Q. Can I use sawdust as mulch after my garden has been planted?

A. Sawdust can be used as mulch and is often recommended around blueberries and other plants that prefer an acid soil. It is best if the sawdust has been composted for at least six months (preferably one year).

If not, it can tie up available nitrogen in the soil and create a deficiency in the plants growing there. You can also add ¹/₂ pound of actual nitrogen per cubic yard of sawdust to avoid the problem. Very fine sawdust can pack down and interfere with the movement of water and air through the soil. Be sure to stir it up periodically to avoid this problem.

A 2- or 3-inch layer is sufficient to help keep weeds down, maintain soil moisture and moderate soil temperatures. Avoid piling it up around the stems of plants, especially if it has not been composted thoroughly.

To determine how much of a given fertilizer is necessary to apply ¹/₂ pound of actual nitrogen, divide 0.5 by the percentage of nitrogen in the fertilizer.

For example, 10-10-10 is 10 percent nitrogen, 10 percent phosphorus and 10 percent potash, always in that order. 0.5 divided by 0.1 equals 5 pounds of 10-10-10 fertilizer added to a cubic yard of sawdust. It is helpful to choose a fertilizer that contains only nitrogen, such as urea (46-0-0). It will take about 1 pound of urea to add one-half pound of nitrogen.

Q. I have noticed a yellowish-white crust on the surface of the soil and sides of my clay pots on many of my houseplants. Is this some kind of fungus?

A. What you are seeing is the accumulation of soluble salts from fertilizers and the minerals dissolved in hard water. Fertilizers are salts, just like table salt, except that they contain plant nutrients. As the salt builds up in the soil, it becomes difficult for the plants to take up water.

To minimize this buildup, be sure to water houseplants until the water comes out of the drainage hole(s) every time you water. It is easiest to put smaller pots in the sink or bathtub and water them there. Use a turkey baster to empty the saucers of plants too large to move so they do not sit in the drainage water. Otherwise, the soluble salts will be reabsorbed as the soil takes up the water in the saucer.

Plants that grow in the same pot for a long time should be leached every six months or so by running water equal to twice the volume of soil through the pot. If salt has accumulated on the top of the soil, remove the top ¹/₄ inch of soil before you leach.

Be careful not to overfertilize houseplants. As a rule of thumb, do not fertilize them at all through the winter months when they receive less light and are not actively growing. Use a water-soluble fertilizers such as Miracle-Gro or Peters formulations from April through September. Once a month should be more than adequate for most foliage houseplants. Use a balanced formulation such as 20-20-20.

Those grown for their flowers should be fertilized more often, perhaps every two weeks. Be sure to use a formulation with a higher phosphate (the middle number) content to avoid pushing vegetative growth at the expense of flowers.