Soil structure

Soil structure refers to the way in which the sand, silt and clay particles are arranged relative to each other.


In soil with a good structure, the particles of sand and silt are held together in aggregates (small clumps) by clay, humus and calcium. The large empty spaces between the aggregates (macropores) allow water and air to circulate and plant roots to grow down into the soil. The small empty spaces (micropores) hold the water the plants need. This “ideal” structure is called granular, or crumbly.

Infiltration rate

The rate at which water can enter the soil surface is called the infiltration rate. The rate is important because it affects the rate at which a soil may “recharge” with water and because it affects the likelihood of surface run-off and hence erosion occurring heavy rain or irrigation.

You can test the infiltration rate of your soil with a simple test: pour 25mm of water into a ring inserted 10 cm into soil. Record time for 2nd and 3rd inch.

If the takes more that 2mm per minutes, you have a problem with infiltration rate which causes soil and water losses and results in losses in production.

An infiltration problem occurs if water does not enter the soil rapidly enough during a normal water cycle to replenish the soil with water needed by the crop before the next rain or irrigation. The end result is a decrease in water supply to the crop.

Flocculation improves infiltration rate of the soil.

Image: Infiltration rates in different soil types – clay soil has a slower infiltration rate that sandy soil.

Soil organic carbon improves water infiltration rate (Kaur et all, 2015)

Image: Soil organic carbon improves water infiltration rate (Kaur et all, 2015).

Flocculation

Flocculation refers to a process during which soil particles dispersed in a solution contact and adhere each another, forming clusters, flocks, flakes, or clumps of a larger size.

The problem of flocculation and stability of soil suspension is very important for soils. Stabble aggregates can be formed only in soils containing clay that will flocculate. If clay remains dispersed, the soil is puddled. Puddled soils are sticky when wet and hard when dry.

Flocculation (left) is important because water moves through large pores and plant roots grow mainly in pore space (Choudnary & Kharche, 2018)

Flocculation (left) is important because water moves through large pores and plant roots grow mainly in pore space (Choudnary & Kharche, 2018)

Root growth and soil aeration require a porous conditions in soils. If percolating rainwater leaches out elecrolytes from the soil, clay particles may become dispersed. As the soil becomes dry, caking or soil compaction may occur. The latter reduces the pore spaces, which inhibits soil aeration necessary for adequate root growth. Therefore, a flocculating concentration of electrolytes should be maintained in the soil.

To reach such a condition, the soil should be limed, although acidic soils are usually flocculated because of their high content of Aluminium, Iron and Magnesium. However, high Aluminium concentrations, though beneficial for flocculation of clay, are harmful for plant growth.

Calcium and Magnesium are also known to have high flocculation powers on the negatively charged clay particles and will reduce the toxic effect of high Aluminium concentrations.

Key ratios

Optimizing the key ratios in between the essential minerals improves soil structure and promotes plant growth.

Key minerals Target ratio Description

Calcium (Ca) to Magnesium (Mg)

3:1 in sandy soil

7:1 in heavy clay soil

The calcium to magnesium ratio is critical as it determines soil structure and associated gas exchange. Soil aeration is essential as oxygen is required for soil microbes and plant roots while carbon-dioxide is needed for photosynthesis. Calcium flocculates and helps to form stable aggregates, resulting to good spore space for water, roots and microbes.

Magnesium(Mg) to Potassium (K)

1:1

Mg to K ratio is almost as important as Calcium to Magnesium ratio. An ideal stimulates Phosphorus intake and ensures optimal plant availability of both Mg and K.

Phosphorus (P) to Sulfur (S)

1:1

An appropriate P to S ratio helps to optimize the performance of these two key anions. Sulfur is critical for healthy root growth, protein formation (plant immunity) and chlorophyll density.

Phosphorus (P) to Zinc (Zn)

10:1

An ideal P to Zn ratio will ensure maximum performance of both minerals. Too much either inhibits the other, however this should not be targeted in high P soils as Zn would tie up other elements

Potassium (K) to Sodium (Na)

4:1 or 5:1

The K to Na ratio is critical to ensure availability of the second most abundant mineral in plants (K). If Na exceeds K in terms of base saturation, then Na will become detrimental for plant health.

Iron (Fe) to Manganese (Mg)

1:1 or 2:1

The Fe to Mg ratio is important to ensure optimum uptake of both minerals. Ideally, Fe should be always higher than Manganese on a soil test.

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