Nutrients in plants

Soil qualities

Root-biological allies

Roots and their biological allies:

Roots imbibe water for the plant and take up dissolved minerals; however the longer the roots are in a given area the less effective they are at obtaining new nutrients as they deplete their local zones of absorption

Plants can increase their efficiencies by utilizing biological allies: fungal mycorrhizal and bacterial symbiotic associates.

Mycorrhizae

Plants have developed a symbiotic relationship with a number of fungi species in order to more efficiently take in water and mineral nutrients. Mycorrhizae or fungal extensions that grow into the root. also extend into the soil extracting water and phosphorous. This relationship evolved early in the evolution of the land plants, perhaps aiding the first plants with their minimal root systems on non-weathered rocky soils.

There are two general types of mycorrhizae: ectohyphae and endohyphae.

Ectohyphae	Endohyphae	Both
They remain outside the individual cells but grow through the intracellular spaces of the cortex. Mostly found in woody plants.	Enter into the individual cells. Once inside the cells they branch. However, the cellular membranes are not broken. Mostly found in herbaceous plants.	Hyphae are cheap to construct--they are basically hollow tubes. They can quickly grow to where the water is. They have high surface area and thus maximize absorption.

endomycorrhizae in cells

ectomycorrhizae surround cells

 
Root Nodules

Bacteria and other prokaryotes of root nodules are important for fixing nitrogen for the plant. Without these symbiotic organisms (cyanobacteria, eubacteria, actionomycetes) plants would never be able to draw enough nitrogen out of the soil. Root nodules which contain them are most common in those plants which are the first to colonize an area (i.e. alders and legumes). The prokaryotes obtain necessary nutrition and protection while the plant receive much needed fixed nitrogen.

The following series of reactions are completed under anaerobic conditions, otherwise oxygen atoms would oxidize the N. The large enzyme nitrogenase is responsible for the breakdown of N2 gas and subsequent reductions, an energetically expensive set of reactions.

Additionally, many other microbes are involved in N useabilty:

Organic N makes up about 5 percent of the soil organic matter (humus) by weight and about 98 percent of the total soil N. Although organic N is not available to plants, soil organisms convert a portion of it each year to inorganic forms (ammonium and nitrate) that are readily used by plants.In warm, moist soils with a pH above 5.0, the majority of ammonium N is converted to nitrate N by soil organisms rather quickly (within days). Therefore, most N taken up by plants is in the NO 3 - form, although NH 4 + is taken up when present in the soil solution. The nitrate ion (NO 3 - ) carries a negative charge which prevents its retention by the negatively- charged soil colloids. Since it is soluble and mobile, the nitrate ion is readily and easily available to plants.
Nitrate moves in the soil solution and can be leached below the plant root zone when soil moisture is excessive. The loss of nitrate by leaching is a common problem on coarse-textured, sandy soils

Related pages:

Nutrients in plants

Soil qualities

Root-biological allies