Very few areas of the world have ideal soils and in most places amendments are necessary to achieve the best outcomes. Australia is the driest continent and has some of the most infertile soils, and we on the coastal sand plain of southern Western Australia epitomise this less than favourable situation. Our sands retain almost no water or nutrients, are often water repellent, calcareous and alkaline, they lack structure, and have virtually no organic matter or clays and only a small microbial population. Many of these features are exacerbated with our precipitation being mainly outside the principal growing season of most fruit trees. This seasonal rainfall distribution is opposite to plant evapo-transpirational needs and humidity requirement for good fertilisation of flowers in many species. Overall a bad starting position, but not hopeless as all these negatives can be significantly improved given sufficient management attention and time. Maybe we should count our blessings rather than lament our woes as we don’t have to endure many of the less-resolvable situations in other sub-tropical areas of the world. For example we don’t have devastating cyclones that can all too quickly destroy crops or mature trees, heavy clay soils that require active drainage and are very difficult to work, topsoils which may be more rock than soil or with subsoil consisting of solid layer bedrock or clay pans, extensive flooding, overwhelming salinity, heavy (or even any) frosts or serious and frequent hail, and we do have plenty of sunlight for photosynthesis.
The following comments are mainly focussed on management of perennial fruit trees for optimal yield and fruit quality. Annual vegetables, broad acre farming and even commercial scale orchards have many different challenges, resulting in strategies which may vary somewhat. Usually in our scene we have the benefit of some form of irrigation throughout the hotter months of the year, and we work with untilled soil. Soil is a complex system influenced by many interacting factors, and it’s important in trying to improve plant growth outcomes to identify which factor/s are limiting and which strategy best targets the solution without negatively impacting on other desirable features.
The three main steps to take are to add clays and organic matter (OM), and get soil pH into the slightly acid range preferred by most fruit trees. Considering our challenges in turn:
This is almost zero, plus the breadth profile with depth is very narrow ie the water essentially goes straight down as a narrow cone from wherever it’s applied. Where dripper irrigation is used this means the cross-sectional area moistened is limited, with negative effects on lateral root growth for searching out nutrients. Sand is very wasteful of precious water as it allows rapid penetration below the root zone, and so more frequent watering is required. Worldwide, good fertile soils (variously-named loams) often have 20% or more clay content, and clays are very good at adsorbing water. Trying to replicate this proportional content in our sands starting with almost zero clay would be prohibitively expensive - either by trucking out the original and replacing it all with good loams or by adding prodigious amounts of clay. Thankfully such a massive exercise is not necessary as much smaller amounts of clay additions are sufficient for a good long term solution.
The main clay types are referred to as 1:1 and 2:1, with well-known examples of these being kaolin and bentonite respectively. While both can be obtained relatively cheaply and locally, it should be realised they can vary considerably in their properties depending on their formation geology at different locations. Most fertile loams consist of varying mixtures of these two types of clay instead of just one or the other. Generally, bentonite can absorb about 15 times its own weight in water whereas for kaolin it’s only 1-2 fold. Adsorbed water can then be slowly released back into the soil for plant use, so leading to a reduction in the necessity for frequent watering. A clay content of 1-2% in the top 30cm soil is often sufficient to improve this parameter dramatically., This equates to roughly 3-5kg per square metre. If you have open ground then you can speed up the integration process by digging in the clays. Left on the surface at this application rate, the next time it rains you’ll have a messy covering that interferes with water penetration deeper down. If existing plants don’t allow tilling then it should be added in portions, with time allowed between applications for more passive forms of distribution eg by water or fauna such as earthworms. Non-Na forms of clays (usually Ca) are best as they don’t exacerbate Na toxicity problems, which would otherwise have to be rectified by extensive watering to leach out the Na; in the meantime sensitive plants could suffer damage. One disadvantage of bentonite over kaolin is that it is a swelling clay, so that it swells up when wet and cracks when dry. But this phenomenon only becomes marked with high clay contents, ie 20% or more, and this level is not recommended or necessary here.
To a lesser degree OM also contributes, as unlike mineral sand grains, OM is porous and can internally absorb some water. This holds whether it has been incorporated into the soil or is surface-only. The benefits of adding surface mulch (immature or particulate OM, POM) are well known, not only in terms of soil water retention and reduction in evaporation but also in moderating soil temperature and suppressing weed growth. The irregular shape of POM means it doesn’t pack very well, leaving pockets and pathways throughout the soil that water can be trapped in. To keep these pathways as open as possible it is important to avoid compaction. If immature POM is added only as a surface layer it should be coarse enough to not act as a wick via capillary action to draw up water from below and actually hasten evaporation.
Again in our sands, this property is almost zero. The most important variable of all soil constituents and the easiest to rectify poor nutrient retention is OM. Our undisturbed sands start off with virtually none, excluding loose surface litter such as leaves and twigs, whereas fertile loams have 2-5%. Peat bogs might have OM as high as 80% but this suits only a very small fraction of plants. Even 30% OM can be counter-productive, so more than 5% is not necessarily better. Unlike clay mineral additions which give long term effects, relatively fresh OM is continually broken down by soil biota, so there is a continuing need to replace it, and also to gradually move from the starting level of nothing towards 5%. This can take several years. After all, the native soil took millennia to get where it is and this can’t be altered instantly. Fresh mulch or POM loses more than half its carbon (C) mass (respired carbon dioxide) as soil biota break down the plant material, absorb it within their collective biomass and excrete metabolic products. In undisturbed conditions these reactions can take considerable time, depending on moisture levels, aeration, microbe populations and C/N ratio. Composting is a process which can fast-track the conversion into more stable forms. The end goal of fully mature compost is humus material which can have a soil lifetime of years.
Along the way to this favoured goal there will be a mix of POM with humus. These two components, the immature forms within POM aggregates and the humus, are together the major factors in nutrient retention. Essential plant nutrients are mostly absorbed as cations (positively charged) with a few being anions (negatively charged); boron is the only essential nutrient that is absorbed uncharged. The role of humus and POM is to temporarily hold these ions ie prevent them from being quickly leached below the root zone by bulk water flow, and then release them over time for uptake by plant roots. The capacity for holding these ions is called cation exchange capacity (CEC) and anion exchange capacity (AEC); CEC is usually much greater than AEC. The importance of OM in addressing this issue can be gauged from CEC values for different soil components: sand, 2-4 mEq/100g; kaolin; 2-10; bentonite, 80-120 and OM, 100-300 whereas a typical loam might be 20-25. So OM is the most efficient way to improve this factor. Nutrients are therefore principally retained through the OM and POM CEC and also on a longer time scale by release from dead and decaying microbes, roots and other soil fauna. As with incorporating clays for water adsorption, tilling of added OM is fastest, but if not possible, it can only be applied on the surface in batches. Fresh OM, like fresh manures, should never be placed closer than 30cm to tree trunks to minimise disease problems. Another potential problem in applying POM to soils is that of N drawdown if the material is poor in N. It should be apparent that for a normal household property, buying a few 30kg bags of compost is not going to get there at all.
Clays contribute to nutrient retention but are not the key determinants when OM is sufficient. With heavy clay soils (ie 40% and above) their relative contribution may be more important, but this is not a goal to be pursued as they can become unworkable and drown plant roots. Bentonites have essentially a permanent CEC which is much greater than for kaolins, and in the latter, most of the CEC is subject to soil pH, only really being significant under alkaline conditions not favourable for most species. However kaolins do coat POM well, slowing down microbial decomposition of POM and thus prolonging OM CEC in situations where OM is not added very frequently. For example in broad acre farming, the main means of providing OM might be alternating years of legumes and crop. In this situation, mixtures of the two clay types can perform better than either alone. Kaolins are typically formed in highly weathered soils and often contain significant proportions of Fe and Al oxides, and although these also bind nutrients they may cause toxicity problems without knowledgeable management.
As our sands dry out during fierce summer heat and virtual absence of natural precipitation, they become hydrophobic and are not easily wetted. As a consequence, applied water remains on the surface without penetrating down to the root zone or may form isolated furrows where water is channelled so that only a very small portion of the whole root volume may be in moist soil. The problem is due to the presence of metabolic products from microbial activity which are hydrophobic and coat the sand grains. With prolonged watering such as during our wet winters, these sands will eventually wet, but each time they dry out the same repellency problem recurs. In addition to being a problem for optimal plant growth, land care agencies are concerned as it contributes to wind and water soil erosion.
The easiest way to improve things is again by addition of clays, with most studies showing the kaolins are better than the bentonite types. When wetted, kaolins appear to disperse better than bentonites which have more of a tendency to clump, and the dispersed form is able to form a more complete coating of sand grains. Bentonite preferentially adsorbs in isolated crevices on the sand grains, so leaving many hydrophobic surfaces still exposed, leading to a reduced effect. Only small quantities need be added for dramatic amelioration of repellency –often 1% or less, which will be achieved when clays are added for improving water retention.
OM also helps, but to a lesser extent. As POM is broken down it also develops a hydrophobic coating, but the loose packing with many air spaces and channels means it’s harder to completely coat the aggregate, allowing water to penetrate where it then has more time to slowly soak in before quickly flowing away.
The most common constituent in sands is silica, but here on the west coast some areas can also have limestone as solid outcrops and underground layers, or rock pieces dispersed in the sand, or pulverised limestone grains mixed in with siliceous sand. Limestone is calcium carbonate, which is alkaline and results in a soil pH which is unfavourable to the growth of most fruit tree species (generally pH 6-6.5 preferred). Depending on its relative level in the soil it can represent a considerable challenge to get pH down into the desired range and may take years to achieve. Only on smaller plots can it even be contemplated. Acidifying fertilizers such as ammonium sulphate will gradually have some effect, but if the reserve of limestone is large, it means the pH will be buffered at high levels till most is dissolved. A more long term strategy is to use powdered sulphur; this is slowly broken down by soil thiobacteria over several months to form sulphuric acid, but again, the buffer reserve has to be overcome to achieve other than marginal effects. Incorporation of S throughout the top 30cm of soil will speed up its acidifying effect, but again, with pre-existing trees this may not be sensible and it can only be surface applied, resulting in a much slower effect. Optimising soil pH can’t be done in one step as each location will be different with more or less limestone and other material. Instead, a series of S additions will be needed and then progress checked with a soil pH meter. Granular S may have the advantage that it’s not blown around by the wind when applying, but the acidifying effect is even slower.
Structure is important because it assists plant root penetration throughout the soil, soil aeration and water penetration; our loosely flowing sands have almost none. Good soil structure can also slow decomposition of POM, important with low frequency application of OM. OM deep within larger POM aggregates is less accessible to saprophytic microbes if coated by clay particles, and as mentioned above, maintaining this OM, although immature, has a major impact on nutrient retention and therefore soil fertility. Clays assist by coating sand grains and OM particles with films when wet, and then as they dry out, sticky, adhesive gel states bring and hold grains and particles together. Decomposable OM also assists aggregate formation independently of clays through metabolising fungal hyphae and filamentous algae enmeshing the particles and secreting catabolic enzymes and polysaccharides for particle decomposition; these hyphae/filaments/molecules help tie and bind soil particles together. Only small concentrations (1-5%) of clays and OM are required to achieve these positive outcomes, so what is added to resolve issues above serve duty on this factor also.
Other contributors to soil structure include root exudates and fauna such as earth worms which create casts of mucilaginous binding material and tunnelling burrows, giving air and water better access throughout the soil. Solutions to achieving good soil structure illustrate some of the complexity and interacting nature of the many factors involved in soil fertility. You’ll know you’re on the right track when you squeeze a handful of moist soil and on release it stays compacted; sand just runs through your fingers.
Fertile soil has a thriving and diverse community of microbes and larger fauna. These break down OM which would otherwise not be available to plants, and convert it into chemical forms that have very stable high CEC, or store nutrient and metabolic products as biomass for subsequent slow release. Diversity also enables competition with otherwise damaging soil pathogens, often keeping them under control without external intervention. Other microbes such as N-fixing bacteria in legumes can directly convert gaseous N into ammonium ions suitable for plant absorption, making them less dependent on supply such as with synthetic fertilizers or manures. Symbiotic mycorrhizal associations occur in probably 80% of dicotyledonous plants and the extensive nutrient absorption capacity of fungal hyphae greatly assists nutrient supply in the plant, at the cost of having to supply photosynthetic carbohydrates to the fungi. This relationship is invaluable for both parties. All these life forms need soil conditions in which they can survive, function and reproduce.
Soils where only chemical fertilizers are used to supply nutrients gradually have smaller populations over the years. Generally all these various life forms fare better in more natural environments where air, moisture, OM and favourable niches are available. OM is a very important factor in meeting these requirements, as it provides food, essential CEC-adsorbed nutrients, keeps some moisture, maintains a maze of air pockets for aerobic metabolism, and also provides niches in the soil battleground where valuable microbes may be protected from predation. Clays also assist in improving water penetration and retention, nutrient storage and preservation of POM. The clumping behaviour of bentonite seems to have a better pore structure than kaolin for creating favourable microbial niches on sand grains. Soil pH also influences which microbes thrive and function – some do better in acid conditions and others when alkaline. A good indicator of progress is when a spade full of soil contains many earthworms.
Given these considerations, how do we weigh it all up? We should appreciate that different locations may face slightly different or additional conditions that might further impact on management strategies. For example, those very close to the ocean may experience salt spray which then washes into the soil, making it more sodic. Or ground water for irrigation might be saline or strongly alkaline or very high in Fe (unsightly staining problem in addition to nutrient effects). Or you may have larger amounts of solid limestone than elsewhere. Or you might live in a low-lying area close to the river where ground water is brackish and the soil has poor aeration.
But taking the average for our coastal plains area, as a first step you should probably aim to incorporate a mix of kaolin and bentonite for your clays. Your OM should be as mature as possible and nutrient rich in essential elements. Otherwise with low OM nutrient levels, the CEC buffering action will mean it’s harder for plants to benefit and considerable proportions of applied nutrients will be stored rather than being readily available for plant use. You’ll need to undertake a long term build-up programme to get very stable humus up towards 3-5%. Even when you approach this level you’ll still need to regularly add POM; this provides food (C) for all your soil microbes and fauna. When the clays and OM are both in the system and you intend to add OM frequently, you might find water repellency is not a major problem and bentonite alone may be sufficient for good outcomes. With established plants, both OM and clays will have to be applied in batches over time. If you have a collection of many different in-ground species, it won’t be possible to match soil pH exactly with the preferred level of each plant, so the best you can do is aim for the most common value of 6-6.5. You need to periodically monitor progress towards this target with a meter. Once plants are in-ground, you should adopt a no-till strategy as this not only avoids possible root damage to roots but minimises disruption of soil structure.