01/06/2024
Soil Health is the undisputed foundation for all plant and crop health
Soil Fertility
1% carbon (in 15 cm soil) houses approximately 750 cubic meters of water per hectare and 50 mm rain on covered soil is equivalent to 200 mm rain on uncovered, bare soil.
With the water monitoring probes, we have observed that soil becomes wetter at night (or on cooler days) compared to warm days. The drippers create a wet spot on the ground, causing much water to evaporate before it can infiltrate the soil.
What if we could increase the carbon content in the soil by 1%? Then we could store 3 weeks' worth of water in the soil. Moreover, if we cover the soil with organic material, water loss due to evaporation can decrease by up to 4 times. On average, soil carbon levels are mostly below 1%.
What makes soil fertile?
Soil fertility is not what we see on a soil analysis. The fertility of soil is determined by its:
1. Chemical composition,
2. Physical composition,
3. Biological composition.
The chemical composition refers to what we get from a soil analysis. The amount of macro-elements in the soil: nitrogen, phosphates, calcium, potassium, etc., as well as the micro-elements like zinc, boron, etc. in the soil.
The physical composition of soil refers to its texture and structure.
Texture: Sand, loam, or clay soil.
Structure: What does the soil look like? Is it layered or not? How are the sand, clay, and loam particles arranged in the soil?
The biological composition refers to all the living organisms in the soil: soil microbes, plants, and plant roots. The biological composition greatly influences how the soil looks and how many minerals are available.
The influence of soil biology on the chemical composition and physical structure of soil
I noticed that darker soils on a farm require less fertilizer than lighter-coloured soils. The reason for the darker colour is the presence of carbon in the soil. The darker the soil, the more carbon it contains. Carbon in the soil includes physical carbon, organic acids, enzymes, proteins, sugars, and carbohydrates. Soil microbes, which influence soil biology, require:
1. Water,
2. Oxygen,
3. Organic material to multiply
Darker soils with more carbon have more soil microbes and better levels of soil biology. Soil biology, soil chemistry, and soil physical structure are interdependent. A change in soil biology alters the chemical composition and structure of the soil.
Soil biology determines soil structure and prevents soil compaction
Soil microbes secrete a sticky substance called glomalin. This sticky substance causes sand, clay, and loam particles to stick together, forming a granular soil structure known as soil aggregates. Soil aggregates are larger than individual sand, clay, or loam particles. These larger structures allow more air into the soil than soils without aggregates. Soil aggregates prevent soil compaction. More oxygen (air) in the soil allows soil microbes to multiply. As soil microbes increase, so do glomalin secretions. More glomalin means larger and more soil aggregates. Thus, fertile soils are well aerated with a granular structure.
How does soil compaction occur?
Soil aggregates are also formed by fungi in the soil. Fungi live in carbon-rich soils and are very sensitive to salts and fungicides. Soil compaction occurs when soil aggregates are broken. The main causes of soil compaction are:
- Soil cultivation actions like ploughing,
- Synthetic salt fertilizers,
- Fungicides.
With soil cultivation, soil aggregates break down into their respective clay, loam, and sand particles. When the soil is irrigated or it rains, the smaller soil particles dissolve in the water and settle. The smaller particles then form a compaction layer in the subsoil. This compaction layer is known in the industry as a plough pan. The compaction layers result in poor soil drainage and salt build-up above this compaction layer, and soil microbes do not like salts. Salts in the soil can come from irrigation water or synthetic fertilizers applied. Due to the salts, there are fewer soil microbes and therefore less glomalin formation in the soil, leading to soil structure degradation.
With soil compaction, oxygen in the soil decreases and microbe numbers decline further. The process repeats itself, ultimately resulting in compacted soils. The standard practice is then to loosen the soils by ploughing or ripping. Ripping compacted soils only treats the symptoms of soil compaction, not the cause. After ripping, the soils return to their compacted state. Ripping soil is thus only a short-term solution.
See the products and services section later in the manual for more solutions on how to reduce soil compaction. Plant roots also cannot penetrate the compaction layer, limiting plant root development. By increasing soil biology, we address the cause of soil compaction and not just the symptoms.
Soil biology determines plant-available minerals in the soil (the soil chemical composition)
Many minerals in the soil, like phosphates and sulphur, are negatively charged. Clay particles are also negatively charged. Clay particles can hold positively charged minerals like calcium, potassium, etc., but not phosphates or Sulphur. Carbon and carbon compounds can hold negatively charged minerals. Positively and negatively charged minerals in the soil bind to each other. However, plants can only take up minerals if they are not bound to each other. The soil pH and acids can break these bonds so that the positively and negatively charged minerals can be taken up by plants.
Soil microbes respire and secrete carbon dioxide. The carbon dioxide binds with water in the soil to form carbonic acid. The acid breaks the mineral bonds, making the minerals available to plants. The more respiration in the soil, the more minerals are made available by carbonic acid. Soil microbes also take in many minerals trapped in the soil and then secrete them again. The minerals secreted are usually bound to amino acids. Plants can take up minerals bound to amino acids.
Nitrogen is formed in the soil by nitrogen-fixing bacteria. The bacteria use the roots of example, legumes as hosts. Nitrogen-fixing bacteria need a carbon/organic source of food and air (which consists of oxygen and nitrogen) to fix nitrogen so that plants can take it up.
When soil microbes multiply in the soil, more minerals become available for plant uptake. Thus, the need for fertilizer decreases. Fewer fertilizer applications reduce salts in the soil and increase soil microbes' numbers. Soil microbes depend on carbon in the soil and increase plant-available minerals in the soil.
Soil fertility affects pest and disease pressure
Have you noticed that certain plants, vines, or trees in a block are less susceptible to disease than others, or certain blocks have fewer pests and diseases? I have particularly noticed in the field that plants on poorer soil sections in the block are more prone to diseases.
Energy in a plant is first used to feed the plant for survival. If there is still energy available, the plant will use it to make itself resistant to pests and diseases. The plant's energy levels can be measured by the sugar (in Brix) in the leaves using a refractometer. When the Brix reading of leaves, e.g., on vines and trees, is more than 10-12, there is a very small chance that the plant will become sick. The Brix reading actually measures how effective photosynthesis is in the plant. Thus, environmental factors like cloudy or rainy days also affect a plant's Brix level.
Plants with higher levels of nitrates are more susceptible to pests and diseases.
Why do nitrates make a plant more susceptible to pests and diseases?
There are mainly two forms of nitrogen that a plant can take up: Nitrates and ammonium. Nitrate nitrogen is easily absorbed by plant roots along with water. For nitrates to be converted into amino acids, the plant uses a lot of energy. The energy is provided by a molecule in the plant, namely ATP. The 'P' in ATP* represents phosphates. Energy levels in the plant decrease with the uptake of nitrates. This is why we often find that, e.g., vines run out of energy during flowering, causing excessive fruit drop.
Plants need both the nitrate and ammonium forms of nitrogen. Ammonium nitrogen uses less energy than nitrate nitrogen in a plant. When fertilizer in the ammonium form (e.g., urea, ammonium nitrate, etc.) is applied and the soil is low in organic material, the ammonium nitrogen in the soil is converted to nitrates before the plant can absorb it. This process is called nitrification (see the following diagram).
The ammonium form mainly comes from soil microbes. Plants that take up nitrogen in the ammonium form need less energy to convert it into amino acids. Amino acids are the building blocks of proteins, which ultimately help form sugars and carbohydrates in the plant. When plants' energy drops due to the uptake of nitrates, the plant has less energy to protect itself from pests and diseases.
On a chemical level: When a plant takes up nitrates, it secretes hydroxide ions through the roots. This secretion causes the plant's pH to decrease (it becomes more acidic). Plants with a low pH are more susceptible to plant diseases like fungi. Fungi prefer a low pH environment. When the plant absorbs most of its nitrogen in the ammonium form, a hydrogen ion is secreted through the roots, causing the pH in the plant to increase, making the plant more resistant to fungal diseases.
In summary: When soil fertility increases, and soil microbes multiply, plants become more resistant to pests and diseases, reducing the need for chemical control.
How do you increase soil fertility?
To increase soil fertility, you must increase the carbon levels in the soil. Over time, I have seen that carbon in the soil struggles to increase when:
1. No carbon and/or organic material is applied to the soil, or
2. Carbon inputs/application of organic material are made, but farming practices like ploughing, synthetic fertilization, and chemical pest control limit the build-up of carbon in the soil.
Based on this, I asked myself: How can you farm without:
- Ploughing,
- Synthetic fertilizers, or
- Chemical pest control and be sure that production will not decrease? The biggest shift in thinking I underwent regarding plant nutrition was that plant nutrition is not just fertilizer. As organic material inputs increase, the need for fertilizer decreases without a decrease in production. When fertilizer is judiciously reduced, soil biology increases, further reducing the need for fertilizer. The more nutrients a plant take up, the more resistant the plant becomes to pests and diseases. The need for chemical pest control decreases. The application of organic material leads to more soil microbes. More soil microbes reduce fertilizer. Less fertilizer leads to more microbes, creating a positive cycle that increases soil fertility.
* ATP (Adenosine Triphosphate) is a nucleotide that plays a crucial role in energy transfer and storage in living cells, including plant cells. It is often referred to as the “molecular unit of currency” of intracellular energy transfer.
Structure and Function
ATP is a nucleotide consisting of an adenine base, a ribose sugar, and three phosphate groups. The three phosphate groups are linked to each other by high-energy bonds, which can be broken to release energy. This energy is then used to drive various cellular processes such as muscle contraction, nerve impulse propagation, and chemical synthesis.
Role in Plant Cells
In plant cells, ATP plays a vital role in various physiological processes, including:
Energy storage and transfer: ATP is used to store energy obtained from light during photosynthesis and to transfer energy to other parts of the cell.
Cell signaling: ATP is involved in signaling pathways that regulate various cellular processes, such as root hair growth, pollen tube growth, and biotic/abiotic stress responses.
Cellular metabolism: ATP is used to drive metabolic processes such as glycolysis, the citric acid cycle, and the electron transport chain.
Extracellular ATP Signaling in Plants
Extracellular ATP (eATP) has been shown to play a role in plant signaling pathways, inducing various cellular responses such as increased cytosolic free calcium, nitric oxide (NO), and reactive oxygen species (ROS). eATP is believed to be involved in modulating plant growth and development, as well as responses to biotic and abiotic stress.
Conclusion
In summary, ATP is a crucial molecule in plant cells, playing a central role in energy transfer, storage, and signaling. Its breakdown and re-synthesis are essential for various cellular processes, and its extracellular release can trigger signaling pathways that regulate plant growth and development.
Credit – S Steenkamp
