Advice for the Home Gardener from the
Help Desk of the UC Master Gardener
Program of Contra Costa County
Client's Request: I have some sandbags that we filled at a City facility. A few of them have broken open, and I wonder if the sand is bad for the soil. I suspect if the sand is from an ocean beach, it is possible it could contain too much salt. How can I tell if it is OK for my garden? If it is OK, I may break open some remaining bags and set a few pavers in it.
Help Desk Response: Thank you for contacting the UC Master Gardener Program Help Desk with your questions about using sand in your garden.
Presuming that you have clay soil like most of us do in Contra Costa County, the addition of sand to your soil will not be beneficial for soil health and in fact can harm your plants. The reason for this all relates to the soil texture. Soil texture is the relative proportions of sand, silt and clay particles. Sand particles are the largest (can be seen with the naked eye), silt particles are in between in size and clay particles are very small. The relative compositions of these three particles determines your soil texture class. If you wanted to determine your soil texture you can do a simple feel method to give you an idea. There is a link below on how to do this and an even more extensive link on soil texture from Colorado State.
Whether you have sand, silt or clay soil, will affect the size of the pores within the soil. Soil pores are the spaces between particles that hold water and oxygen. Sand, because it is a larger molecule, has large pores. Clay, because it is a smaller molecule, has tiny microscopic pores. The pores are where water or oxygen is available to the plants. Generally, sandy soils have larger pore size, hence they feel lighter when you are working with them. Larger pores hold less water and nutrients but have better aeration (more oxygen). This is typically appreciated as these soils usually have good drainage. Water that is applied rapidly moves through the topsoil and into the deeper layers. Clay soils, on the other hand, have very small microscopic pores. These pores hold water longer, hold onto nutrients but have limited aeration. This is why these soils are typically called heavy. Because these pores are very small, water cannot drain as well and clay soils typically have poor drainage.
The problem with mixing a small amount of sand into clay soil is that you create an even heavier soil than clay alone and it has even smaller pore sizes. This is because the large spaces between sand molecules are then filled with tiny clay particles. This results in a mixture that resembles concrete. In this mixture there is little to no water or oxygen available for plants to use and their roots may not be able to penetrate at all. You can avoid this problem by incorporating 50% sand into a clay soil, however that would mean removing half the topsoil in your garden and replacing it with sand, which is not recommended or practical. There is a great summary of the problem of adding sand to clay soils below. If you did need to improve your clay soil in some way, the recommendation would be to add organic material such as compost and mulch. The link below provides guidance on how to manage clay soil in the home garden.
So, given that information about adding sand to our native clay soils, you did ask about whether you could do that and put pavers in it. If you were creating a walking path, it might not matter to you if your soil underneath those pavers had poor texture and would not support the growth of plants. It is worth considering that development of this very heavy soil mixture, similar to concrete, under the pavers may lead to water runoff during heavy rain which then needs to be absorbed by adjacent areas. It also may limit the growth of larger perennial plants, such as trees and shrubs, in that vicinity depending on how much of their root zone would be affected.
In looking at the Contra Costa County Sandbag program website, there is no indication where the sand comes from. Therefore, I cannot be sure what its salt content is. Too much salt is not ideal for plants, but adding the sand to your native soil would already be detrimental to your garden. If you do choose to add the sand to your garden, you might call the city to see where they sourced the sand. Conversely, you could rinse the sand before using it and dispose of the water in an area separate from your garden in case it was excessively salty.
I hope this helps answer your questions. Happy Gardening!
Help Desk of the UC Master Gardener Program of Contra Costa County (SES)
Note: Contra Costa MG's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, email: firstname.lastname@example.org, or on the web at http://ccmg.ucanr.edu/Ask_Us/. MGCC Blogs can be found at http://ccmg.ignore.edu/HortCoCo/ You can also subscribe to the Biog.