Healthy Soil & Soil Structure Information

Soil physical fertility is determined by its ability to satisfy the essential growth requirements of the crop planted in it. These requirements include storage and supply of water, nutrient elements, and oxygen — all made available to the plant through its roots. Good soil physical fertility is indicated by the presence of adequate water and air to promote prompt seed germination and good root growth, and by its minimal need for seedbed preparation.

The physical fertility of soil is influenced by its properties and processes. The important properties include soil structure and soil texture. The physical processes that affect fertility include particle aggregation, water infiltration rate, waterlogging, and erosion. There is a dynamic interplay among these factors, each influencing and in turn being influenced by the others.

Measuring physical properties requires sophisticated instruments and highly-trained technicians. From the farmer, it demands considerable time and thoroughness in sampling procedure. Adding to the difficulty, commercial labs are more focused on chemical tests and evaluate only few physical properties. Consequently, farmers may have to resort to qualitative observations in the field and use formulas in reference books.

Soil structure refers to soil aggregation and pore-size distribution and describes how the individual soil particles stick together to form larger aggregates. Soils with relatively few aggregates present are considered to have poor structure. Aggregate formation is strongly influenced by the amount of organic matter in the soil because of its ability to bind particles together. The spaces (or pores) between aggregates create a channel for drainage of water; if there is adequate space for drainage, waterlogging is minimised. Soil can retain water in its finer pores; this prevents rapid evaporation to the wilting point. Larger pores allow water and air to flow in the soil, and weaken soil enough for roots to grow. Smaller pores are the points of storage for available water and nutrients. A balance of small to large pores (and therefore of small and large aggregates) is needed for good fertility. Aggregate stability helps determine the ability of the soil to withstand the impact of weather action: falling rain; surface flow of water; and wind erosion.

The tests applied to assess soil structure include, among others, those that measure water holding capacity, dispersible clay, infiltration rate of rain or irrigation water into the soil, and qualitative observations on soil appearance on and below the surface.

Soil texture is easy to estimate in the field. Sandy soil has low water-holding capacity but needs little tilling; in contrast, clayish soil has high-water holding capacity and requires more tilling. Loamy soil has adequate water-holding capacity and favourable pore-size distribution conducive to plant growth.

Soils that are easy to till have roughly the same amounts of large and small pores. Farmers can measure soil bulk density easily. Carefully take a core sample, then remove and weigh the soil. Line the hole left by the core sample with thin plastic and fill it with water, taking note of the exact water volume (which is equivalent to the soil volume). The ratio of soil weight in grams to the soil volume in cubic centimeters is the bulk density. Good, well-aerated soils have bulk density values ranging from 0.9-1.35 g/cm3.

Soil bulk density depends a lot on soil organic matter (SOM) content: SOM is porous, with many air spaces, thus it tends to lower the soil density. The degree of compaction also impacts bulk density and highly compacted soils have values more than 2 g/cm3. If bulk density increases over time, this is a sign of declining soil quality due to loss of SOM or compaction.

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Helen M. Disler
Farming Secrets
http://www.farmingsecrets.com/
Email: info@farmingsecrets.com

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