Paddy on acidic or low-CEC soils: why fertiliser seems to disappear

Some paddy fields never respond the way they should. Plants stay stunted, roots are poor, and fertiliser seems to disappear with little effect on yield. Two soil problems are behind many of these fields: strong acidity, and low cation exchange capacity (CEC).

Acid sulphate soils, found in parts of Malaysia's coastal rice land, contain pyrite, which produces very high acidity and releases large amounts of aluminium and iron. The main agronomic problems are aluminium toxicity, reduced phosphorus availability, nutrient deficiencies and iron toxicity.^[1]

CEC describes how well a soil holds on to positively charged nutrients such as potassium, calcium, magnesium and ammonium. Clay and organic matter carry negative charges that hold these nutrients in the root zone so they are not easily lost through leaching. A soil with little clay or organic matter has a low CEC and holds few nutrients.^[3]

• Aluminium toxicity at low pH. When soil pH is below about 5, aluminium dissolves and becomes toxic to rice. Raising the pH above 5 reduces aluminium in solution to a minimal level.^[2]
• Phosphorus locked up. Acid sulphate soils reduce phosphorus availability, so phosphorus fertiliser gives less return than expected.^[1]
• Low CEC, especially at low pH. CEC is lowest in very acidic soils and increases as the pH is raised by liming.^[3]
• Sandy soils with little organic matter. Sandy tropical soils have poor nutrient-holding capacity and low CEC. In these soils, organic matter is the main determinant of fertility and nutrient storage, and nutrient leaching is a high risk.^[4]

• Test first. Measure soil pH, exchangeable aluminium and CEC before deciding how much lime or fertiliser to use. The Department of Agriculture runs a soil fertility advisory service that farmers can apply to.^[5]
• Lime according to acidity. Liming with ground magnesium limestone (GML) is the common practice among Malaysian farmers on acid sulphate soils, and the amount needed depends on how acidic the soil is — more acidic soils need more. Researchers in Malaysia have also tested calcium silicate alongside GML because it reacts faster and moves deeper into the soil.^[1]
• Build organic matter. On low-CEC soils, organic matter improves nutrient storage and the soil's ability to hold nutrients. Return straw and add compost or other organic amendments where you can.^[4]
• Split fertiliser on light soils. Because low-CEC soils leach easily, apply fertiliser in smaller amounts matched to crop demand rather than all at once.^{[3, 4]}
• Re-test after liming. Liming raises both pH and CEC, so a follow-up test shows whether the soil now holds nutrients better and whether more correction is needed.^[3]

This is where the NEXGROW rates for paddy come from. The product booklet sets the paddy rate by soil CEC: the lower the CEC, the higher the rate, because a low-CEC soil has less organic matter and nutrient-holding capacity to start with.

NEXGROW is a bio-organic, phosphate-solubilising fertiliser — it converts insoluble soil phosphorus into forms plants can absorb, and adds organic matter and beneficial microbes to the soil. It does not replace liming on strongly acidic soils. You need your soil's CEC from a soil test to pick the right row below; rates are taken directly from the booklet.

On paddy land, test before every major change: before liming, before choosing a NEXGROW rate, and when a field underperforms its neighbours. Ask for pH, CEC and, on suspected acid sulphate soil, exchangeable aluminium. The DOA soil fertility advisory service is one route; our team can also arrange a test.^[5]