If you think nothing of pouring used soapy water onto your garden, think again. Your household cleaning products may bear reassuring labels such as "green," "non-toxic" and "environmentally friendly," but they're still detergents. The active ingredients that clean your clothes and dishes are toxic to plant growth. They're toxic to the mites and aphids that prey on plants, too, but great care must be taken when soaping the pests away.
Growth Improvement Through Pest Control
Your plants' health can only improve when spider mites, aphids, mealybugs and other soft-bodied bugs stop munching on the leaves. Sap-sucking aphids stunt new growth; spider mites eat up new leaves and buds. Getting rid of these pests can restore your plants to vibrant growth. You can do this with diligent application of insecticidal soap. Soap disrupts the cell membranes of these insects and attacks their protective, moisture-preserving wax coat. Dilute insecticidal soap according to instructions. Make sure you spray each plant thoroughly, undersides of the leaves included, in order to coat every insect present. Insects that are born or that arrive after the soap dries will not be affected. The Garden Helper recommends spraying daily in order to catch every generation of mites.
Dehydration Through Leaf Destruction
Soap can hurt your plant in the same way it kills insects. It'll strip the wax coating right off your plants' leaves, allowing them to dehydrate no matter how much you water them. Loss of water slows leaf growth and causes leaves to curl, yellow, turn brown or simply drop off. Flowers will also drop prematurely.
To be safe, don't make your own pest-control soap spray at home. Unlike insecticidal soaps, which include ingredients selected to maximize impact on specific pest populations while minimizing plant injury, household detergents and liquid soaps are an unknown quantity. They may or may not kill your pests, and there's no way of knowing how much to dilute them to make them safe for your plants. And some soaps, such as dry dish soaps and clothes-washing detergents, may not be safe at any concentration.
Detergent in the soil can also affect your plants' growth. Many household detergents contain sodium, which increases the saline toxicity of the soil. Plants do not thrive in saline conditions. Detergents can also disrupt the natural "percolation" of the soil--the ability for water to move through the soil and reach the plants' roots. Lanfax Labs discovered that in four out of five cases, leftover laundry water penetrated the soil to a noticeably lesser extent than clean water. The more detergent in the soil, the less water will reach the plants' roots, and the quicker the plants will show signs of dehydration.
Finally, remember an important purpose of detergents and liquid soaps is to remove dirt from clothes and hands. Accordingly, soapy water is likely to wash necessary nutrients out of the soil and away from your plants' roots, damaging them through nutrient imbalance and deficiency.