Show If you have an area in your yard that doesn’t seem to respond to your fertilizing efforts, check your soil pH. Soil pH is the measure of acidity (sourness) or alkalinity (sweetness) of a soil. The pH scale goes from 0.0 to 14.0. The most acid soil is 0.0 and the most alkaline is 14.0. Halfway along the scale, 7.0, is neutral, neither acid nor alkaline. A soil gets more acid as the pH values decrease from 7.0 to 0.0 and is more alkaline as pH values increase from 7.0 to 14.0. For example lemon juice is 2.4 pH value or acid; water is 7.0 or neutral; soap solution is 9.3 or alkaline. Soil pH can affect plant growth in several ways. Bacteria that change and release nitrogen from organic matter and some fertilizers operate best in the pH range of 5.5 to 7.0 making this the optimum pH range. Plant nutrients leach from the soil much faster at pH values below 5.5 than from soils within the 5.5 to 7.0 range. In some mineral soils aluminum can be dissolved at pH levels below 5.0 becoming toxic to plant growth. Soil pH may also affect the availability of plant nutrients. Nutrients are most available to plants in the optimum 5.5 to 7.0 range. PH can also affect the structure of the soil, especially in clay soils. In the optimum range clay soils are granular and easy to work with. However, if the soil is either extremely acid or alkaline clay, soils tend to become sticky and hard to cultivate. A pH soil test will tell you whether your soil is within the optimum range or whether it will need to be treated to adjust the pH level. Although the optimum range is 5.5 to 7.0 some plants will grow in a more acid soil and some at a more alkaline level. PH is not an indication of fertility, but it does affect the availability of fertilizer nutrients. The soil may contain adequate nutrients yet plant health may be limited by an unfavorable pH level. On the other hand, builder’s sand, which is devoid of nutrients, may have optimum pH for plant growth. To correct the pH of or “sweeten” an acid soil (5.5 to 0.0) use lime or dolomite. Lime contains mainly calcium carbonate and dolomite contains both calcium carbonate and magnesium carbonate. Ground limestone and dolomite are less likely to burn plant roots than hydrated lime and is therefore recommended for home use. The greater the amount of organic matter or clay in a soil, the more lime or dolomite required to change a pH level. The best results will be achieved if you incorporate the lime uniformly at least six inches into the soil. If soil is too alkaline you should determine if it is due to a soil characteristic or lime application. It is very difficult, if not impossible, to change appreciably the pH of naturally alkaline soils by using sulfur, ammonium sulfate, or similar acid forming materials. If this high pH is due to applied lime or dolomite, acid forming materials like sulfur or ammonium sulfate can be applied. To decrease the soil pH use superfine dusting or water soluble sulfur. Repeat applications of sulfur should not be made more often than once every two months because soil sulfur oxidizes and mixes with water to form a strong acid that can burn the plant roots — so use it with caution. It takes approximately 1/3 the amount of sulfur to decrease the pH one unit as it does limestone to raise soil pH one unit. Our soil on Marco Island is mostly alkaline due to the high sand and shell content. Using sulfur coated fertilizers is a helpful practice. These coated fertilizers are the slow release kind which is a more environmentally friendly way to fertilize and it slowly adds some much needed sulfur to you landscape. Various soil textures are sand, loamy sand, sandy loam and sandy clay loam. If you aren’t sure which soil texture you have ask for a determination when you have the pH checked at your county extension office. Eileen Ward and her husband Peter own and operate a lawn maintenance and landscaping company. In this article we will look at pH and acidity and how it affects the plant growth. First of all, we’ll look at what acidity and pH really are. Acidity is essential for life on earth. Acidity often determines the characteristics, quality, absorbability and solubility of many substances. This is how enzymes, which are responsible for almost all biological processes in organisms, work, but only with the correct acidity[1]. A small fluctuation in the blood’s acidity is deadly. What is pH?The pH (pondus Hydrogenii) indicates a solution’s acidity or alkalinity[2]. The pH value usually varies between 0 and 14. A solution with a pH value between 0 to 7 is acid and one between 7 to 14 is alkaline. Vinegar and cola have a pH value of less than 3. Soda and soap have a pH value higher than 8. A pH value of 7 is considered neutral. Pure water at room temperature has a pH of 7. The pH of tap water is generally a little higher due to the presence of calcium. pH and plantsMany natural environments such as our skin, plant substrates and nutrient mediums are mildly acidic and have a pH value of between 5 and 6.5. If we look at the things that people like we see that they are generally mildly acidic or neutral substances such as water. Plants also prefer mildly acidic substances. A pH value of around 5.5 occurs so often in nature that some plant experts regard this value as 'neutral'. Why is acidity important for plants?Acidity has a substantial influence on the absorbability and solubility of a number of food elements (see figure 1). In addition acidity has considerable influence on the structure, breakdown of organic substances, and the micro life in the ground. The pH also influences the way in which food elements, heavy metals, and pesticides are flushed out of the ground. A pH value that is too low or too high can be detrimental to your plants, so it is important to get it right. But how do you know when the pH is wrong? By experience! So to help you, we’ve set out some of the symptoms you might observe and here you can read about EC and pH using CANNA AQUA: Signs of low pH in the soil:Signs of high pH in the soil:
What determines the pH?One of the most important factors determining the pH value in a solution or in the substrate is the buffering capacity. The buffering capacity in this instance means that there is a sort of balance present that continually restores itself. For example, if one puts a drop of acid into 1 litre of tap water that has a pH of 7 it will have little influence on the acidity. However, if one puts one drop of acid in 1 litre of demineralised water (battery water), the pH will immediately fall dramatically. This is because tap water contains bicarbonate while demineralised water doesn’t. Bicarbonate is the most important buffering substance for pH values between 5.5 and 7.5 in water[4]. Bicarbonate and acidityBicarbonate binds itself to acid in the solution which releases carbon dioxide into the atmosphere. This is how the acid is neutralised and the changes in the acidity will only be minor so long as there is still bicarbonate present. With a pH value of 5.3 all the bicarbonate has been used up and the solution has no more buffer. The pH is now unstable and it will change immediately if acid is added (see figure 2). The amount of acid that is needed to get a feeding solution to the correct acidity can therefore be calculated based on the bicarbonate content. The bicarbonate content of tap water is generally given by the water company in milligrams per litre[5]. The buffering capacity and the substrate’s acidity depend on its composition and freshness. The presence of organic material, calcium and bicarbonate generally determine the pH. Clay always contains calcium carbonate and has a relatively high pH value which is difficult to change, while peat and sandy soils are acid[6]. Plant and acidityThe plant itself also has great influence on the acidity. The roots will secrete either acid or alkaline substances depending on the crop’s stage of development, the food available, the differences in root temperature and light intensity. So you see why the pH of the root environment can constantly fluctuate. A sophisticated feeding balance during the different phases of development will keep the pH in the root environment within acceptable limits. Micro life, CO2 levels, and algae growth can also have an effect on the acidity of the root environment and the nutrient tank[7]. Measuring the pH valueIt is quite easy to measure the pH – you need some pH indicators such as litmus paper or a pH testing set. These are relatively cheap but are not always accurate and can sometimes deviate by 1 to 2 pH units. pH meters are generally more expensive and the accuracy depends on the type of meter and regular calibration with calibration fluid. Taking samplesThe pH of the water used to water the plants is important but the acidity around the roots is essential. So when you measure the pH it is very important to take the sample in the correct way to get good results. The sample has to represent the average acidity in the root environment. It is easy to take samples and measure the pH in a recirculation system, simply measure the recirculated feeding solution. In substrate systems without recirculation, feeding solution is drawn out of the substrate (rock wool, agrofoam) at a number of locations. Experts have been discussing the question of where to take the samples from for a year and a day. We recommend, just like a number of reputable laboratories do, taking samples from the places where the roots are which is under and around the drippers. Take small samples from as many places as possible. Always take all samples at the same time and preferably after the second drip-feeding during the light – daytime – cycle. You can best measure the acidity of your sample using the '1:1.5 volume extract' method. You can easily do this yourself by making the growing medium so wet that the water runs through your fingers when it is kneaded and squeezed quite hard. Use a 250 ml measuring beaker for example. Fill the measuring beaker to 150 ml with demineralised water. Add growing medium until the volume is 250 ml (photo 3). Shake it well and let it stand for a few hours. Then filter it and measure the pH. The correct pH values for every mediumWhen cultivating in substrate pH values of between 5.0 and 6.4 are fine for the root environment. There will not be any adverse effects if the values are a little higher or lower. Immediate adverse effects will only be seen with values lower than 4 and higher than 8. pH values lower than 4 often cause immediate damage to the roots. In addition, heavy metals, including manganese and iron are absorbed so well that they can poison the plant (necrosis). Values between 7 and 8 are not immediately harmful for the plant. Nutrients such as iron, phosphate, and manganese are less available then which will lead to deficiencies (chlorosis and development problems) in the long run. Correcting the pH valueCorrecting the acidity is most easily done by lowering the acidity of the feeding solution with nitric acid during the growing phase and phosphoric acid during the flowering phase or, as the case may be, to raise it with caustic potash, potassium bicarbonate or soda[8] and CANNA RHIZOTONIC. Ensure that the pH in the solution that is used does not fall too far below 5.0. When growing in rock wool the fibres will be harmed causing a lot of alkaline material to be released at very low pH values. In addition, the pH is more difficult to control due to the absence of bicarbonate. A high pH in the root environment can also be caused by bicarbonate that has built up. To remedy this maintain 20% drainage or rinse through with a more acid solution. It is useful to note the pH measurements from both the solution added and the feeding solution in the substrate. You will get a good idea of the progression of the pH and the effect of the measures taken.
Figure 3: illustration of the pH values of tap water from various areas with differing levels of bicarbonate. We added 33 ml of nitric acid (38%) to each 100 litre sample of each water type. The pH curve drops faster after pH 5.3 because for this type of water the acid neutralises all the bicarbonate. Below pH 5.3 the acidity level will accelerate fast. |