Gardening: Stop guessing, start testing your soil

Soil organic carbon content is a strong indicator of soil fertility. Photo: 123rf.com

Just as regular health checkups are crucial for maintaining good health, testing the chemical properties of your garden soil is essential for understanding its nutrient status. This knowledge enables informed decisions regarding fertiliser applications.

It is important to send soil samples to accredited soil laboratories for analysis, focusing on soil pH, organic carbon, cation exchange capacity (CEC), macronutrients (nitrogen, phosphorus, potassium, calcium and magnesium), and micronutrients (boron and zinc). If your soil has been subjected to persistent heavy fertilisation, salinity tests are also advisable.

Soil pH is a critical factor affecting nutrient availability. In Malaysia, soils tend to be acidic, with pH values often below 5, which can hinder nutrient absorption and negatively affect beneficial bacteria. The optimal pH range for nutrient uptake is between 5.5 and 6.5. To raise the soil pH, adding lime (such as limestone, dolomite, quicklime and hydrated lime) is effective, with a recommended quantity of approximately 400g per square metre. Thoroughly mixing the lime into the soil ensures the best results.

Soil organic carbon content is a strong indicator of soil fertility. A content below 1.5% suggests low organic matter, while above 5% indicates high levels.

Biannually applying 3-5kg 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 3% (equivalent to 5% 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. Many Malaysian soils fall below the ideal CEC range of 8-10cmol(+)/kg, which indicates poor nutrient retention and fertility. CEC can be enhanced by adding organic matter or lime. Note that 1cmol(+)/kg represents one centimole of positive charge per kilogramme of soil, which is a metric for the total exchangeable cations a soil can hold.

Soil nitrogen with levels above 0.26% 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 2% carbon and 0.1% 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 3-4 times the amount of magnesium and potassium 2-10 times the combined amount of calcium and magnesium.

In Malaysian soils, micronutrients, except for boron and zinc, are usually sufficient. Boron and zinc levels should not fall below 0.5mg/kg and 1.5mg/kg, respectively.

Soil salinity, indicated by electrical conductivity (EC), reflects the salt content. Sodium levels should be under 0.1cmol)(+)/kg (23mg/kg). However, Malaysian soils rarely exhibit high EC levels, typically well below the 1dS/m threshold. Gypsum (calcium sulfate) is a common treatment used for soils with high salinity.

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.

Assoc Prof Dr Christopher Teh Boon Sung heads the Department of Land Management, Faculty of Agriculture, Universiti Putra Malaysia. His field of specialty is in soil and water conservation.