Crop growth on saline soils is affected by high levels of soluble salts (NaCl). Salt accumulates in the topsoil during dry phases and is often very high at the start of the rainy season and rises towards the end of the wet season. Appropriate timing of the cropping season is therefore essential. Tolerant varieties are increasingly available and should be used wherever possible. Salinity may cause K and Ca deficiency, in some coastal areas B toxicity occurs. Accompanying crop management options include:

• Good nursery practice (described earlier in this lesson) improves stress tolerance on saline soils. Transplant older seedlings (40 to 45 days) because they are more salt tolerant. Compensate the reduced tillering ability of old seedlings and the higher seedling mortality with higher transplanting density. Do not wash roots before transplanting, treat roots carefully to avoid root damage, and transplant within a few hours after uprooting.

• Apply organic matter whenever possible, especially rice straw helps to recycle potassium and improve the plant K status.

Plants affected by salinity

• Use medium rates of inorganic NPK fertilizers, high rates may have negative effects especially on soils with high salinity. Application of potassium is especially important on saline soils. Urea or ammonium sulfate are both adequate N sources. Apply N topdressed at critical growth stages (10 to 14 days after transplanting, at mid-tillering, and at panicle initiation), basal N is used less efficiently on saline soils.

Crop growth on alkaline/sodic soils is affected by the high pH, limited availability of P, Zn, and Fe, and unfavorable soil structure. Tolerant varieties are increasingly available and should be used wherever possible.

• Good nursery practice (described earlier) improves stress tolerance on alkaline soils.

• Alkaline/sodic soils can be improved trough the application of gypsum. Combining the use of gypsum with the application of organic matter substantially reduces the quantity of gypsum needed, and even organic matter alone can improve alkaline/sodic soils tremendously.

Plants affected by alkalinity

• Zinc should be applied regularly, either to the nursery seedbed a few days before transplanting or incorporated in the soil before seeding or transplanting. On alkaline soils with severe Zn deficiency, the residual effect of applied ZnSO4 is small, and therefore Zn must be applied to each crop (about 30-60 kg Zincsulfate per hectare). On most other soils, blanket applications of ZnSO4 should be made every two to eight crops (15-30 kg Zn per hectare).

• Use fertilizers that generate acidity where possible (e.g., replace urea with ammonium sulfate). Apply N topdressed at critical growth stages (10 to 14 days after transplanting, at mid-tillering, and at panicle initiation), basal N is used less efficiently on sodic soils. The application of gypsum may reduce P availability and result in an increased requirement for P fertilizer.