Soil conditions for cultivation
Creating and Maintaining Optimal Soil Conditions for Cultivation
Introduction
The soil system has its own chemical reactions and biological activities. Our cultivation practices interfere with this, not always positively. It is important to understand the possible effects of our actions to mitigate lasting disturbance.
Soil must have certain characteristics to allow a beneficial relationship with plants; however, diverse crops and different growth stages may have specific demands. We have options to optimise soil conditions through careful adjustments and management,
To determine if a soil is suitable for cultivation, an initial assessment involving digging, observation and testing is necessary, keeping in mind general crop requirements.
Soil Characteristics and Parameters
Soil texture, determined by the size of its particles, significantly influences its properties. Fine particles result in a ‘heavy’, clay-rich soil. While this type retains water well, it can be difficult to work, and the topsoil tends to form easily a crust. Conversely, larger particles create a sandy soil that drains water and nutrients rapidly and lacks significant structure, although it is easily loosened.
Lighter, sandier soils (low water-holding capacity) require more frequent irrigation and fertilizer applications compared to heavier, clay-rich soils (strong water-holding capacity).
Loam, or loamy soil, with medium-sized particles, is often the preferred type due to its balanced characteristics (water retention, drainage and workability).
Soil structure refers to the arrangement of soil particles and the resulting cohesion, pores and channels. The structure is crucial for regulating air circulation, water retention, and drainage of excess water.
The composition of the soil reveals the content of organic and inorganic components (organic material, nutrients and other chemical elements).
These three primary characteristics largely determine a soil’s suitability for cultivating specific crops. However, by working the soil, incorporating additives, and implementing appropriate practices and management techniques, adjustments and improvements are achieved.
Soil properties can vary considerably across land areas, even one acre, especially on sloping terrain. Therefore, soil sampling for analysis should consider different conditions at various locations. Furthermore, topsoil and subsoil layers can exhibit differences in characteristics.
Former use of heavy machinery like tractors can have caused soil compaction. Unlike soil working with bullocks or horses, repeated passes of tractor wheels, particularly in moist or wet conditions, can create a compact layer beneath the topsoil, often around 25 cm deep. This problem is exacerbated at field edges and corners where machinery frequently turns. Consequently, many areas have a shallow topsoil (20 cm or less) and poor drainage. Breaking this compacted layer is essential for improving soil suitability.
The widespread adoption of chemical fertilisers has unfortunately led to the neglect of organic manure application. Moreover, fertilisers have not always been applied prudently, and accumulation of certain elements has taken place with serious element inequilibrium in the soil. As a result, many soils have become mineralised and depleted of vital organic matter. Leaving soils uncultivated and bare leads to rapid and substantial loss of organic material, particularly in tropical and subtropical climates.
Organic matter is the foundation of biological activity within the soil, driving the natural release of nutrients. Moreover, organic matter significantly enhances the soil’s water-holding capacity.
Soil texture and structure can be assessed through visual and physical examination. A wet soil sample pressed in the hand will behave differently based on its texture: clay will stick together and retain water, loam will exhibit less cohesion, and sand will fall apart with water readily draining. Digging a pit to a depth of 2 feet allows for observation of soil layers and conditions at different depths.
To assess soil composition and nutrient content, laboratory analysis of soil samples is necessary. For this purpose, multiple samples are collected from areas with seemingly uniform soil. After removing the top 1-2 cm of soil, samples are taken from the top down to a depth of 20-25 cm. These collections are then mixed to create a composite sample of approximately 500 grams, ideally with medium humidity. In areas with varied soil types, each distinct area shall be sampled separately. The analysis typically includes determination of pH, organic matter content, electrical conductivity (EC), major nutrients (Nitrogen (N), Phosphorus (P), Potassium (K), Magnesium (Mg), Calcium (Ca)), and micronutrients (Zinc (Zn), Copper (Cu), Iron (Fe), Manganese (Mn), Boron (B), and Molybdenum (Mo)), as well as other parameters like Sodium Adsorption Ratio (SAR), Carbon to Nitrogen ratio (C/N ratio), and Cation Exchange Capacity (CEC).
Improving the structure of both topsoil and subsoil can be achieved by tilling the soil with appropriate equipment, crucially when the soil has the correct moisture content. Various tools are available to break up compacted layers and large clumps in both topsoil and subsoil. Soil clumps of different sizes promote drainage and aeration. A topsoil that is too fine can easily form a surface crust and suffer from poor aeration. Cultivators and rotavators loosen the soil without turning over, while disc and share ploughs turn the soil over. Utilising all these implements correctly and with minimal passes is key to creating better soil structure. Special equipment behind the tractor can be used to break sub-soil layers.
Care must be taken, especially during land levelling, to preserve the topsoil by setting it aside while levelling the lower soil. Then the topsoil is set back again. Healthy topsoil harbors a balanced biological composition and activity, which can take several months to a year to develop. The ‘no-till’ practice is founded on the principle of minimising disturbance to the topsoil. Implementing this practice effectively requires the soil to be in excellent condition initially, followed by appropriate crop rotation and management.
Incorporating organic matter and compost helps to stimulate biological activity within the soil, which is most concentrated in the top 20-30 cm. By focusing on this layer, we can create a highly favorable environment for root growth. It is also important to manage damaging insects and nematodes. Various control products, both chemical and biological, are available, and practices like crop rotation and fallow periods can be implemented. Nematode control during cultivation is challenging. Introducing beneficial fungi or bacteria into the soil can help suppress the development of soilborne pests and diseases that threaten crops. However, these beneficial organisms must be protected from chemical contamination.
For more in-depth information, refer to my article: ‘Using Biologicals’.
Even in a seemingly uniform topsoil layer of, for example, 40 cm depth, there will be a gradual decline in biological and root activity from the surface downwards due to reduced oxygen levels. Consequently, most activity is concentrated in the upper 10 to 30 cm. However, creating more homogenous conditions in deeper soil layers can encourage the roots of many crops to grow deeper if conditions are favorable and the transitions from the topsoil in terms of biological activity, composition, and structure are gradual. Roots tend to avoid growing through sharply contrasting soil layers. When roots can extend into deeper layers, plants are better equipped to withstand drought and temperature fluctuations.