Organic matter makes up only a small part of a soil (at best 5%), yet the organic matter packs a mighty punch in the orchard.
When orchardists build up organic matter in soil, they contribute more in the long-term to healthy soil and trees than does any other resource—far greater than the 2–5% of organic matter suggests.
Here, we show why organic matter is important, what it is, where it comes from, and how the orchardist can build it up in soil.
Why is organic matter important in soil?
Organic matter in soil affects the growth and production of fruit trees chemically, biologically and physically, as follows.
- As a slow release fertiliser, organic matter directly and indirectly provides nutrients to the trees and cover crops
- Provides nutrients and energy for the many organisms (microorganisms, animals) in soil
- Helps to buffer soil against big swings in pH when chemicals are added to soil
- Improves structure and resilience of soil, and so affects the supply of water and air to roots and microorganisms
- The added organic residues also act as slow-release fertilisers so help to replace the many nutrients that are removed from the orchard in harvested fruit.
The nutrients attached to the humus are not easily leached down to the ground water away from the roots. However, the orchardist still needs to add inorganic fertiliser to make up for nutrients removed at harvest.
What is soil organic matter and where does it come from?
Organic matter in soil ranges from fresh unchanged organic materials through to highly decomposed organic materials.
So the organic matter consists of soil biota (living organisms and roots), plant fragments, and polysaccharides, proteins, fats, lignin, waxes, resin, humus and charcoal.
Most organic matter is found in the surface soil, where most of the roots are, and so comes mainly from plants and soil biota. Normal healthy roots continually release organic exudates into soil. Other plant residues, such as leaves and shoots, plus organic mulch or manures added by the orchardist, also contribute to organic matter in soil.
Relationship between soil organic matter and soil organic carbon
Sometimes a soil test shows organic matter as organic carbon; to calculate percent organic matter, multiply the percent organic carbon by 1.6.
Soil biota and decomposition of organic matter
Soil biota in the orchard include roots, and beneficial and harmful organisms, and range from the smallest viruses (0.25 micron) to bacteria (0.3 micron) to actinomycetes (0.5 to 2 micron wide) and fungi (less than 15 microns wide) to the largest animals, e.g. earthworms (> 10 mm wide). Together, the soil biota can weigh up to 21t/ha.
Soil biota use the organic residues for food and energy, and successively decompose complex organic molecules and release other organic compounds that other biota can use to build into new organic molecules, or store in their own bodies.
At the same time the biota release carbon dioxide, energy, water; and inorganic materials such as nitrogen, phosphorus and sulphur; and micronutrients that plants and biota can use.
There is generally a high turnover of biota in soil, which continues to add organic matter. Fine roots and hyphae do not live long in soil, but are fairly quickly replaced by new roots and microorganisms.
Amount of organic matter in soil
The amount of organic matter depends on:
- amount and quality of the organic residues returned to soil
- soil texture, depth and density
- soil pH
- temperature, water content, and aeration of soil
- microbial population
Organic residues of high quality, i.e. with a low ratio of carbon to nitrogen (C/N of <18), are decomposed quickly as there is enough nitrogen to stimulate microbial activity.
Organic residues with a high C/N (> 60) are decomposed very slowly.
Simple sugars, simple proteins and starch are easily decomposed, whereas hemicellulose, cellulose, fats, waxes and resins are slowly decomposed.
Lignin comes from cell walls of plants and some algae, and is decomposed very slowly.
Charcoal is inert so is not further decomposed.
Eventually the complex organic matter, humus is produced and darkens the soil.
Limits
For each soil, there is a limit to the amount of organic matter that can be stored in soil.
Organic matter is more quickly decomposed in sandy soils, so they contain less organic matter than do clay soils.
The deeper the soil, the more organic matter it can hold.
Soils that are less dense are usually better aerated and drained, are warmer with a longer growing season—so that plants grow better, releasing more organic residues into soil and supporting more biological activity.
Soils in wetter and cooler climates generally contain more organic matter than do soils in drier and warmer climates. This is because plants grow better with high rainfall and a long growing season, adding more organic residues; yet bacteria are less active and decompose the organic residues slowly.
However, very wet soils, such as swamps, contain little air and biological activity, so decomposition is slow, and percent organic matter is high.
Soil pH also affects the supply and decomposition of organic materials in soil. It is best to aim for a pH of about 5.5 to 6.5 so that the plants grow well and add plenty of organic residues to soil.
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 (continued next month)
See this article in Tree Fruit May 2015
