Mass flow is the movement of dissolved nutrients into a plant as the plant absorbs water for transpiration.  The process is responsible for most transport of nitrate, sulfate, calcium and magnesium.

Diffusion is the movement of nutrients to the root surface in response to a concentration gradient.  When nutrients are found in higher concentrations in one area than another, there is a net movement to the low-concentration area so that equilibrium is reached.  Thus, a high concentration in the soil solution and a low concentration at the root cause the nutrients to move to the root surface, where they can be taken up.  This is important for the transport of phosphorus and potassium.

Root interception occurs when growth of a root causes contact with soil colloids which contain nutrients.  The root then absorbs the nutrients.  It is an important mode of transport for calcium and magnesium, but in general is a minor pathway for nutrient transfer.

The actual pathway of nutrients into the root itself may be passive (no energy required; the nutrient enters with water) or active (energy required; the nutrient is moved into the root by a "carrier" molecule or ion). 

• High CEC means more nutrients are held on the soil, decreasing their mobility and uptake
• Low CEC means that more nutrients are in the soil solution, making them available to plants but also increasing the likelihood of leaching

CEC is defined by measurement of the amount of positively-charged ions (cations) which can be bound by a given weight of soil.  Cations bound on the soil surface can exchange places with cations in the soil solution, making them available to the plants and subjecting them to leaching.

Examples of cations include: K⁺, Ca²⁺, Mg²⁺, NH₄⁺, Cu²⁺, Fe²⁺ or Fe³⁺, Mn²⁺, Al³⁺, Zn²⁺

A larger CEC implies a greater capacity to retain K⁺, Ca²⁺, Mg²⁺, and NH₄⁺.  Soils with large CEC are typically high in clay minerals and soil organic matter (OM), which have a lot of negative charges.  CEC increases with pH, due to variable charge on the organic matter; the CEC measured at the pH of the soil is called the effective CEC.  The CEC is calculated from exchangeable cations, and is only seldom measured in a soil testing lab.

Low CEC means that fewer nutrients can be held by the soil, implying a need for more frequent nutrient additions. As CEC increases, more nutrients are attached to soil particles, and fewer remain in the soil solution.  Since the nutrients in soil solution are available to plants, this means that while there are plenty of nutrients in the soil, the plants may not be able to take advantage of them.  At the same time, they are less likely to leach.  Addition of cations to the soil, through acidification, liming, or fertilization, will release cations into the soil solution as the new cations swap places on the CEC.

As discussed in PO 4, nutrient mobility in the soil affects the ease of its uptake by plants, and the likelihood of its leaching through the soil.

Nitrogen mobility depends on the form it is in.  Nitrate (NO₃^-) is very mobile in soil water, and can be easily leached.  Ammonium (NH₄⁺) can be held on cation exchange sites, and is not susceptible to leaching.

Phosphorus is typically immobile in soil, unless soil levels rise above the soil's ability to bind it.

Calcium, magnesium, and potassium are considered immobile, since they are held onto cation exchange sites.  Only when excess cations are added to replace the nutrients' place on the CEC are these released.

Sulfur is commonly found in the anion sulfate form (SO₄^-), which does not bind to cation exchange sites and is thus mobile in most soils.

Texture is defined as the proportion of sand, silt, and clay in the soil.  High clay content increases CEC and thus the ability to hold nutrients, while high sand content decreases the CEC and nutrient holding capacity.  Sandy soils also have large pore spaces, allowing more leaching of nutrients. See the next page for a diagram of various soil texture characteristics. A diagram provided in the next few pages demonstrates the different components of soil texture.

Structure is defined as the arrangement of soil particles into aggregates.  Good structure is essential for water and nutrient movement, penetration, and retention.  Large spaces between aggregates allows soil water (and the nutrients dissolved therein) to move more freely, resulting in leaching losses.  Small or no spaces between aggregates, especially due to compaction, prevents water from moving through the soil profile, resulting in runoff.

Drainage and aeration have effects on nutrient loss and solubility.  Poorly-drained soils are poorly aerated, as they flood easily.  This promotes nitrogen loss through denitrification, while excessively-drained (sandy) soils promote leaching losses.  Flooded or very wet soils increase the solubility of minerals like iron and manganese.

Moisture is important for root growth and nutrient uptake.  Adequate moisture will improve uptake of nutrients by diffusion and root interaction, and will increase organic matter decomposition, which releases N, P, and S.  Low moisture can result in the formation of insoluble nutrient-containing compounds.

pH affects nutrient availability by changing the nutrient form.  For instance, the different forms of N (affected by pH) have different leaching capabilities; other nutrients may become adsorbed or desorbed, precipitated, mineralized, or immobilized at different pH values.  Many nutrients are more available in slightly acid soils: P is most available at a neutral pH (about 6.5); Mb is available at high pH and can be toxic to plants.  The pH is also important in N transformations, such as mineralization, nitrification, and N fixation, as the bacteria involved are pH-sensitive.

Temperature affects the plant's ability to grow, and thus affects nutrient uptake.  Ideal temperatures vary by plant species and cultivar.  The soil temperature also influences microbial activity, an important part of organic material decomposition.

Nitrogen is an essential and often growth-limiting plant nutrient. Crops take up and release N through a series of processes known as the Nitrogen Cycle.  N availability limits the productivity of most cropping systems in the US, and a deficiency in nitrogen leads to yield declines or even complete crop failure.  Excessive applications however may contribute to acid rain, destruction of the ozone layer in the stratosphere, the greenhouse effect, eutrophication of surface waters, contamination of ground water, and fish and other marine life kills, as well as blue baby syndrome in infants and amphibian mortality and deformations. The nitrate concentration in ground and surface waters is an important water-quality index; the U.S. Environmental Protection Agency (EPA) has set the Federal Standard for the maximum permitted amount of nitrate N in drinking water at 10 mg N/L or 43 mg NO₃^-/L.

It is important from both an economic and an environmental standpoint to manage N optimally. Thus, the two primary objectives of N management are:

1. To have adequate inorganic N available during the growing season
2. To minimize the availability of inorganic N during the fall, winter, or early spring, when N may be transported to surface and groundwater.

Fixation by clay refers to association of nitrogen with the soil.  Since the soil has a negative charge, the ammonium ion (NH₄⁺) can be bound to the soil particle.  Depending on the type of clay, this ion can be trapped in the actual structure of the clay mineral and become unavailable to plant uptake.

Ammonification and Mineralization is a process that converts organic N in manure, crop residues and soil organic matter to ammonia and ammonium.  Annual mineralization rates vary, though in general about 1.5-3.5% of the organic nitrogen in the soil will be mineralized each year.  The exact rates depend on soil temperature, moisture and aeration status; most rapid mineralization occurs in hot climates (68-95°F), well-aerated soils, and moist soils.  If large amounts of N-rich organic materials with narrow C:N ratios (<15-20) are added, significant levels of NH₄⁺ can be produced.  This will then be converted to nitrate via nitrification, absorbed by plants, fixed or held by the soil, or converted to ammonia and lost to the air via volatilization.  In NY, about 60-80 lbs N/acre is mineralized from soil OM each year

Nitrification is the process by which microbes use enzymes to convert ammonium (NH₄⁺) to nitrate (NO₃^-) to obtain energy.  It is a two step process, with a different species of bacteria performing each step.  Nitrate is most readily available to plants and is the preferred N form.  Nitrification is most rapid when soil is warm (67-86°F), moist, and well aerated (late May, June).  However, it will not occur when soil temperature drops below 41°F or goes above 122°F.  Because the process releases H⁺ ions, nitrification lowers soil pH.

Volatilization is the production and loss of ammonia gas from ammonium.  Ammonia volatilization increases with soil pH, as the high H⁺ concentration promotes the conversion of nitrate to ammonium.  Volatilization losses may be high for unincorporated urea fertilizer or manure (urine).  The high level of evaporation assists this loss.  Incorporation of manure and fertilizers can reduce ammonia losses by 25-75%.

Denitrification occurs when NO₃^- is converted into gaseous forms of N.  The process is common in poorly drained (anaerobic) soils, even those that are tile-drained, and in warm conditions.

Immobilization is the reverse of mineralization.  Microbes compete with crops for NH₄⁺ and NO₃^- for their own survival; when nitrogen is scarce the microbes convert inorganic N forms into their own organic forms, preventing plants from taking the N up.  This commonly occurs in aerated soils (as opposed to denitrification, which occurs in anaerobic soils), particularly with high carbon-to-nitrogen (C:N) ratio.  This happens when materials like straw, sawdust, etc. are incorporated.  Immobilization ties up available N in microbial tissue, which must be "re-mineralized" to become available to plants again.

Leaching is the loss of NO₃^- from the soil with water movement.  Since nitrate is an anion, it does not attach to soil particles and thus easily leaches from the soil.  Total losses are determined by water movement and nitrate contents of the soil.

Plant uptake occurs when nitrate is available and conditions are aerobic (i.e. not wet or flooded).

Symbiotic fixation is the conversion of N₂ from the atmosphere to plant protein.  Atmospheric N is fixed in a symbiotic process carried out by microorganisms, the Rhizobium bacteria which form root nodules in legumes.  This nitrogen becomes available when N fixers die.  The process requires energy and the enzyme nitrogenase (Fe, Mo, P, S), so if a plant-available N source is present, the crop will use that instead of fixing N from the air.

Soil conditions affect the degree to which all nitrogen cycle processes occur.  Fixation in particular is an important process for legume management, and is influenced by many soil properties.  As pH drops, fixation decreases; very little will occur below a pH of 5.0.  Excessive wetness or dryness will cause fixation to slow, and even to stop under severe drought conditions.  The population of correct Rhizobia species is essential, as different bacterial species require different plant hosts. Good fixation is dependent on correct inoculation of the legume seed with the appropriate Rhizobium.  The soil nitrogen level can influence fixation; as readily-available N from other sources increases (manure, fertilizers, biosolids, OM), the amount of N fixed will decrease.  Rhizobia are aerobic and require oxygen to function, so under wet conditions, N fixation will decrease.  High soil organic matter content will also cause a drop in N fixation, especially when coupled with high available N. 

• Minerals are provided to plants through soil content, plant residues, organic matter, and applied nutrients.  Soil characteristics such as texture, pH, and temperature affect the form and availability of nutrients.
• The nitrogen cycle is an example of how different soil conditions and processes change the form of N, which may increase or decrease the availability of the N to plants, and may contribute to environmental loss.