Soil Fertility

Fertility and Forest Soil

Forest managers generally take a passive rather than an active approach to managing forest soil fertility

In other words, unlike agriculture, forest managers do not generally amend (ie fertilize) forest lands

This does not mean that soil fertility is not of great importance however

Fertility and Forest Soil

For most forestry operations the management of soil fertility involves:– recognizing the differences between sites

in terms of fertility, and especially nutrient holding capacity

– maintaining fertility through the use of appropriate forest harvesting and silvicultural treatments

Fertility and Forest Soil

In intensive forest management operations the management of soil fertility may involves:– creating optimal growing media for the production

of greenhouse container stock– measuring and amending the fertility of forest

sites for uses such as bareroot nurseries, and seed orchards

– designing specific treatments to enhance fertility (ie peatland drainage)

Soil Macro and Micro Nutrients

Soils require various elements in order to conduct the processes of food manufacturing (photosynthesis), growth and differentiation of cells, cellular respiration

Macro-Nutrients are the elements that are required in the greatest amount by plants

Micro-Nutrients are essential but are required in smaller amounts

Macro Nutrients

– MACRO NUTRIENTS (primary) OFTEN LIMITING FOR PLANT GROWTH

• Nitrogen (N)

• Phosphorous (P)

• Potassium (K)

– MACRO NUTRIENTS (secondary) USUALLY NOT LIMITING FOR PLANT GROWTH

• Calcium

• Magnesium (Mg)

• Sulfur

Micro-Nutrients

Micro-Nutrients occasionally limiting for forest growth– Copper, Iron, Manganese, Boron, Zinc

Micro-Nutrients with no reported deficiencies for forest growth– Chlorine, Cobalt, Silicon, Sodium,

Vanadium

Roles of Macro-Nutrients Nitrogen

– building proteins– critical to photosynthesis– promotes lush vegetative growth

– Available to the plants in cation form as NH4+ or as an

anion as NO3-

– Nitrogen Fertilizers• Ammonium Nitrate (NH4NO3), 34-0-0• Ammonium Sulphate (NH4)2SO4), 21-0-0-24S• Urea (CO(NH2)2), 46-0-0 • Organic – manure

Roles of Macro-Nutrients Phosphorous

– critical to cellular respiration and energy transfer– promotes rooting

– Available to plants as a soluable anion PO43-

– Fertilizers are derived from rock phosphate treated with acid

– Called superphospate, triple super phosphate etc. (Chem analysis is P2O5)

– Manure is also phosphate rich

Roles of Macro-Nutrients

Potassium– Acts as a catalyst in cellular respiration– Available to plants as a soluable cation K+

– Original source is mineral potash– Most common form of fertilizer is Muriate of

Potash or KCl– Wood ash is an organic form of pottasium

fertilizer

Critical Limits and Luxury Consumption At a certain point the availability of a single

nutrient element may be critical In other words there is simply not enough

available to permit plant survival and growth

In certain soils an abundance of certain nutrient elements exist in a state of luxury of consumption where far more nutrient is available that the plant can ever utilize

Concept of Optimal Levels

Between the critical limit and the point of luxury consumption exists a range in which plants will survive and grow

A theoretical optimal level of a specific element can be calculated (experimentally) for a specific plant (crop)

Concept of the Limiting Factor

The ability of a plant to benefit from increased supply of a certain nutrient element or other factor (ie light, moisture, warmth) is based on the interaction of the supply of all other elements and factors

The factor which is in shortest supply is deemed to be the Limiting Factor

Concept of Limiting Factor

Analogy; man needs food, water, shelter to live; if you are on the desert, with a tent but there is no water; you are going to suffer and die even if someone builds you a house or gives you more food.

Concept of Limiting Factor

Plants growth, and vigor will be adversely affected by the element/factor that has lowest availability vs demand; additions of other elements beyond the optimal level will not improve the growth or vigor of the plants.

Nutrient Cycling

Sources of Nutrient Elements– atmospheric content (nitrogen, oxygen,

hydrogen, carbon)– geological content (phosphorous, calcium,

potassium and other nutrients except Nitrogen)

– organic content; decomposition, throughfall

Nutrient Cycling

Available Nutrients– Only nutrients in solution or capable of moving

into solution are available to plants– Elemental Nitrogen is the source of all nitrogen

for plant growth but is unusable by most plants– Certain plants, (legumes), bacteria, are able to to convert

nitrogen to usable nitrate or ammonium– Also lightning changes atmospheric nitrogen into a

soluable form which reach the earth through precipitation– Chemical fixation; production of fertilizers

Nutrient Cycling

Nutrient storage– In Clays

• The nutrient holding capacity of a mineral soil is directly proportional to the amount of clay in the sample

• Clay is formed of layers of oxygen and silicon atoms (silicates); various cations (positively charged elements) can adhere to negatively charged sites on the broken edges of clay particles

Nutrient Cycling

Nutrient storage– In Organic Material

• Organic Material is important for its role in – releasing nutrients back into the nutrient cycle

– storing nutrient cations

– holding moisture

– The combined capacity of clay and organic combines to store cations (positively charged nutrient elements) is termed the soils cation exchange capacity

Nutrient Cycling

Nutrient storage– Lots of nutrients are stored in the living

plants in an ecosystem– The relative amount, compared to other

storehouses, is important when considering the impact of forest harvesting.

Nutrient Cycling

Nutrient Losses– Crop Removals (tree harvesting)– deep leaching and surface runoff– volatilization (forest fire)

Nutrient Cycling Nitrogen Cycle (example)

– Original source is atmospheric nitrogen– Atmospheric nitrogen is converted into soluable nitrates

through electrical storms

– N2 is converted to NH4 (fixation) and then to NO3

(nitrification), which is the plant soluble form, primarily by the action of bacteria (also some plants i.e. legumes)

– Nitrogen is taken up and stored in plant structures and then released back to the ecosystem; nitrogen can also be lost back to the atmosphere through volatilization

Carbon/Nitrogen Ratio

Nitrogen is extremely important to the survival and growth of plants

Carbon is a basic building block of plants but it is brought in from the atmosphere

However – In order for Nitrogen to be available for the uptake of plants there is a critical balance in the soil known as the carbon/nitrogen ration

Carbon/Nitrogen Ratio The bacteria which breakdown organic

matter also consume nitrogen Certain organic matter has a high C/N

ratio (ie sawdust is 500:1) other matter (vegetable waste) is low (approx 10:1)

When the C/N ratio in the soil exceeds 30:1 then the efficiency of the bacteria is decreased and the amount of available nitrogen in the soil decreases and therefore plants suffer from nitrogen deficiency

Soil Acidity Measured by Soil pH Normal ranges from 4 to 7.5

• (normally 4-6.5)

Generally conifer ecosystems perpetuate a more acidic condition than do hardwoods

Strongly acid soils have slower nutrient cycling and may render some elements such as phosphorous unavailable for plant growth

Soil Acidity

some nutrient elements (micro nutrients) such as boron, copper, iron, and manganese are more available when soil is more acidic.

Generally more acidic soils (ie lower pH) have lower cation exchange capacity

Soil Acidity

Soil pH in land can be adjusted through the addition of lime (raised) or lowered through the addition of Aluminum or Sulfur.

Wood ash naturally increases the pH of soils thereby making nutrients more available immediately following forest fire

Forest Operations and Fertility

Harvesting– Site Disturbance

• rutting, compaction, erosion

– Nutrient Removals• The impact of harvesting depends on the size

of the nutrient reserves on the site, the rates of nutrient cycling, the amount of nutrient removed, and the amount of time between subsequent entries

Forest Operations and Fertility

Site Preparation– Creation of Micro-sites– Site preparation or site damage– Prescribed burning

Species Selection

Tending

Fertilization and Soil Amendment It is customary in agriculture to amend

soils annually to replace nutrients removed with a harvested crop

In forestry the period between harvests is sufficiently long that in most cases natural processes of nutrient cycling, soil weathering replace the nutrients that are removed with the crop

Fertilization and Soil Amendment For certain specialized forestry

operations such as the management of seed orchards or the production of nursery stock, direct fertilization may be required

In order to fertilize affectively the manager must first know the current levels of various nutrients in the soil

Fertilization and Soil Amendment Based on fertility trials, experience and

published results, the manager will establish fertility targets for the site and the crop being grown.

The manager will amend the plow layer with fertilizer to move the soil towards the desired fertility target.

This may involve one or more amendments