Sandy soils, such as the coastal soils of Terengganu and Kelantan, support agriculture but hold nutrients poorly. Sand grains carry little charge and these soils are usually low in organic matter, so their cation exchange capacity (CEC) is low. Applied potassium, magnesium and calcium are held weakly and can be washed below the root zone after heavy rain. The problem is often not the fertiliser itself but the soil’s limited ability to hold cations. Building organic matter, including humic substances, is one of the ways growers try to improve this.
The leaching penalty in light soils
CEC comes mainly from clay minerals and organic matter. Sandy soils are low in both, so their CEC is much lower than that of loams and clays. Applied K⁺, Mg²⁺, Ca²⁺ and trace cations are not held strongly by the soil and can move down with percolating water after heavy rain.
Growers often compensate by applying fertiliser more often in smaller doses. The longer-term approach is to build the soil’s organic matter and, with it, its capacity to hold nutrients through the season.
Humic acid and soil CEC
Humic acids are large organic molecules with carboxyl (COOH) and phenolic (OH) groups that carry negative charge, which is why organic matter contributes so much of the CEC in sandy soils. SoilBoost EA is a leonardite humic soil conditioner (a typical analysis gives pH 3.84 and 0.45% sulphur as sent) that supplies humic substances to the root zone.
Field data on how much a given humic product raises CEC in Malaysian sandy soils are limited, and published plant responses to humic products vary widely, with many studies showing little effect (Rose et al., 2014). Measure the change on your own soil rather than assuming a figure.
Organic amendments as a complement
Hyacinth Plus is a composted water hyacinth soil amendment. It adds organic matter, which also carries exchange sites and helps the soil hold water; it is a soil conditioner, not a source of NPK. A humic soil conditioner and an organic amendment work towards the same goal: building the organic fraction that light soils lack.
Application on sandy soils
Rates depend on soil test results, crop and rainfall. Agree rates for SoilBoost EA and Hyacinth Plus with your agronomist, or ask us for a site-specific recommendation; product rates are given on the product pages. Where possible, incorporate the amendments into the topsoil before planting and water them in.
The dual benefit: CEC and water holding
Organic matter improves not only CEC but also water-holding capacity. Sandy soils hold little water between field capacity and wilting point, so crops reach water stress quickly after rain stops. Building organic matter can widen that window, which matters most in years with a late monsoon or an early dry season.
Monitoring the response
Test soil CEC and exchangeable K before application and again after 6 and 12 months to see whether the soil is responding. If K still leaches, check whether the amendment is mixed through the root zone or sitting at the surface. Root pits at 12 months show whether roots are growing deeper.
Organic matter breaks down faster in warm, well-aerated sandy soils than in clay-rich soils, so gains are not permanent. Humic substances are more stable than fresh compost, but regular additions are still needed to maintain soil organic matter.
Costs and benefits on light soils
Whether humic and organic amendments pay on light soils depends on how much fertiliser you are losing, the value of the crop and local prices. Compare your own soil tests and fertiliser records before and after; we can help you set up a simple comparison.
Integration with fertiliser timing
On light soils, split fertiliser applications remain sensible, especially in high-rainfall areas such as the east coast, Sabah and Sarawak, so that leaching between applications does not leave the crop short at peak demand. Building soil organic matter works alongside good timing; it does not replace it.
Reference
Rose, M.T., Patti, A.F., Little, K.R., Brown, A.L., Jackson, W.R. & Cavagnaro, T.R. (2014). A meta-analysis and review of plant-growth response to humic substances: practical implications for agriculture. Advances in Agronomy 124: 37-89. https://doi.org/10.1016/B978-0-12-800138-7.00002-4