Hope all of you had a Merry Christmas, Happy New Year, and have been able to dig out from the snow storms. In Fertilizing Hay Crops Part 2 the five sources of nutrients for hay crops were discussed. From these five sources nutrients can move into the plant roots through the soil or into the above ground portion of the plants through leaves and shoots. In the soil they are three routes that nutrients travel to reach the plants roots: root interception, mass flow, and diffusion.

The crops that farmers grow are made up of mostly carbon, oxygen, and hydrogen. The carbon comes from the CO2 in the air and the hydrogen and the oxygen mostly come from water. Almost all nutrient movement into the plant is dependent on water in one of many ways. Above and below ground, the nutrients must become soluble (suspended) in water before moving into the plant. Once the nutrients become soluble they can take one or more of the different paths to the plant.

Most crop fertility research and recommendations are based on the three pathways through the soil solution. Figure 1 below shows the proportions of these paths that 12 nutrients move through the soil to a corn plant’s roots. While corn is not alfalfa, the chemistry and proportions of nutrient flows through these paths will be similar to alfalfa and other forage crops.

Root interception of nutrients occurs as the roots grow in the soil. These nutrients are essentially “run over” by the roots and taken up when the roots come into contact with them in the soil solution. For anyone who has seen a bad movie where a blob-like monster absorbs everything in its path it’s easy to picture this. On more serious note, nearly 40% of the calcium (Ca) taken up by the plant is dependent on this pathway as shown by Figure 1. When combined with appropriately timed sub-soiling and/or deep plowing, calcium applications have the potential to increased the depth and volume of root growth and improve soil drainage. Surface applications are also effective over a longer period of time and are usually made more frequently in no-till and reduced tillage systems. This can be confusing because when farmers hear calcium, they think lime, and when they hear lime they think raising the soil pH. Lime application does raise pH, but it’s the carbonate (CO₃^2-) that binds with the soil acids (H⁺) to raise the pH of an acid soil, not the calcium (Ca²⁺). Calcium’s role in plant nutrition will be discussed more in-depth in a future article.

Most nutrients reach the roots plants through the mass flow of dissolved nutrients in the soil water that is flowing towards the roots. This flow of water is driven by the transpiration rate of water out of the plants leaves. This is part of the reason plants can grow really fast in warm weather.  An easy way to think of this mechanism is a vacuum cleaner. When the sun is up and the plants are taking in CO₂, the vacuum is on and the water is pulled into the roots and out the leaves. The pictures in Figure 2 below demonstrate this well. As the water moves through the plant the nutrients are absorbed.

From the website http://www.fijiaquifer.com/transpiration.htm

Besides the mass flow vacuum and direct root contact plants have another below ground mechanism to gather many of the nutrients needed for plant growth. That mechanism is diffusion, the flow of nutrients from higher concentration in the soil solution to lower concentration at the root. Many people can recall an elementary science class where food coloring is added to water and eventually colors the whole container-this is diffusion. Phosphorus (P) and Potassium (K) are very dependent on this type of chemistry as shown by Figure 1 above. Their dependence on diffusion and their medium (for P) and high (for K) requirements in total amounts of P and K are two reasons why these are the nutrients most often applied to alfalfa fields and there’s a response in yield. When the concentration of the nutrients in one part of the soil is increased it will diffuse to an area of lower concentration, hopefully near the plants roots.

Next up Part 4: Nutrient Movement Into the Plant: Foliar Application Chemistry and Best Management Practices. Post a comment below or send an email to bverbeten@gmail.com if you have a specific interest or question on this topic.