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Monday, July 1, 2013

Most Desert Plants Do Not Need Sulfur Applied to the Soil

Q. Do desert-adapted shrubs like Texas Rangers, Cassias etc benefit from adding sulfur soil?  Will they perform better in a pH around 7.5 as opposed to 8.0?   I know it's probably not necessary but I'm just wondering if the add'l sulfur in the soil will help them thrive better or if it's overkill.

A. We have to remember that the pH scale is exponential - like the scale used to measure earthquakes. So changing the pH from 8 to 7.5 is huge. Our soils are chocked full of calcium carbonate or lime which wants to force the soil to a pH around 8.2.

Because there is so much lime in our soils, lowering the soil pH is always just a temporary solution to the problem.

Yes, sulfur in moist, warm soils will slowly bring the soil pH down from 8 to some lower value. So will decomposing organic matter. Mineral sulfur (not sulfates) will produce acidity as the sulfur changes to the sulfate form.
sulphur granular
Sulfur granules
When most of the sulfur has been converted to sulfate the pH will begin to rise again rather rather quickly. Then you apply more sulfur and it will work again in the same manner.

Two years after spreading granular sulfur in a desert landscape. It still has not broken down. 

When organic matter is mixed in the soil and is decomposing, the micoorganisms responsible for breaking down this organic matter release carbon dioxide. This carbon dioxide mixes with soil water and produces carbonic acid which also lowers the soil pH. However, it also only works only in warm, moist soils.

The question becomes is it really necessary? In some cases probably not. The pH of the water conducted from the roots to the leaves of most plants is about 6.8. So any time you can keep the soil close to pH 6.8 you are better off.

Some plants are more fussy about soil pH than others and demonstrate this fussiness through problems such as the leaf yellowing due to iron chlorosis. These fussy plants need the soil modified with sulfur or organic matter or the additions of iron in a form that works at a higher pH. At a pH of 8 the only chelate applied to the soil that really works is EDDHA. This is the reason I mention it so often.
pH stability diagram for different iron chelats. Note that
EDDHA chelate remains stable as the pH increases.
Borrowed from this website

Many desert plants would also prefer to have their roots surrounded by a soil at a pH of 6.8 but can tolerate soils much higher than this. Plants not from desert soils, like photinia and Indian hawthorn apparently do not handle soils with a high pH very well.

What can you do? The plants you mention are tolerant of desert soils and so probably not have problems. Watch your plants. If they demonstrate they are having some problems (yellowing of leaves, unhealthy weak growth) and you have planted them correctly, watering and fertilizing them, then apply some sulfur or wood mulch at the surface that will decompose and let the soil slowly adjust its pH during the warm months.

If they are doing well without it then don’t bother. If yellowing leaves are the problem then apply the iron chelate, iron EDDHA, to help get iron inside the plant in a form it can use. I would do it case-by-case.

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