Soil pH Facts
pH affects plant growth and nutrient availability. pH can affect the availability of nutrients. pH can affect the absorption of nutrients by plant roots pH values above 7.5 cause iron, manganese, copper, zinc and boron ions to be less available to plants. pH values below 6 cause the solubility of phosphoric acid, calcium and magnesium to drop. pH values between 3 and 5 and temperatures above 26 degrees Celsius encourage the development of fungal diseases.
Why pH Varies The ratio in uptake of anions (negatively charged nutrients) and cations (positively charged nutrients) by plants may cause substantial shifts in pH. In general, an excess of cation over anion leads to a decrease in pH, whereas an excess of anion over cation uptake leads to an increase in pH. As nitrogen (an element required in large quantities for healthy plant growth) may be supplied either as a cation (ammonium – NH4+) or an anion (nitrate – NO3), the ratio of these two forms of nitrogen in the nutrient solution can have large effects on both the rate and direction of pH changes with time. This shift in pH can be surprisingly fast. Daylight photosynthesis produces hydrogen ions which can cause the nutrient acidity to increase (lowering the pH). At dusk photosynthesis stops and the plants increase their rate of respiration and this coupled with the respiration of micro organisms and the decomposition of organic matter uses up the hydrogen ions so the acidity of the solution tends to decrease ( pH rises )
Most varieties of vegetables grow at their best in a nutrient solution having a pH between 6.0 and 7.5 and a nutrient temperature between 20 and 22 degrees Celsius
In low light ( overcast days or indoor growing environments) plants take up more potassium and phosphorous from the nutrient solution so the acidity increases (pH drops). In strong intense light (clear sunny days) plants take up more nitrogen from the nutrient solution so the acidity decreases (pH rises). pH can be controlled in two ways.
Extremes in pH can result in precipitation of certain nutrients. For plant roots to be able to absorb nutrients, the nutrients must be dissolved in solution. The process of precipitation (the reverse of dissolving) results in the formation of solids in the nutrient solution, making nutrients unavailable to plants. Not all precipitation settles to the bottom of the tanks, some precipitates occur as very fine suspension invisible to the naked eye. Plants can tell us their problems through leaf symptoms (e.g. iron [Fe] deficiency) when it’s too late. Iron (Fe) is one essential plant nutrient whose solubility is affected by pH which is why it is added in a chelated form (or daily), Fe deficiency symptoms occur readily. At pH values over 7, less than 50% of the Fe is available to plants. At pH 8.0, no Fe is left in solution due to iron hydroxide precipitation (Fe(OH)3 – which eventually converts to rust). As long as the pH is kept below 6.5, over 90% of the Fe is available to plants. Varying pH of summer lettuce nutrient solutions also affects the solubility of calcium (Ca) and phosphorus (P). Due to calcium phosphate precipitation (Ca3(PO4)2) the availability of Ca and P decreases at pH values above 6.0. All other nutrients stay in solution and do not precipitate over a wide pH range. Poor water quality could exacerbate any precipitation reactions that may occur. Generally in the pH range 4.0 to 6.0, all nutrients are available to plants. Precipitation reduces Fe, Ca and P availability at pH 6.0 and over .
Adjusting pH The addition of acids or alkalis to nutrient solutions is the most common and practical means to adjust pH, and can be easily automated. There are ways to minimise pH variations and they are worth some consideration. Nitrogen is the essential inorganic nutrient required in the largest quantity by plants. Most plants are able to absorb either nitrate (NO3-) or ammonium (NH4+) or both. NH4+ as the sole source of nitrogen or in excess is deleterious to the growth of many plant species. Some plants yield better when supplied with a mixture of NH4+ (ammonium) and NO3- (nitrate) compared to NO3- alone. A combination of NH4+ and NO3- can be used to buffer against changes in pH. Plants grown in nutrient solution containing only NO3- as the sole nitrogen source tend to increase solution pH, hence the need to add acid. But when approximately 10%-20% of the total nitrogen is supplied as NH4+, the nutrient solution pH is stabilised at pH 5.5. NH4+ concentration needs to be monitored as it has been shown recently that micro-organisms growing on plant root surfaces can convert the NH4+ to NO3-. Since hand-held ion-selective electrodes for measuring both NH4+ and NO3- are now available, it should be possible to accurately monitor and maintain a predetermined NO3-/NH4+ ratio throughout the life of the crop. Phosphorus is required in large amounts by plants. Interestingly, there are two forms of fertilisers containing both K and P – KH2PO4 mono-potassium phosphate (MKP) and K2HPO4 di-potassium phosphate. Equal quantities of both can be used to maintain the pH at 7.0. Using a higher proportion of K2HPO4 increases pH. MKP can be used to lower the solution pH. Buffers are solutions which resist pH change and are used to calibrate pH electrodes. Buffers can be added to nutrient solutions in an attempt to maintain pH stability. One such buffer is called 2-(N-morpholino) ethanesulfonic acid – abbreviated to MES. Many of the companies who claim better pH control with their ‘specially’ formulated nutrient solutions add MES to their mixes. It is important to remember when using MES, that after MES addition the pH is low and needs to be adjusted to your required level with an alkali such as potassium hydroxide (KOH). Another method of pH stabilisation is to use ion- exchange and chelating resins. Generally, these resins are small beads which have nutrients absorbed or chelated onto them – the nutrient solution circulates through the beads or the beads can be suspended in the nutrient tank. As plants absorb nutrients, more nutrients are released by the resins. The aim is to achieve controlled release of nutrients into the solution in an attempt to mimic the way the soil releases nutrients. Ideally, such release can adequately supply the growing plants’ nutritional requirements and maintain pH stability.
Is pH Adjustment Critical? pH is not as critical as most hydroponicists believe. The main point is to avoid extremes in pH. Plants grow on soils with a wide range of pH. For most plant species there is an optimum pH in the region of pH 5 to pH 6.