What effects does over-grazing cause on soil?

Answer 1

Overgrazing, which is a factor of time and poor management from allowing livestock to basically continuously graze a pasture, affects the soil by increasing compaction and decreasing organic matter in the soil. Less roots growing and dying back means less organic matter is going to be present in the soil. With less roots or root material going into the soil also decreases water infiltration rates, which means that water from torrential or rain events is mostly going to run off instead of soaking in to the ground. Runoff creates flooding or pooling of water in excess in low areas. Erosion is also likely to occur, mostly in severely-overgrazed areas where soil is exposed to the elements, and little to no litter or green plant material is available to cover the ground and protect the soil. Overgrazing also encourages more heating of the soil, which impacts soil biota, from mycorrhizal fungi to earthworms.

Answer 2

Overgrazing is a function of time. Despite what the dictionaries tell you (most which are actually wrong), overgrazing actually means "to defoliate a plant when its energy stores have been depleted in attempt to regrow after the first defoliation event."

Overgrazing occurs when either animals are allowed to continuously graze a pasture, or animals are on a piece of ground for too long, allowing for a "second bite" before a plant is able to recover.

So, what effects does overgrazing have on the soil?

Basically, it's these:

• Increased erosion
• Compaction
• Loss of organic matter
• Loss of soil biota populations
• Reduced water infiltration
• Water runoff/flooding issues

Increased Erosion. Because overgrazing decreases the amount of litter and green plant material covering the soil, more soil is exposed to wind and water. Severely overgrazed pastures, like that with sheep and horses (cattle can actually starve on what sheep and horses can still graze on), are a big problem with erosion because plants are continually grazed down to their growth points, to the point that it effectively kills the plants. These dead plants decompose or their litter oxidizes, exposing more soil to the wind and the rain. It becomes a negative feed-back loop of overgrazing begetting plant death begetting soil exposure begetting erosion.

Compaction. Compaction problems are two-fold: Animals continually coming back to their favourite plants before those plants have a chance to recover fully can press into the soil more so than if that area received adequate rest. Without adequate rest then, the roots of these overgrazed plants are very shallow, such that they do not have a chance to grow roots down deeper into the root surface to push down organic material and attempt to break the compaction layers, if they are able to that is. Roots instead become root-bound at the top two inches of the soil, a big problem that tends to happen with grasses. History of breaking up the sod via tillage can also contribute to compaction issues, because the pore space between soil aggregates are so greatly diminished that water has a difficult time trying to percolate down into the deeper soil layers.

Loss of Organic Matter. When there are less roots and plants are not able to get into a flux system of root growth and death with response to proper grazing management practices (short duration grazing followed by a long rest period), there is less organic matter available in the soil. There are soil bacteria that feed on organic matter, breaking it down further and/or releasing carbon into the air. This process is in fact normal, but what's not normal or healthy about this is the lack of roots being able to grow deep into the soil surface, die and contribute to organic matter while new ones grow back and push ever deeper, or as deep as genetically and physiologically possible.

Loss of Soil Biota Populations. The less green and dead plant material covering the soil, the warmer the soil becomes and the less food is available to those organisms to feed on. While there may not be a complete loss of soil biota, there can be a shift away from the beneficial organisms, such as mycorrhizal fungi and earthworms, causing the whole system to suffer to the point of crashing. During hot periods, soil biota can suffer if soil surface temperatures get above a certain point, such as above 25ºC (102ºF).

Reduced Water Infiltration. The capacity of soil to have good water infiltration means a covering of the soil surface by dead plant material, plus a living root in the soil, and root channels created by roots of current or decomposing plants. If a pasture does not have this, water has no place to go except along the soil surface, down a hill into a low area. When there are less roots going deeper into the soil because of inadequate rest and compaction, that means less water is going to follow those roots down into the soil. Less roots deeper in the soil profile also means there is poorer porosity of the soil itself, which also decreases water infiltration rates. A lack of litter cover to catch water droplets can mean that the soil will dry out sooner; soil moisture is not being retained, water from rain is not allowed to soak in. So, instead, it evaporates or runs off.

Water Runoff/Flooding Issues. Following poor water infiltration capability means that water is going to run down into low areas instead of being soaked into the soil, and remaining in that soil. If an overgrazed pasture has suddenly got quite a bit of ponding after a heavy rain event (and this goes for a huge number of croplands that are tilled as well), it is not because of the soil becoming "saturated," it will be primarily because of poor water infiltration.