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How pH And TDS Levels Affect Plant Water

Many of the water quality problems that gardeners face stem directly from what comes from the tap. Local municipalities need to provide water safe enough for human consumption. In general, humans are not adversely affected by water that has a high pH, but plants often are.

Similarly, high levels of salts in tap water can certainly be a problem for both human and plant life, but it is expensive to remove these minerals. Two aspects of the water supply—pH and total dissolved solids (TDS)—affect many aspects of plant growth and vigor.

TDS measures various salts that have been dissolved in water. These dissolved minerals cannot be removed by traditional filtering, but only through membrane, reverse osmosis or distillation. The quantity of TDS in the soil or growing medium will move in the direction of the water. If you keep applying water that is high in pH and high in TDS to your soil, the root systems of the plants will eventually have trouble taking up many of the nutrients you have applied. If the water supplied is too acidic (the pH is too low), nutrient uptake can also be adversely affected.

If you are starting with water that has issues, it is a constant battle to adjust for this. For example, if you lived in Seattle, Washington, your influent water pH would likely be in the 7.5-7.8 range. Though the pH level for nutrient uptake for plants is optimized at around 6.5, this range is not too bad.

The mineral and salt levels of Seattle’s water supply are also low, with a TDS of around 50 ppm. However, if you lived in Las Vegas, Nevada, you would find that your influent water was quite different, with a pH in the range of 8.0-8.3 and a TDS in the range of 600-800 ppm. This is due to the high mineral content of water flowing through the Colorado River and the accompanying high pH (basic) aspect of that water.

Nutrient Availability

Check out any pH & Nutrient Availability Chart and you'll see that pH that is too low or too high inhibits most of the iron, zinc and manganese from being absorbed by plant roots. These micronutrients are essential for basic plant function as well as the ultimate taste of the crop. You can also see that the availability of nitrogen drops off when the pH of a grow media or solution is too high or too low. Since nitrogen is the key to vigorous leaf growth, it affects yields considerably, so it is absolutely essential to balance your pH levels to ensure this essential nutrient is readily available for plants.

Phosphorous is the key plant macronutrient for producing better roots, fruits and flowers, and it is also severely affected by large swings in pH. In general, aim for a pH between 6 and 7.5 in the soil or nutrient solution.

Orthophosphates (the form of phosphorous taken up by plants) originate largely from minerals or organic sources already in the soil or added in the growing process. The phosphorus cycle is no less complex than the nitrogen cycle.

There are many factors that affect the availability of phosphorus in the soil, but pH is a major player. When pH levels are low, phosphorus is combined with iron and aluminum compounds. Maximum phosphorus availability occurs at a soil pH between 6.5 and 7.

One additive I recommend is bone meal, which is a popular form of phosphorous (11%) and calcium (22%). Bone meal is derived from animal or fish bones and commonly used in a powdered form on root crops and bulbs. It also contains 2% nitrogen and many micronutrients.

I also recommend monoammonium phosphate (MAP), which has an N-P-K rating of 11-52-0. This is a great ratio for rooting plants that scores low on the salt index. Maintaining the right pH range, in conjunction with supplying grow media that scores low on the salt index, will certainly assist in our efforts to produce maximum yields of healthy, tasty crops!

Salts from Nutrients

Fertilizers are primarily salts, and although they are an absolute necessity, over time they can present a problem as the salts build up in the soil/media or on the roots. These salts are taken up by the plant and when they are too abundant, the plant will attempt to push them out. It often does this by moving them to the extremities of the leaves. That’s why, when a plant is over-fertilized, the symptoms are often damaged or burned-looking tips and margins of the leaves.

So, while trying to maintain an optimal level of nutrients for your crop, you certainly don’t need any unnecessary extra salts coming in from your water supply. One alternative method for providing nutrients other than through the grow media or soil is foliar application. Liquid nutrient formulas are available and formulated to provide their nutrients through leaf tissue. Some growers have found a successful balance between root and foliar feeding to achieve optimal nutrient levels while minimizing excessive salt levels at the roots.

Salts can be harmful to a growing plant and can be especially damaging to seeds in the sensitive germination period. The newly formed roots of a seedling are tender and easily damaged. For this reason, it is important to minimize the use of fertilizers during this period and in the immediate vicinity of the seeds. The vigor of the future plants, as well as higher yields, will depend on this.

Some fertilizers have a higher salt index then others. You will notice by reviewing a salt index table that the SI for nitrogen, phosphorous and potassium (the three macronutrients) falls generally into different ranges. Nitrogen, the nutrient for producing foliage growth, is the highest, and potassium generally is a bit higher in salts than phosphorous, which is used for producing root, flower and fruit growth.

So, when considering that seeds are root producers first, phosphorous (lower in salts) is a safer and preferred nutrient. Holding off on significant amounts of the other macronutrients for starting seeds makes sense.

Controlling pH Levels in a Hydroponics System

Monitoring your garden’s pH levels can be done through the use of meters in your grow media throughout the grow cycle. Measuring is the easy part, but controlling pH can be a troublesome issue.

Fortunately, when the pH gets too high or too low, there are options to help correct this and keep the nutrients flowing into the plant. You can add chemicals like acids to reduce pH, or basic-type amendments like sodium hydroxide and other specialized products to increase it.

Again, most plants do best when the soil or nutrient solution is between 6 and 7.5. This is the range where all of the nutrients become optimally available to plants. Start with small adjustments. Measure, then measure again.

High levels of pH can also be kept in check through a solution or grow media that fosters a lot of biologic action. While you work long hours to produce a bigger and better crop, billions of bacteria, fungi and perhaps many insects and other organisms are working underground, making your crop’s success possible.

These organisms decompose organic matter (lowering pH) and transform nutrients into forms your crops can use. They help build good soil tilth, enhance crop growth and control harmful pests. If your pH levels are high, consult other growers to find out what will work best for your garden’s scale.

Additionally, there are many products designed specifically to help growers adjust the pH of their hydro solutions up or down. Using these to correct potential pH problems as your crop is growing and maturing is important. You do not want your crop to only flourish in the early stages then to begin to decline as the flowers are budding or the fruit is setting.

Maintaining the right pH range and choosing a fertilizer lower on the salt index will provide many benefits in your effort to produce maximum yields of a healthy and tasty crop.


Controlling TDS Levels in a Hydroponics System

Controlling salt levels needs to happen through careful selection and application of the fertilizers you use. One technique that can help reduce salts if they become too high is leaching.

Even influent water with relatively high levels of TDS will still be much lower than the salt level in a mature grow media, and flushing your soil out to waste (slowly but thoroughly) can reduce the salt level considerably.

The addition of gypsum (calcium sulfate) to soil at appropriate levels can provide many benefits in the area of leaching. It helps prevent crusting and benefits seed emergence. As you would expect, leaching will also reduce the levels of nutrients in the media. The buildup of salts is one of the reasons many growers start each crop with new soil.

Being a top-notch gardener or farmer first requires a love for what you do and a lot of little adjustments to keep improving and getting the most from each crop. Fortunately, there are so many different products, from nutrients, to amendments, to meters and other diagnostic tools that can help give growers the upper hand.

As you keep doing your research and gaining new ideas, practices and knowledge, you will be able to select the best products or services to master each of these aspects. Growing successfully is a science—take notes and read them later. Learn and get better with each crop.

Source: Maximum Yield, March 12, 2020,


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