Cherry guava

George asked why his cherry guava was not setting fruit although it produces plenty of flowers, and why the growing tips were dying. He didn’t mention which area he lived in, or what fertiliser and watering program the tree has received. There is a variety of causes for these problems.

Perhaps the tree is too young to set fruit. Cherry (or strawberry) guava trees (Psidium cattleianum) are slower growing than tropical guavas, and do not start to produce fruit until about 3 – 4 years after planting out –

Or, maybe frost has affected the flowers. Frost can also cause dying back of young tips, even though mature cherry guavas are reasonably frost hardy.

Although cherry guava are not too fussy about soil, they do grow very well in fertile soil. A lack of calcium will cause both dying of young tips, and premature shedding of blossoms – in other words, soil may be too acid to provide enough calcium. I’d advise testing soil around the tree and, if necessary, raise the soil pH.
Some trace element deficiencies can also cause die back of growing tips, but an application of seaweed extract tea to the soil around the tree should solve this problem.

If the soil is fertile and soil pH is suitable, perhaps the soil has become water repellent and the tree is unable to absorb the nutrients it needs. Or, watering may have been erratic and the tree may be water stressed.

What’s soil pH?

Soil pH is the measure of the acidity or alkalinity of soil from an extremely acid pH of 0 to an extremely alkaline pH of 14. A soil pH of 7 is neutral, neither acid not alkaline. Knowing the pH of your garden soil is important because soil pH controls the availability of nutrients and the number of microorganisms that improve soil structure. Plants can only absorb nutrients as electrically charged “ions” that attach themselves to clay and organic matter ions with the opposite electrical charge. Depending on the level of acidity or alkalinity of soil, varying amounts of different nutrients can be taken up by plant roots. At some pH levels, nutrients can become bound to other elements, or to soil, and become “locked out” and unavailable to plants. All the major nutrients are only freely available to plants within a narrow soil pH range of 6.5 to 7.5, where essential trace elements are also available, and aluminium is locked out. (See pH Table below) Most vegetables and exotics will remain healthy if grown in a pH range of 6.0–7.0, but potatoes and strawberries do best when pH is around 5.5, and Brassicas and beetroot require a pH close to neutral. However, few plants will survive when the soil pH is below 4.5 where major nutrients are strictly limited and trace elements become available in toxic quantities, or above 9.0 where calcium becomes insoluble.
On the pH scale, the “p” stands for potential, and “H” is the chemical symbol for hydrogen. The more acidic your soil is, the more hydrogen ions in your soil. As hydrogen ions are replaced by calcium ions on the charged sites, soil pH rises. Just to make it confusing, the pH scale is shown as a negative logarithm so that the more hydrogen ions in topsoil, the lower the pH number. Because soil pH is expressed as a logarithm, a pH of 6.0 is ten times more acid than a pH of 7.0, and a pH of 5.0 is a hundred times more acid than 7.0. Adjusting pH without the buffering effect of decomposed organic matter is difficult.
Testing for pH level
The only way to find the exact pH of garden soil is to test it. The Manutec Test Kit is quite economical to use, is available from most large nurseries. I’ve found this test kit to be very reliable and have used it since it was first developed by the CSIRO.
Testing involves taking samples of topsoil from across the growing area, and mixing them thoroughly in a bucket. A small sample of the mixture is placed on a supplied sheet and moistened with a liquid dye. The damp mixture is then dusted with barium sulphate, and the resulting colour matched to a pH range on the kit’s colour chart.
***All garden soils should be tested at least annually, because exudates from plant roots and the decomposition of organic matter release hydrogen ions into the soil, replacing calcium ions and increasing acidity.


Changing soil pH

If the pH of garden beds needs adjusting, organic gardeners have a distinct advantage over “chemical” gardeners, because mature compost has a pH of about 6.5 where all the major nutrients are freely available to plants, essential trace elements are available, and aluminium is locked out. Adding mature compost to topsoil when preparing beds will help to lower pH of alkaline soils, and raise the pH of more acid soils, as well as buffering plant roots from an unsuitable pH in surrounding soil. Where the amount of mature compost is limited, green manures and well-rotted manures will break down to add nutrients, microorganisms and humus to topsoil. Worm castings and other solid organic fertilisers provide nutrients in easily absorbed form. Garden beds should be prepared a month before planting to allow soil chemistry to achieve a balance.

To raise soil pH
In all acid soils, pH can be raised by the combined use of organic matter and the addition of calcium ions in the form of dolomite or lime.
Agricultural lime – (Calcium carbonate) is finely ground limestone (chalk). Mined limestone, i.e. not chemically treated, is a safe choice to raise pH in garden beds. Although it takes several weeks to have an effect, it is longer acting than other sources of lime, and can be watered in around plants. Agricultural lime can be worked into the top 15 cm of soil when preparing garden beds. It takes less lime to raise the pH of sandy soils than it does to change clay soils. To avoid an excess amount of calcium in soil, apply as recommended in the test kit, and test soil a month later.
I must say here that I have not found the application rate recommended by Manutec for “organic soils” to be accurate, if soils contain compost. It may have been calculated for soils where only manures are added.
Dolomite – (Calcium magnesium carbonate) is limestone with a higher proportion of magnesium than agricultural lime, and is applied in the same way. It is a good way to raise soil pH on sandy soils with fairly low organic matter content because both calcium and magnesium leach easily from these soils. In soils with high magnesium content, such as in South East Queensland, agricultural lime is the preferred way to raise soil pH.
Quick lime – (Calcium oxide) is made by heating limestone in a furnace to remove carbon dioxide. It is very caustic and unsuitable for garden use.
Hydrated or slaked lime – (Calcium hydroxide) is also known as brickies or builders’ lime because it is used to harden mortar. Hydrated lime is made by soaking quick lime in water to form hydroxides. It is more soluble and faster acting than agricultural lime, but its effects do not last as long. This lime can burn roots and should not be used on beds that contain plants. It should also be applied a month before organic matter and fertilisers or nitrogen can be lost through conversion to ammonia. Gloves and a mask should be worn when applying hydrated lime because it is very drying to skin and throat. Apply hydrated lime to the soil surface, and water it in.

To lower soil pH
Adding organic matter as compost, green manures, and animal manures, without including lime or dolomite, can be enough to adjust the pH of slightly alkaline soils because organic matter produces hydrogen ions as it decomposes.
Manure from cows, horses and sheep that have grazed on herbicide-free pasture can be used more liberally on alkaline soils. It has been calculated that 2–3 kilos of manure per square metre of bed area will reduce soil pH from 8.0 to 7.0. Manures release hydrogen ions as they break down, replacing calcium ions on the charged sites.
Elemental sulphur, sometimes sold as flowers of sulphur, will assist organic matter in reducing soil pH in more alkaline soils. Elemental sulphur is available from produce stores, and some nurseries. For soils with a sandy structure, apply at 35 g per square metre, or 100g per square metre for clay soils. Test soil after one month, to see if further applications are necessary.
Please note – Lime sulphur is a fungicide, not a soil conditioner.
Acidic fertiliser can assist when alkaline topsoil contains some organic matter and herbicide-free manures are not available. Multicrop’s Ecofish liquid fertiliser is registered by NASAA as an input for organic cultivation. The concentrate is very acidic and diluting it in water should modify the acidity, somewhat. It can be watered into the soil or used as a foliar feed for plants in alkaline soils.

Feeding citrus

I have revised this post to provide more detail because there have been a few enquiries recently about fertiliser requirements for citrus, particularly trace elements.
Time to fertilise
There is no set time of the year for fertilising citrus as different species produce fruit in different seasons. Many citrus trees are producing crops at this time of year (through winter) and it is not a good time to give them a good dose of fertiliser. As citrus go through a growth cycle after fruit has matured, a good general rule is to apply fertiliser to the soil surface, under the outer part of the canopy, after fruit has been harvested, then cover the fertiliser with about 5-8 cm of organic mulch. Avoid scratching fertiliser into the soil surface as citrus roots lie close to the soil surface, and never apply fertiliser to dry soil, as it will burn tree roots. Repeat the fertiliser application in approximately six months, but avoid applying fertiliser in very hot weather.
Suitable citrus fertilisers
Citrus trees require a good supply of fertiliser that contains a full range of both major and trace elements for good growth, and pest and disease resistance. Trees to five years of age can use up to 500 g of complete organic fertiliser per year, as the inclusion of organic matter in soil will make nutrients more readily available. This should be divided into 2 applications. Older trees may require a little more. Very young trees should receive a proportional amount, as they will also benefit from the occasional application of manure tea. Too much high nitrogen fertiliser will attract aphids, scale, the citrus butterflies and citrus leaf miner. Over fertilising can also kill citrus trees.
Compost is the best fertiliser for citrus, but worm castings, poultry-based fertilisers, and well-rotted manures will also keep trees healthy.
Major nutrient elements are: nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Part of the role of phosphorus in plants is to promote root formation and early growth. Heavy applications of high nitrogen fertilisers can make phosphorus unavailable to plants. Phosphorus is also only available within a narrow pH range. Keeping the soil pH around citrus trees close to neutral will improve their growth, and phosphorus from organic sources is more readily available to plants. Citrus have a fairly high requirement for the major element magnesium. Signs of magnesium deficiency appear first in older leaves where yellowing begins at the outer edges of the leaves and moves inwards, resulting a green V shape at the stalk end. It can also cause cupping of leaves and lack of sweetness in ripe fruit. This problem is common in citrus in autumn when fruit it maturing. This deficiency can be corrected quickly by watering in some Epsom salts: about 250 g for a young tree up to 2 kg for a fully-grown tree. If pale leaves have occurred on your citrus trees in the past, in future, apply one handful of dolomite per square metre of tree canopy when fertilising after harvest. In soils of SE Queensland that contain high quantities of magnesium, this problem will only occur where far too much potassium has been applied, or where soil has become quite acidic.
Yellow leaves in late winter, or early spring, are often caused by cold soils, if the tree has been adequately watered and fertilised. This problem will correct itself as the soil warms, and the tree begins to extract nitrogen from soil.
Trace elements
As mentioned above, citrus trees require a full range of trace elements. These are: iron, copper, zinc, molybdenum, manganese and boron. Their availability to plants is dependent on soil pH and the presence of organic matter in soil.
Copper deficiency will cause fruit drop and can, in more serious cases, cause gum to form inside and outside fruit, and on shoots. Iron deficiency reduces citrus crop size and causes leaves to gradually become pale green, and then fade to pale yellow, preventing the tree from manufacturing carbohydrates. Zinc deficiency reduces fruit bud formation, and manganese deficiency prevents vitamins forming in fruit. Zinc and manganese deficiency both produce yellow mottling between veins on young leaves, but in zinc deficiency leaves are smaller than normal and bunch together. Boron is essential for flower production and fruit quality, but boron toxicity can be a problem where laundry grey water containing borax has been used for irrigation.
Trace element deficiencies can occur if your soil, or the fertiliser you have been using, does not contain a particular micronutrient. However, they are most commonly caused by your soil being too acid or alkaline where nutrients are locked into compounds that plants can’t absorb. Humus in soil has a pH of around 6.5 where all nutrients are available to plants. Humus is also able to hold trace elements in a form that is easily absorbed and prevent nutrients leaching away through soil.
It is unwise to apply trace elements individually because, as the name implies, they are only required in tiny amounts, and excess applications can be toxic to plants, causing another range of problems.
To avoid trace element deficiencies, add a moderate amount of compost to the soil surface around your citrus trees. Organic mulch will also produce humus after friendly microorganisms break it down, and earthworms distribute it through soil. Also apply a liquid seaweed extract, at weak black tea strength, in autumn and spring to ensure your trees have access to a full range of trace elements. Seaweed also contains a good quantity of potassium to improve fruit quality and build plants’ resistance to pests, disease, frost and drought by strengthening cell walls. If you are experiencing serious problems with the health of your citrus trees, I suggest you test and correct your soil pH, or problems will continue.
Reasonably priced Soil pH test kits are available from most nurseries.


Recent storms have not only limited gardening time; they have played havoc with our power supply and phone lines. Consequently, computer work and posting on the internet has been difficult. However, I did find time after the Full Moon to get the asparagus bed ready for spring.

I always wait until our asparagus foliage has developed a yellowish-brown colour before cutting back the plants to several centimetres above ground level. Asparagus plants withdraw nutrients and carbohydrates from the foliage and store them in the roots during their period of dormancy; in the same way that bulbs store nutrients to provide spring growth. Cutting back asparagus (and spring bulbs) while foliage is green will weaken the plants.
After cutting back the plants, I remove any weeds and test the pH of the bed, as asparagus prefer a soil pH of 6.5 for good growth. The soil pH in our bed was 7.0 so it will not be necessary to add any dolomite to the bed this year. Then I give the bed a thorough watering and a good drink of seaweed extract tea (see post on Seaweed tea). This delicious vegetable and medicinal herb originated along coastal areas and riverbanks, and will appreciate the full range of trace elements that seaweed provides. They are also salt-resistant plants and are one of the vegetables that will do well where soils or water supplies are saline. However, they don’t particularly like heavy clay soils, and mixing some well-washed river sand through the topsoil before planting crowns will assist spear production.
Asparagus are fairly heavy feeders with a high nitrogen requirement. I fed mine with some organic poultry complete fertiliser, some semi-mature compost to provide food for the large family of earthworms in the bed, and some not too fresh horse manure. Manures are slightly acidic and will help bring the pH back a little. I then covered the bed with 5-7 cm of fluffed-up organic mulch. Apart from an occasional watering to keep the bed just damp, the asparagus will not require any attention until spears start to appear in spring.