Less tillage means more benefits
It’s hard to explain why less tillage is better, without defining and describing some terms and their meanings. There are a number of things one needs to understand before discussing the impact of tillage on the root environment.
Plant growth is affected tremendously by the physical characteristics of soil — the types and characteristics of the pores and solids. Pores are especially important. Roots must grow in existing pores, plant-available water is stored in pores, excess water drains through pores, nutrients move to the plant roots in pore solution, and carbon dioxide and other toxic gases escape while oxygen and other beneficial gases enter via the pores.
Organisms and their activities are fundamental to soils. The life and death of plants is the basis for organic matter, which largely impacts the physical and chemical suitability of a soil for crop growth. For example, living roots improve soil porosity because young roots extend into micropores (small pores) and then expand laterally to create macropores (large pores) as they extend. Simultaneously, the roots release exudates that cause soil particles to release nutrients essential for continued root growth. This mineral breakdown contributes to pore enlargement. Finally, the exudates help glue together mineral particles into soil aggregates.
The exudates from the roots and the microorganisms bind individual soil particles (sand, silt and clay) together to create aggregates. This binding of particles into aggregates is what allows pore space in soils. If all soil particles were separate and not glued together, there would be nearly no pore space, very little holding of water, limited room for root growth and limited exchange of gases. This would be a very restrictive environment for roots, and crop production would be extremely limited. Non-aggregated sand would have only large pore space, which would not be able to hold water.
How tillage interferes
An interconnected network of pores associated with loosely packed, crumbly, highly aggregated soils allows rapid infiltration and easy movement of both water and air through the soil and provides habitat for soil organisms.
Tillage breaks apart the soil aggregates and collapses the larger pores. Then you end up with not enough large pores. Heavy equipment can collapse the larger pores also. This happens in varying degrees. Each tillage pass breaks some aggregates and collapses some pores. Each time compaction occurs, the same two things happen. So soil can be anywhere from nicely aggregated with good pore space to limited aggregation and limited pore space.
Even with limited pore space, soil will range from a nice balance of pore size down to mostly only small pore size. This is why limiting tillage, postponing tillage until one to two days after the soil moisture is down to field capacity (field capacity is still too wet for most soils), and growing cover crops all contribute to a better environment for cash crop roots.
Field capacity is when all the water that’s going to drain by gravity has drained, and the water left in the soil is what is held there in the smaller pores. Most of this water is available to the roots. As the layer of water in the pores becomes thinner and thinner, eventually what is left is being held by the soil so tightly that the roots cannot take up enough. This is called the wilting point. At this point most water is held on particle surfaces and pores are empty.
So, in poorly aggregated soil, you reach the wilting point much sooner during a dry spell than in soil with good structure.
Probably more important is that with good soil structure, you get greater root growth with greater soil profile exploration for water and nutrients. Roots find more water because they are able to explore more soil. The nutrients enter the root only in solution. So finding more water means finding more nutrients, too.
The amount of organic matter in a soil reflects a long-term balance between organic matter additions and losses. The additions come from new growth and its incorporation into soil. Mechanisms of incorporation include direct ones like root growth, as well as indirect ones such as when leaf litter is taken underground by soil fauna such as worms and ants or by tillage.
The losses derive from decomposition and erosion. Rates of decomposition are largely controlled by the same factors controlling new growth (temperature, availability of oxygen, water and nutrients).
Less loss of organic matter
Tillage also introduces air that has oxygen, which oxidizes, or burns up, organic matter. OM supplies nutrients to plants, helps soil hold water, helps macropores form and is something you want to conserve and not lose over time. If you tested for organic matter in a long-term pasture or in old fence lines, you find higher OM content than in the tilled part of the field nearby. With less tillage, there’s less loss of organic matter.
Tillage leaves soil particles exposed to the forces of wind and water. Transported by wind and water, detached soil particles leave the field or settle into pores within the field causing surface sealing and reduced infiltration. When this happens, less water is available to plants, and runoff and erosion increases.
Over time with use of no-till, or at least minimum tillage, and cover crops, the OM will increase and along will come better soil structure, better infiltration of rain, better water-holding capacity and more resiliency to withstand long periods without rain. In other words, soils with good structure and good levels of OM won’t reach the wilting point as soon as fields with less positive soil structure and lower amounts of OM.
Healthy soils, those with high organic matter, high microbial activity, stable structure and high water infiltration rates, will produce higher yields long term. Cover crops are one practice that helps produce such soils for corn and soybean farmers.
Johnson is an ISU Extension field agronomist covering central Iowa. Contact him at firstname.lastname@example.org.
• Integrated Agronomics Newsletter by Mark Johnson from Dec. 30, 2014. Search for No. 0227 at extension.iastate.edu/ag/newsletters-integrated-agronomics.
• “Maximizing Yield Potential by Optimizing Soil Management Practices,” by Mahdi Al-Kaisi, ISU department of agronomy, and Mark Hanna, department ag and biosystems engineering, at crops.extension.iastate.edu.
• “Do You Know How Water Moves Through Soil?” at YouTube, t.co/mq7YWcrlJA.
This article published in the January, 2016 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2016.
Field Conservation Maintenance/Practices