What is an effective way to retain moisture in sandy soils and unirrigated sites?

Planting in shallow basins and creating soil berms downslope is an effective strategy for retaining moisture in sandy soils and unirrigated sites. This method takes advantage of the natural topography to collect and hold water, which is essential in sandy soils that typically drain quickly and do not retain moisture well. The shallow basins allow for better water infiltration and reduce surface runoff, while the berms help to slow down and redirect water flow, making it more accessible to the plants. This combination creates micro-ecosystems that enhance moisture retention, ultimately supporting plant growth in areas where water is limited.

The other options do not focus on methods that effectively enhance moisture retention in sandy soils. Synthetic fertilizers primarily serve to enhance nutrient availability but do not address moisture concerns. Watering deeply and infrequently is a general irrigation practice that may not be practical in unirrigated sites and does not promote lasting moisture retention in the soil. While chemical soil additives can improve certain soil properties, they may not be as effective in the context of sandy soils as physical modifications like basins and berms.