Mechanisms of Nutrient Uptake

Plant uptake of nutrients can vary from nutrient to nutrient.  Some require plants to use energy for absorption and others are capable without.  To achieve an adequate amount of each, they may require different application approaches and a better understanding of how they are taken into the plant. 

Soil nutrient uptake into the plant occurs via three main avenues: root interception, diffusion and mass flow.  Root interception is the physical contact between roots and soil.  Nutrients such as calcium, magnesium, zinc, iron and manganese are absorbed through this pathway.  Promoting good soil structure, uniform application of nutrients and strong root structures all benefit root interception.  Uniformity is crucial to create the most surface area contact with the plant roots.  A very small amount, as little as one percent, of nutrient uptake is through root interception.

Diffusion is the movement of nutrients from a high concentration to a low concentration.  This is typically observed with a high soil solution concentration to the lower root surface.  The movement of nutrients is then ceased when an equilibrium has been met.  It does not require plant energy for uptake of diffused nutrients and is especially important for potassium and phosphorous uptake.  Placement of these two nutrients can rely heavily on tillage programs, soil types and environmental factors.  The soil test levels must be built up to adequate levels for proper diffusion to occur.  This can be important in no-till situations where surface applications will accumulate near the soil surface, but not near the root zone.  Banding partial recommendations near the root zone, especially in the earlier growth stages, can prove beneficial. 

The broadest category of nutrients utilized by the plant are taken in through a process called mass flow.  This is the movement of dissolved nutrients.  As the plant transpires, more soil solution is being brought into the plant through the root system.  Transpiration can have a large effect on nutrient uptake.  It requires soil moisture to regulate plant temperature, and water/nutrient transport.  Temperature, humidity, light and wind all play a part in the rate of transpiration.  Mass flow is responsible for nitrogen, potassium, calcium, magnesium, sulfur, boron, copper, iron, manganese, zinc and molybdenum uptake. 

Some plant nutrients will be taken up via multiple avenues.  Note that soil moisture is the key driving factor with 99% of soil nutrient uptake.  If soil test levels report adequate to the crop being grown, but it is experiencing low uptake (tissue) levels there are a few variables to consider.  Is there proper soil moisture, is pH limiting accessibility, root growth, environmental factors such as weather, and was the fertilizer placed in the right area for the plant to access it?