Article by Bas van den Ende
Consultant in fruit production (ret.).
Every orchardist should understand the importance of soil pH as it influences availability of mineral nutrients in the soil necessary for tree growth, yield and high quality of fruit.
pH is a measure of acidity and alkalinity. The letters pH stand for potential of hydrogen. When we measure the pH of a soil, the terms acid, neutral and alkaline refer to the relative concentrations of the hydrogen ions, written as H+, and the hydroxyl ions, written as OH‾ in the soil solution. In order to distinguish between the relative degrees of acidity or alkalinity of a soil, a pH scale from 0 to 14 is used. At the middle of the scale (pH 7.0), soil is neutral in reaction, while below 7.0 the reaction is acidic, and above 7.0 it is alkaline (basic).
The lower the pH value, the more acidic the soil, and conversely, the higher the pH, the more alkaline. Since pH is a logarithmic function, each pH unit represents a tenfold increase in relative acidity or alkalinity. For example, a soil with a pH of 5.0 is 10 times more acidic than one with a pH of 6.0. Also, a soil with a pH of 8.0 is 10 times more alkaline than one with a pH of 7.0 and 100 times more alkaline than one with a pH of 6.0.
The pH of your orchard soil can affect the availability of nutrients or elements (plant food) in the soil, and the solubility of toxic elements, such as aluminum. Soil pH can also affect vigour of roots, nitrogen cycling, and soil microorganisms. A pH between 6.5 and 7.5 gives a maximum availability of the primary nutrients nitrogen (N), phosphorus (P) and potassium (K), and a relatively high availability of the other nutrients. In strongly acidic soils (pH below 5.5), the solubility of aluminum and manganese and other heavy metals is high, and yields are frequently reduced by toxic levels of one or more of these elements. At the other end of the scale are the strongly alkaline soils (pH above 8.5) These contain precipitated carbonates (calcareous soils). Tree growth on these soils may be limited by inadequate supplies of iron (lime-induced chlorosis) and zinc. Although fruit trees will grow and produce a crop over a wide range of soil pH, management is much easier if pH is maintained in the 6.2 to 7.0 range. This article will deal with soils that are acidic (pH lower than 7.0).
The pH is conventionally determined in a suspension of soil and distilled water (1:5 ratio of soil:water). This is called water pH. Most soil testing laboratories also report pH values as calcium chloride pH for greater accuracy, by mixing the soil in a weak calcium chloride solution instead of distilled water (also at a 1:5 ratio of soil:calcium chloride). With the later method, pH values are usually 0.5 to 1.0 units lower than values in distilled water are. The optimum pH in calcium chloride (written as CaCl2) is 5.5. Values in this article are referred to as water pH. There are many methods to test the pH of your soil, including pastes, probes, reacting paper, pH testing kits, and the pH meter with electrode. For orchard soils, the most reliable and proven method is the pH meter with electrode, done in a reputable laboratory. Do-it-yourself testing can be inaccurate resulting in over-liming or under-liming of soil when you attempt to correct an acidic soil.
Variations in soil pH come about slowly since a well-formed soil possesses the ability to oppose transformations towards high acidity or high alkalinity. This property is known as buffer capacity and is related to the soil’s content of organic matter and the amount of clay and silicates. For example, heavy clay and clay loam soils are highly buffered, because large quantities of calcium, magnesium and potassium can be absorbed into them without changing the pH of the soil solution. The effect of acidic soils is more obvious in sandy soils than in heavier loams, because sandy soils have a lower buffering capacity, making the use of lime more critical.
What makes soil acidic?
Soil acidification is caused by a combination of a) biological reactions of the different forms of nitrogen fertilizer, and b) of leaching calcium, magnesium and potassium from the topsoil into the subsoil and through removal of these nutrients by the trees. As these nutrients are removed from the soil particles, the nutrients are replaced with acid-forming hydrogen and in very acidic soils also aluminum ions. These are the ions measured when the pH of a soil is determined.
Ammonium sulphate, urea, and liquid fertilizers such as Green N™ and EASY N™ will lower the pH of the soil. Only calcium nitrate and liquid calcium nitrate (Nitro Plus™, EASY Cal™) do not lower, and sometimes increase, the pH of the soil.
Calcium carbonate in the soil acts as a buffer against acid formation. Calcium carbonate restricts the development of acidic soils, because, as the acidity of the soil increases, so does the solubility of calcium carbonate. The calcium in the soil removes (exchanges) hydrogen ions, which combine with oxygen from the carbonate to form water. Carbon dioxide is released in the process. This is why calcium carbonate, or agricultural lime (hereafter called lime), is used to raise the soil pH. Lime is not a fertilizer; it does not supply nutrients to the trees, although some brands of lime can contain small amounts of magnesium that can be beneficial. Lime makes nutrients already in the soil available for extraction by the roots of the trees. Dolomite or dolomitic lime contains both calcium carbonate and magnesium carbonate. Use dolomite only when the soil is acidic and requires magnesium. Gypsum has little direct effect on soil pH and cannot be used to correct a low soil pH. Gypsum (calcium sulphate) is used to improve the structure of soil. Mixing lime with gypsum can impede the action of the lime.
Lime dissolves very slowly in the soil. Lime applied to the surface of the soil does not raise the pH, because lime only penetrates a few centimeters into the soil. Earthworms can carry lime a little deeper, but not enough to affect the entire root zone. In orchards with mature trees under microjet or drip irrigation, there is no practical way of physically incorporating lime into the wetting pattern.
The best, safest and cheapest way to correct a low soil pH is to mix lime with the soil. Your last and only chance to use lime in this way, is when you prepare your soil before
you plant the trees. Whether in a new or replanted orchard, it is essential to have the soil tested for pH to a depth of 400 mm. Then add enough lime to raise the pH to 6.5. It is not a bad idea to add a little more lime to the newly-formed tree line, in a band of 1.0 to 1.5 metres wide, to raise the pH to 7.5. This lime should also be mixed into the soil to a depth of 400 mm, to give the trees protection against future acidification. Never let your soil pH drop below 5.7.
How much lime should you apply?
Do not apply lime willy-nilly. While it is important to apply lime to an acidic soil, it is wrong to apply it indiscriminately. Over-liming, or liming soils that are not acidic, can lead to poor growth mainly due to a reduction in the availability of nutrients from the soil to the trees. Acidity cannot be assessed by feel or appearance, so a test must be used to determine the pH. Have your soil tested by a reputable laboratory to determine how much lime you should apply to the whole depth of tillage. Sometimes this may mean that you have to sample the soil yourself. The sample must correctly represent the area of the soil being sampled. The result obtained can be no better than the sample collected and analysed. No single set of sampling instructions can apply to all situations, but here are a few general hints:
- Use a proper soil sampling tube, not your shovel. A mechanical pasture sampler on the side of a car is not good enough for sampling soil in an orchard, unless the sampler can go deep into the subsoil. A mechanical sampler on the side of a car usually cannot take soil samples from the middle of the tree line in an existing orchard.
- Remove the layer of organic material before you sample the soil. Sample 2 depths. Take a composite sample from the topsoil, and another composite sample from the subsoil. Sample the subsoil to a depth of 200 mm. If the subsoil shows a severe pH problem (pH less than 5.0) , re-sample to a depth of 400 mm.
- If the topsoil is deep or cannot be distinguished from the subsoil, take a composite sample from 0 to 200 mm, and from 200 to 400 mm.
- A composite sample of a block consists of at least 20 soil cores taken diagonally across the block and put in a container and thoroughly mixed.
- Take about 2 kg of soil and put this in a strong paper bag that has your name, orchard and soil details on it.
When you get the result back from the laboratory that tested your soil sample, it should state how much pure calcium carbonate you should apply per hectare. When you purchase agricultural lime or dolomite, pay attention to the quality of the product you buy. The effectiveness of the lime depends on the purity and ability to react with the soil. The amount of acid that will be neutralized per dollar is a key factor, not dollars per tonne of lime.
Pure calcium carbonate (limestone) is the standard against which other liming brands are measured. It has a neutralising value of 100. The value of the lime is related to this ability to neutralise soil acidity and is called the Effective Neutralising Value or ENV. The ENV tells you how much acidity a certain quality of lime will neutralise compared with pure calcium carbonate. The ENV also takes into account the chemical analysis and fineness of the material. The smaller the lime particles, the faster the lime can react with the acid in your soil. The higher the ENV value, the greater the ability of the product is to raise soil pH.
The best way to compare lime brands is to look at the product ENV and the product cost per tonne delivered and spread on your orchard. Simply divide the cost per tonne by the ENV of the lime. The liming brand that has the lowest figure is the cheapest product on your orchard. Your local lime distributor should be able to tell you the product ENV.
Do not apply more than 5 tonnes of lime per hectare in any one application. Drastic sudden pH changes can bring about major changes in nutrient availability and in the population of soil microorganisms, which may result in a nutrient imbalance. A slower pH change will tend to avoid the nutrient imbalance problem. Too much lime or dolomite may set back growth of trees, because in soil with pH above 8.0, nutrients such as phosphorus, manganese, iron, boron and zinc become less available to roots.
Apply lime evenly and incorporate it into the soil with a good moisture level 4 to 8 weeks, preferably 6 to 8 weeks before planting, to ensure reaction takes place and pH is increased. If you want to apply lime to the surface of the soil, do this about 4 to 6 weeks after you have applied your nitrogen fertilizer in autumn. But remember, lime applied on top of the soil does little to alleviate the acidic situation in the root zone.
You may ask: When I have applied the correct amount of lime during preparation of the soil, how do I maintain the correct soil pH?
Some suggest that you use a maintenance programme of 1 to 2 tonnes of lime per hectare for sandy soils and 2 to 4 tonnes for loam and clay soils. But this may not work, as was explained earlier. My suggestion is to maintain the correct soil pH by using your nitrogen fertilizer judiciously in conjunction with irrigation water. Use liquid calcium nitrate and sometimes a cheaper liquid nitrogen fertilizer which contains a mixture of urea and ammonium nitrate. You can balance these two liquid fertilisers according to soil type and need of the trees. Calcium nitrate contains more soluble calcium than nitrogen, which favours tree growth and quality of fruit. Liquid fertilisers are applied with irrigation water (fertigation), which means you must avoid leaching of nutrients by giving your trees the right amount of water and you must fertigate correctly. Monitor the nutritional status of your trees by way of leaf analysis.
Lime is mainly used in the orchard to preserve the long-term fertility of the soil and to prevent troubles arising through soil being too acidic. Lime is a slow-acting, and should be mixed into the soil for best results.
The benefits of liming acidic soils are broad in scope. Calcium and magnesium are essential plant nutrients. As well, the correction of chemical, physical and biological conditions may result in striking improvements in tree health and production. Phosphorus availability is greatest at a soil pH of 6.5 to 7.0. Toxicities of manganese and aluminum are minimized at pH values of 6.0 to 7.0, and availability of the micronutrients is optimized. Biological activity is improved in soil with a neutral pH or with a pH close to neutral, including such processes as nitrification, nitrogen fixation and decomposition of plant material.