I stole this...... Sounds like you're doing this, but article stresses you need a lot of organic matter. A cover crop won't give you enough........

Improving Sandy Soils

A very sandy soil will grow very little plant life unless organic matter of some type is added to it. The only way for a soil to acquire organic matter in proper proportion is for the garden enthusiast to incorporate it himself. Organic matter improves the texture of sandy soil by filling the spaces between the tiny stones (of which sand is actually composed). The filling of the spaces also increases the water-holding and nutrient-holding capacity of the soil, therefore, holding both water and fertilizer where it is available to plants.

Organic Matter
Organic matter consists of materials derived from plants and includes such materials as animal manures, peat moss, straw, leaves, compost, native peat, old sawdust, garden refuse and sod. Organic matter is a temporary product and must be maintained in the soil. For most gardens the incorportation of organic matter into the soil each year is a simple matter; but for lawns, organic matter can be incorporated only at the time of construction.

Rate of Application
Several months of preparation should go into supplying sandy soils with organic matter to properly improve the soil. Several applications of organic matter several weeks apart should be thoroughly incorporated into the soil and allowed to settle before planting. The organic materials should be applied at the rate of 6 to 7 cubic yards per 1000 sq. ft. of soil surface. This would amount to about a 2 inch layer of organic matter spread over the soil surface. This would be incorporated into the top 5 or 6 inches of soil. Lighter applications are of little value.

Incorporate thoroughly
The organic matter can be spaded, forked or plowed in, but should be thoroughly mixed with the existing soil. The most satisfactory method is roto-tilling. It is best to accomplish this incorporation several months prior to planting to allow time for the organic matter to break down and settle itself in the soil. If available, some topsoil mixed in along with the organic matter would further benefit sandy soils. When using native peats, have your soil checked for lime requirement because this material is usually quite acid.

I have to add at least 4" of compost to our raised beds each year because that's how much the beds drop. The organic matter in the compost is broken down more each year and you have to replace it and not just little piddly amounts either.

You are faced with several challenges as you garden in sand. Sand is coarse and allows rapid water infiltration but also drains at a commensurate rate. Sand, unlike clay or organic matter, does not adsorb water since it is not negatively charged. It will also not adsorb cations, those positively charged compounds needed in abundance by plants. As the responder above noted, massive amounts of organic matter are indicated.

You are also confronted by the fact that organic matter is rapidly degraded by soil microbes in the presence of moisture, heat and oxygen. Gardening in a well-aerated soil in Zone 8 with plenty of moisture means that you must add lots of organic material and do it often to be successful.

g'day mcaren,

all you can do is to continue to add organic matter, compost whatever and keep the gardens heavily mulched with green type hay's ie.,. spoilt lucern hay or pasture grass hay, over here we have sugar can mulch as well.

maybe add in some mushroom compost derived direct from the mushroom farm, i hear over there many farmers are glad to see people take it away as they have to pay to dump it, over here it started that way but the farmer soon learnt that us gardeners are prepared to pay for it, good drinking money hey? and of course once money is involved it gets dearer and dearer as the farmer is distanced from the cost saving benefit it gave him when it was given away.

len

Here is a link that might be useful: lens garden page

I don't have any advice for you, but does anyone know whether it would be beneficial add some soil from the yard to the bed?

What you sandy soil needs is lots of organic matter. If you have been adding some and not improvement has been noticed then you have not been adding enough. From experience I know that sand needs lots of OM to get to the optimal 6 to 8 percent in the soil that will retain soil moisture. Sand, without organic matter, will not retain moisture.
Adding "topsoil" is largely a waste of your money as well as time and energy since that "topsoil" will be most likely 95 percent of what you already have, the mineral portion of soil, and maybe with luck have about 5 percent organic matter which you do need. So if you add something with maybe 5 percent OM to the soil you already have and that does not have enough OM how much OM are you really adding to that soil?

Just an idea. What is your native soil like? I think you said acidic, but is it clay, loamy or also sandy. I think you also said poor draining so I would suspect some clay. Maybe it would help to use alot more of your native soil and less of the purchased sandy top soil. I think acidic soil is easier to correct than very sandy soil.

Good gardening, Mary

Organic matter (OM), specifically humus increases the CEC of the sandy soil. Increased CEC increases the soil's capacity to buffer water and nutrients. There are three things in addition to adding compost to the soil that will improve your sandy soil.

1. Fertilize your vegetable garden. Otherwise the OM is mineralize to supply plant nutrients. Fertilize the green maunre crop to increase the OM dug in.

2. "Water" your compost with a clay slurry. The clay will increase the CEC of the resulting compost. The clay will also improve the composting process as well as increase the nutrient content of the final compost.

3. Add zeolite to the compost and/or to the soil. This is a high CEC material that does not deteriorate like OM.