues. In this way, soil is much more than just a water storage reservoir, it is an ideal growth medium for plants.

The two most important characteristics of the soil water phase are the amount of water in a given amount of soil and the forces holding water in the soil. Many processes (gas exchange with the atmosphere, diffusion of nutrients to plant roots, soil temperature, microbial activity) are controlled by the amount of water in the soil, while others (efficiency of water absorption by plant roots, amount of drainage occurring, and the extend of movement of water and solutes) are influenced by the forces exerted on water.

Figure 1. Recently tilled soil, not it has fluffy aggregates.

Figure 2. Tilled soil after it is consolidated and crusted after several rainfalls.

Above you can see two different pictures of soils and soil pores.  The photo on top (figure 1) shows a field that has been recently tilled. In this case you can see that the soil is fluffy and pores are well connected, this allows water to more easily move through the soil. You might also note that the tillage has reduced the size of the soil aggregates. This means that if runoff was to occur these particles may be susceptible to erosion since they are smaller and not stuck together. This is in contrast to the soil on the bottom (figure 2). This soil is what a tilled soil may look like after a few rainfall events. You can see the surface aggregates have broken apart from the rainwater crashing on them and a crust has developed. This crust makes a layer where it is more difficult for water to enter the soil which may promote more soil runoff. You can also see that the soil is more consolidated with generally smaller pores.

Thinking about your crop production fields, this might give you some insight into why maintaining surface residue can do a lot to reduce your erosion. The soil particles at the surface are no longer subjected to the pounding forces of the raindrops. Instead, the residue can protect the soil aggregates. This keeps the soil particles bigger and less available for transport in runoff water and also reduces the opportunity for the soil to form a crust since the aggregates weren't broken apart and reformed in a continuous layer.

Take home message - the condition of the soil plays a key role in its ability to retain water and for water to move through the soil, all pores and aggregates are not created equal, and the method we use to apply our manure and how it is protected from these aggregates plays a key role in how manure nutrients can move through soil.