pH (acidity/alkalinity) is an important water-quality indicator. In a farming water body, pH reflects water-quality changes very directly — the vitality of the algae, the state of dissolved carbon dioxide and more can all be judged by the pH value and its daily fluctuation. pH also strongly affects shrimp growth. In the early farming stage, especially at stocking, if pH is too high, the post-larvae find it very hard — or impossible — to moult, slowing their normal growth. In the middle and late stages, particularly in high-density high-position ponds where organic matter is high and algae are badly aged, if pH is too low, moulting takes longer and the new shell needs more time to harden; the longer recovery takes, the more dangerous it is for the shrimp — "stealth death" tends to happen during this window, and it is always the biggest shrimp that die first. What is more, in such nutrient-rich water, low or no oxygen is common, and soft-shelled shrimp are easily attacked by bacteria and viruses at this time, causing various diseases.
There are many ways to measure pH; the reagent method is simple and widely used in farming. What should you pay attention to when measuring, and what results are actually useful? A farming water body is a whole made up of plankton, bacteria, organic and inorganic matter and the farmed animals; life processes run all the time, so water-quality indicators keep changing. Normal water follows a certain pH rhythm: generally pH is lowest in the early morning, rises during the day as photosynthesis strengthens with sunlight, starts falling in the afternoon, and reaches its lowest point again the next morning. Two points matter when measuring. First, measure at fixed times, twice a day — around 6–7 a.m. and 3–4 p.m. — and keep records. The difference between the two readings shows the vitality of the plankton and the intensity of photosynthesis. If the daily pH swing is small, the algae are ageing or dying and photosynthesis is weak — treat the water immediately to avoid incidents such as oxygen shortage.
Since pH is so important, what should you do when it is too low or too high? Here are some practical tips. First, when pH is too low, most people think of lime or fresh water; both raise pH effectively, but they are not always available or stable. A comparatively safe method: top up fertiliser appropriately. Low pH usually appears in the middle and late stages, caused by high organic matter and ageing algae — use "Dijuan" and "Dijiayang" (bottom conditioner/oxygenator products) to break down the surplus organic matter, then apply "Zaoansheng" at half the normal dose to re-fertilise until pH rises to the desired level; lowering the water level in deep ponds works even better. Topping up fertiliser also prevents mid-to-late-stage oxygen shortage. Second, when pH is too high — most farmers do not worry much, and to lower it they think of adding fresh water, which is sometimes impossible due to site conditions. High pH usually appears in the early stage, caused by active algal photosynthesis and low organic matter. Adding fresh water is one good method; another is to supply carbohydrates such as "Huolicarbon" (activated carbon source) — an excellent material for lowering pH. Use about 2–3 jin per mu, once or twice, until pH drops to the ideal value, then apply "Lijunduo" to stabilise the water.
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