Soil Ecology Restoration Group  

 

Possible zinc deficiency on disturbed sites

Zinc is one of the essential micronutrients for plants (Fe, Mn, B, Mo, Cl are others). Zinc is important as an enzyme catalyst, and lack of zinc is often reflected in poor carbohydrate metabolism and protein synthesis. Zinc also stabilizes the association between catalytic and regulatory units of enzymes (Jenny, 1980). This can cause stunting because inadequate auxins (specifically tryptophan and its successor indoleatic acid, IAA) may be produced (Landis and van Steenis, 1998). This may be revealed in reduced internode elongation of stunted foliage, in some cases resulting in resetting and little-leaf syndrome, Chlorosis of younger leaves with interveinal mottling is diagnostic in citrus, but foliar symptoms in other plants are more variable.

Zinc deficiency is not uncommon and is most important in younger tissues. This may make it very important in early growth and plant establishment. Inadequate zinc may reduce growth of roots below critical levels as a result of depressed metabolism. Zinc deficiency may also result in increased susceptibility to disease. In some agricultural crops zinc deficiency prevents flowers from forming, and in others seeds may fail to develop properly (Parnes, 1990). Zinc can be measured in the soil of in plant tissues. Tissue testing is often done with agricultural crops, but no levels of desirable levels of zinc in native plants are available.

Zinc deficiency may be caused by low levels of zinc in the soil or low availability of zinc. The level of zinc needed for healthy coastal sage scrub is unknown, but for seedlings of many trees it is fairly high. For example, the minimum desirable tissue levels for zinc in Pinus radiata was 11 ppm (McGrath and Robson, 1984). Most agricultural crops respond to added zinc when levels drop below 0.5 ppm DTPA extractable zinc (Brown and de Boer, 1983), and levels below 1.5 ppm are of concern (Soil Improvement Committee, 1998).

Plants concentrate zinc and organic matter generally has mulch higher levels of available zinc than surrounding soils. Organic matter in the soil may be required to maintain zinc in a chelated or available form (Parnes, 1990). Low organic matter in soils may be a contributing factor in zinc deficiency.

Zinc availability is very closely linked to soil pH (Pierzynski et al., 1993). With more acidic soils (lower pH) more zinc is available, while with more basic zinc is less available. Zinc uptake is also aided by the beneficial mycorrhizal fungus associations with plant roots (Landis and can Steenis, 1998). Aerated soils also increase zinc availability (Parnes, 1990).

Few studies have been done on normal levels of available or total zinc in soil samples of Southern California. A range of 25-200 ppm total zinc is considered normal for soils (Bowie and Thornton, 1985). Available zinc levels in 461 California soils averaged 1.23 ppm, with a high of 10.4 ppm (Brown and de Boer, 1983). In our testing possibly deficient soils (<1.5 ppm) and deficient soils (<0.5 ppm available zinc) were very common at our restoration sites, table 1.

 

Table 1. Possible zinc deficiency

  n Percent 0.5 ppm or less Percent 1.5 ppm or less
Hungry Valley
Red Rock Canyon
Penasquitos
Scripps Knoll
Elliot Reserve
USIU
6
42
9
31
9
8
100
90
55
42
37.5
33
100
98
100
74
75
66

 

The low zinc levels in disturbed and very disturbed soils in Southern California are compounded by increased soil pH, soil compaction, and decreased organic matter. If we assume a direct linkage between zinc and organic matter the disturbed soils have only one fourth the available zinc, and the very disturbed soils have only one tenth the available zinc of the less disturbed soils. Adding in compaction and loss of soil symbionts zinc availability in degraded soils may drop to 5% of the level in native soils.

How can we add more zinc or make zinc more available? Adding organic matter adds small amounts of zinc and increases zinc availability. Using a more acidic compost, such as pine needles would also help. Reducing compaction with ripping or tillage helps by improving aeration and facilitating growth of mycorrhizal symbionts. Zinc can also be added by using organic fertilizers like poultry manure that have low levels of zinc. A zinc fertilizer such as zinc sulfate, which is 25-35% zinc, can be used (Parnes, 1990). A typical application rate would be 2-20 pounds per acre. Zinc can also be provided in a chelated micronutrient fertilizer, either a dry powder or sprayed as a foliar feed (Peaceful Valley, 1999). We will evaluate zinc fertilization during 1999-2000.