Sulfur (or Sulphur)

Sulfur may be the stuff of brimstone, but it is also an important plant nutrient.

Sulfur crystal by Rob Lavinsky (iRocks.com) CC-BY-SA-3.0

Plants use a surprising amount of sulfur. This secondary nutrient is used in making chlorophyll and certain proteins and enzymes. Sulfur is also part of the arrangement between legumes and rhizobia bacteria that allow them to make use of atmospheric nitrogen.

Plants tend to pull equal amounts of sulfur and phosphorus from the soil. Imbalances can cause problems.

Chemical balance in the garden

Most plants prefer a relatively neutral to slightly acidic pH. Some plants, such as blueberry, prefer more acidic soil. Sprinkling sulfur throughout the garden and then watering it in creates sulfuric acid. This is not the acid that will dissolve your car’s paint, but it will help make many nutrients available to plant roots. Before treating soil with sulfur to adjust the pH, it is important to get a soil test from a reputable lab. Too much of a good thing can be a bad thing. If sulfur levels become too low, some plants, such as clovers, will disappear completely. Sulfur deficiency is seen first in new growth. Leaves are pale and growth is spindly. If sulfur levels become toxic, leaves will be smaller than normal and have scorched edges.

Sulfur as fungicide

This bright yellow mineral has antifungal properties. Dusting plants with sulfur can prevent or counteract many fungal diseases, such as powdery mildew, brown spot, crown rot, and others. Fungi generally avoid acidic environments, which is what makes sulfur so effective as an organic fungicide. Sulfur in the soil also helps reduce salt levels.

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WARNING: 

Do not use horticultural oil within 2-4 weeks of applying sulfur. Sulfur and horticultural oil  are phytotoxic (poisonous to plants) when combined. Also, do not use sulfur on apricot trees. Use fixed copper, instead.