Having just read two references to mulching fruit trees in the July newsletter, I thought I would pass on some information I have gained over the years on the role of mulch from both personal experience and from various readings.

Firstly in response to Mr. Whitman's comments about mulch spreading virus and fungus diseases, I have been mulching trees now for over 12 years and have never had any of the problems he cites. I now have over 1000 fruit trees mulched including citrus trees. Rots can develop in tree trunks because: (a) fresh organic material gives off heat which can burn the trunk and provide an entry point for fungus and (b) organisms which naturally occur in mulch and soil, especially in wet conditions can promote fungal or bacterial rots. These problems can be avoided by keeping all mulching material at least 10 cm from the tree to prevent the trunk from staying moist for too long.

Organic mulching is the covering of soil surface with crop residues, legume or grass hay, there are also of course such non-organic mulches as plastic. Mulching is a method of improving the physical condition and fertility of the soil and changing its microclimate. Subtropical fruit trees have evolved in a microclimate very different to that of the conventional orchard. Using mulches while the canopy cover is being established can buffer the soil against nutritional, biological and ecological disorders or imbalances which are detrimental to tree health.

Mulching results in optimum temperature and moisture conditions for root growth, improved soil structure and physical penetration of plant roots, increased infiltration capacity and erosion control, higher organic matter content and improved weed control. Organic matter can also act as a buffer against the effects of excessive fertilizer applications. In addition to improving the soil structure, readily decomposable organic mulches release nutrients to plants and increase the biological activity of the soil. Grass or trash mulches have been shown to increase available moisture - soil moisture levels and infiltration rates were shown to increase from 30% on bare ground to 90% on mulched soil. Unwatered straw mulches have been shown to have increased soil moisture in the top 15 cm of soil compared with regularly watered bare soil.

Research in South Africa with avocado, macadamia and citrus trees have shown that mulch treatments promoted development of vigorous roots on the surface, soil moisture was much increased and daily soil temperature varied much less under mulches. Young macadamia trees cropped better with mulching. In Russia annual mulching of tea increased the yield by 76% and also reduced weed growth. Dried grass mulch 8-l0 cm thick increased coffee yields by 173-233% in Africa. Yield increases of 146% have been obtained using a straw mulch on peach trees. In another test a layer of straw 4 cm thick reduced evaporation by 73%. A layer 16 cm thick reduced evaporation by only a further 7%. This confirms that more is not always better, in fact thick mulches are undesirable as they hold a lot of water, so reducing the proportion of water reaching roots.

Organic mulches reduce the soil temperature which means that in the summer plant roots can grow right up into the moist cool surface layer of soil. In cooler areas mulch has a major disadvantage - frost damage may be increased. The answer here is to remove your mulch during frost-prone periods.

All of my trees are mulched and although they don't get the care and attention they deserve due to other commitments (family, work, house building, etc.) I have a survival rate that I am very pleased with and I attribute this to mulching. Last year prior to a particularly dry spell I planted 64 persimmon trees. I watered them, mulched them and left them to it. On checking them several months later, the only tree that had died was one that I had missed with the mulch.

Given the fact that organic mulches consist of bulky animal or vegetable matter, readers could be excused for being confused by Sandra Clayton's conclusion in the paragraph on mulching fruit trees the statement that lime, urine and wood ashes are also good. They may be good, but they are NOT mulches, The best use for urine and wood ashes are in the compost heap. Urine is considered by some to be the very best compost activator. It is definitely an under-utilized resource. It is sterile (W.W.1 soldiers are said to have bathed their wounds with it), it contains approximately 94% of the total NPK excreted by - humans (500 litres contains 5.6 kg N, 0.4 kg P, 7.0 kg K) and is free from heavy metals such as mercury, lead and cadmium (these are fixed in the liver and kidney). Wood ashes have an average composition of 1.5% P, 7.O% K, their potassium content very useful in the compost heap of the organic farmer, as most animal manures are rather low in this element. My ceramics friends who use wood ash glazes tell me that wood ashes are very alkaline and caustic and that gloves and a dust mask are essential when handling this material. Perhaps a safer source of K for the compost heap is molasses (approx 5%K) which has the added advantage, like urine, of being a great activator, but then compost-making is another story altogether!

Phil Thomas,
Upper Dungay, Murwillumbah, NSW 2484

DATE: September 1994

* * * * * * * * * * * * *