ANN/THE STAR – Similar to regular health checkups maintaining well-being, assessing your garden soil’s chemical properties is crucial for understanding its nutrient status. This knowledge informs decisions on fertiliser applications.
Send soil samples to accredited labs, focusing on pH, organic carbon, CEC, macronutrients (nitrogen, phosphorus, potassium, calcium, and magnesium), and micronutrients (boron and zinc). For soils subjected to heavy fertilisation, salinity tests are advisable.
Soil pH is vital for nutrient availability. To raise pH, effective options include lime (limestone, dolomite, quicklime, and hydrated lime), with a recommended quantity of around 400 grammes (g) per square meter. Thoroughly mix lime into the soil for optimal results.
Soil organic carbon content is a strong indicator of soil fertility. A content below 1.5 per cent suggests low organic matter, while above five per cent indicates high levels.
Biannually applying three-five kilogrammes (kg) of organic material per square metre is recommended. Organic carbon in soil typically builds up quickly at first, then the rate slows, eventually reaching a plateau, usually not exceeding three per cent (equivalent to five per cent organic matter). Regular application of organic matter is crucial for maintaining soil organic matter levels.
The CEC is a measure of the ability of the soil to retain nutrients. Low-CEC soils are at risk of nutrient loss, necessitating more frequent fertilisation.
Soil nitrogen with levels above 0.26 per cent are generally considered high. Even if the initial nitrogen levels are high, N fertiliser supplementation may still be required to compensate for plant consumption. Balancing the nitrogen and carbon levels is crucial. The carbon-to-nitrogen (C:N) ratio is a vital soil health indicator, with ratios below 25:1 being ideal.
For instance, a soil with two per cent carbon and 0.1 per cent nitrogen has a C:N ratio of 20:1. Ratios below 25:1 imply easy decomposition of organic materials, whereas higher ratios suggest slower decomposition, potentially necessitating the addition of nitrogen-rich fertilisers or organic materials.
Phosphorus in soil is measured as available phosphorus, indicating the amount accessible to plants. Levels below 15mg/kg call for phosphorus fertilisation, whereas levels above 45mg/kg likely do not require additional phosphorus.
Potassium, calcium and magnesium levels are typically measured as exchangeable ions.
The soil should maintain potassium levels above 0.14cmol(+)/kg (55mg/kg) and calcium levels above 0.2cmol(+)/kg (40mg/kg). Balancing these three cations is essential, with calcium ideally being three-four times the amount of magnesium and potassium two-10 times the combined amount of calcium and magnesium.
Soil salinity, indicated by electrical conductivity (EC), reflects the salt content. Sodium levels should be under 0.1cmol)(+)/kg (23mg/kg).
Addressing soil nutrient deficiencies involves several steps. First, adjust the soil pH by applying lime to neutralise acidic soils.
Adding organic matter, such as compost and mulch, is vital for enhancing overall soil fertility. It not only supplies nutrients but also improves soil structure and microbial activity, conserves soil moisture and aids nutrient release and absorption. Targeted fertilisation is necessary for specific deficiencies. Nitrogen levels can be boosted with ammonium nitrate, urea or organic sources such as blood meal. Bone meal or rock phosphate can replenish phosphorus, whereas potash is suitable for potassium.
Gypsum is effective for adding calcium and magnesium. Specialised fertilisers with added micronutrients (boron and zinc) are recommended for micronutrient deficiency.
Regular soil testing is important for monitoring nutrient levels and adapting fertilisation strategies. Balanced fertilisation is crucial for the prevention of nutrient imbalance.
Proper water management is also essential, as it can significantly affect nutrient uptake and nutrient loss. By implementing these practices, soil health can be significantly improved, leading to healthier plant growth. – Dr Christopher Teh
