Organic matter and supply of nutrients: There is a significant correlation between the percent organic matter in soil and soil fertility.
Most well-managed soils should have 2–4% organic matter, depending on the texture of the soil. Less than 1.5% is usually low and can lead to waterlogged or dehydrated soil and imbalanced nutrients.
On the other hand, with more than 5% organic matter in many soils, herbicides often become attached to the organic matter and are inactivated, so the grower needs to apply more herbicide.
Nutrients come not only from the original plant residues but some nutrients become attached to humus (highly degraded organic matter in soil), and are slowly released for plants and other soil biota.
The humus matter provides both negative and positive charges that can hold and exchange nutrients.
Nutrients with positive charges such as potassium, calcium and magnesium adhere to the negative charges on the humus. Nutrients with negative charges such as phosphate and sulphate adhere to positive charges. In both cases the nutrients remain available to plants and are not leached down away from the root zone.
Cation exchange capacity
A soil’s cation exchange capacity (CEC) is a measure of the net negative charge per kilogram of dry soil, and therefore is a measure of the amount of positive nutrients that can be stored.
Soils with a low CEC (less than 5) have a low net negative charge and do not hold positive nutrients in the soil as well as do soils with a high CEC (more than 5). Organic matter, with its negative charge, can help improve soils with low CEC.
Organic matter also decreases the fixation of phosphorus and potassium in the soil, and forms metal-organic complexes that stabilise the micro-nutrients that otherwise might not be available for plants.
Organic matter and stability of soil
Organic matter controls the stability of soil, so that the aggregates do not collapse or erode when wetted quickly.
Organic mulch or growing plants protect the soil from heavy rain, so a hard crust does not form when the soil dries.
Large aggregates (> 2 mm) are held together by a fine network of root and fungal hyphae, and fragments of plants and dead microbial cells become encrusted with clay particles to form stable smaller aggregates.
The organic matter also encourages soil animals to produce tunnels that enable soil, water and roots to move through soil.
Silt and clay soils with less than 1.5% organic matter have small, closely-packed aggregates, and many very small pores. When wetted, any poorly-structured large aggregates break down, i.e. slake, where large aggregates collapse into micro-aggregates. When dry, the micro-aggregates block pores which are too small for water to move through causing the soil to become poorly aerated when wet, and excessively hard when dry.
How should you increase organic matter in your soil?
As you prepare the soil in summer and autumn before you plant trees, till (rip and/or cultivate) the soil then grow ryegrass, but kill it in winter.
Once the trees are planted, do not till the soil again, as repeated tillage oxidises (burns) organic matter and decreases activity of beneficial organisms, worsening the soil structure, and the supply of water, air and nutrients.
Each year manage the soil carefully with a cover crop or organic mulch to:
- add organic residues
- protect the surface of the soil from heavy rain
- decrease loss of water by evaporation
- increase storage of water, and
- increase the activity of beneficial biota in soil.
In winter, allow weeds to grow. Throw the green residues of the weeds onto the tree rows when you slash.
In spring and summer, use herbicides to kill the weeds in the tree rows, so that the weeds do not compete with the trees for water and nutrients. The green and dead weeds on the soil surface protect the soil surface from heavy rain, and the dead roots add organic matter and maintain a soft, stable and porous surface soil.
See this article in Tree Fruit June 2015
