Phosphate toxicity is indicated first by interveinal yellowing of the foliage, followed by blackening of the leaves and twigs. Death will result eventually. If a plant is suspected of being poisoned by phosphate, immediate applications of almost any form of iron will usually cure the problem. Iron and phosphate form a molecular complex, and render each other unavailable, both in the soil and in the leaf. Use a light rate and wait a couple of weeks between applications.
Plants usually respond favorably by producing new, dark green growth.
Iron is usually present in the soil but complexed with another element and unavailable to the plants, especially at higher pH. Applications of sulfur will usually cure the problem in the long run by making the soil more acid. Magnesium deficiency can also be present in Australian natives in many California soils. Epsom salts can be used to cure this problem. A result of adaptation to low nutrient levels is that some of the smaller Australian woody shrubs and perennials will need help with weed suppression in our richer soils, through the use of either mulch or preemergents. If the plants are grown in soils with low nutrient levels, and fertilizing is kept to a minimum, weeds should be much less of a problem.