There are 3 different ways that plants uptake nutrients, mass flow, root interception and diffusion.
Root interception is relatively self explanatory, root hairs grow until they can reach ions and exchange then takes place.
In Mass Flow soluble ions move to the root with soil water.
The last mechanism in which plants get nutrients is called diffusion. Low solubility ions move by random thermal motion, and is very slow, approximately 1 inch per year. These ions move from areas of high concentration to areas of lower concentration.
Through root interception only a small fraction of nutrients is taken up by the plant.
In mass flow, dissolved nutrients move with water towards root surfaces where they are absorbed. Mass flow is a significant mechanism for the uptake of mobile nutrients, such as nitrogen, sulphur, Magnesium and Boron. Nutrient uptake by mass flow is reduced in dry conditions and at lower temperatures because the rate of transpirational water uptake is reduced.
Diffusion is the process by which nutrients spread from areas of high concentration to areas of low concentration. When roots absorb nutrients from soil solution the concentration of nutrients surrounding the root drops. As a result, nutrients in areas of higher concentration in soil solution migrate toward the root. Diffusion is an important process in crop uptake of P and K.
Phosphate exists in the soil water as one or both of the phosphate ions (HPO4, H2PO4). These are the phosphate ions absorbed by plants. Unlike nitrate (NO3-), however, they do not flow as mass flow because they interact with calcium, magnesium, iron and aluminum to form insoluble compounds. All of the phosphorus is not insoluble. There is always some that moves to the root surface by diffusion. In western Canada we have thousands of pounds of unavailable P per acre in most soils, tied to calcium and magnesium in alkaline soils and that bind changes to aluminum and iron in acidic soils. In What kind of soil extreme would P not be tied up? Extremely sandy soils.
This is a complicated section, the main thing to remember from this is that the plants get nutrients in different ways and applying all our fertilizer requirements into the soil at the beginning of the y ear is not only risky, quite frankly, its not the best for maximizing yields in any crop.
P is a critical nutrient for early growth and root development. One of our problems in western Canada is cool soils in the spring. As you can see from the graph soil temp has a huge impact on available P.
If you look at the lowest measurement at 10 degrees C, P availability is around 20%. I would suggest that when your soil is 10 degrees C, in most years you’ve probably got a lot of seed already in the ground.
What is the temp of the soil when you start seeding?
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Cation Exchange Capacity, (CEC), measures the soils ability to hold and release nutrients such as potassium, magnesium, and calcium, as well as other positively charged ions such as sodium and hydrogen. The CEC of a soil is dependent upon the amounts and types of clay minerals and organic matter present. Soils with high CEC will generally have higher levels of clay and organic matter. For example, one would expect soil with a silty, clay loam texture to have a considerably higher CEC than a sandy loam soil. Although high CEC soils can hold more nutrients, good soil management is required if these soils are to be more productive. It usually takes more fertilizer to correct a deficiency in high CEC soils as the size of bank is bigger. Another drawback to high pH soils.
Low CEC soils hold fewer nutrients, and will likely be subject to leaching of mobile nutrients. These soils may benefit from split applications of several nutrients. The particular CEC of a soil is neither good nor bad, but knowing it is a valuable management tool when deciding on your fertility strategy.
