Chapter 15

Plant Nutrition of Greenhouse Crops

Growing Media pH

Growing media pH whether organic amendments (e.g., peat, bark, coir) or inorganic amendments (e.g., perlite, soil, rock wool) is a measure of the growing media’s acidity and alkalinity. The pH scale ranges from 0 to 14. A pH of 7 is a neutral state, representing the value found in pure water. Values above 7.0 are basic, while values below 7.0 are acidic. Growing media pH is influenced by both acid and base-forming ions in the soil. Common acid-forming cations (positively charged dissolved ions) are hydrogen (H+), aluminum (Al3+), and iron (Fe2+ or Fe3+), whereas common base-forming cations include calcium (Ca2+), magnesium (Mg2+), potassium (K+) and sodium (Na+). Plant nutrient availability is strongly tied to the growing media pH. Figure 14.1 shows optimal availability for many nutrients at corresponding pH levels. With the exception of P, which is most available within a pH range of 6 to 7, macronutrients (N, K, Ca, Mg, and S) are more available within a pH range of 6.5 to 8, while the majority of micronutrients (B, Cu, Fe, Mn, Ni, and Zn) are more available within a pH range of 5 to 7. Outside of these optimal ranges, nutrients are available to plants at lesser amounts. With the exception of molybdenum (Mo), micronutrient availability decreases as media pH values approach 8 due to cations being more strongly bound to the media and not as readily exchangeable. Metals (Cu, Fe, Mn, Ni, and Zn) are very tightly bound to the media at high pH and are therefore more available at low pH levels than high pH levels. This can cause potential metal toxicities for crops in acid media. Conversely, “base” cations (Ca, K, Mg) are more, weakly bound to the media and are prone to leaching at low pH. In addition to the effects of pH on nutrient availability, individual plants also vary in their tolerance to alkaline and/or acid media conditions.

Factors Affecting Growing Media pH

There are a number of inherent factors that affect growing media pH such as climate, mineral content, and growing media texture but in a greenhouse environment water quality/alkalinity, media components, and fertilizers applied are primarily responsible for changes in growing media pH.

Water Alkalinity

Alkalinity is one measure of the quality of water used for irrigation. Alkalinity is the measure of the concentration of bicarbonates and carbonates in water which determine the water’s capacity to neutralize acids. Dissolved bicarbonates such as calcium bicarbonate (Ca(HCO3)2), sodium bicarbonate (NaHCO3), and magnesium bicarbonate Mg(HCO3)2); and carbonates such as calcium carbonate (CaCO3) are the major contributors to alkalinity in irrigation water. Alkalinity increases as the amount of dissolved carbonates and bicarbonates rises.

Media Components

The container medium that we use has an initial pH. Acidic media (pH less than 7) include: sphagnum peat, pine bark, coir, and many composts. Neutral media (pH around 7) include: perlite, sand, and polystyrene.

Types of Fertilizers Applied

The type of fertilizer you are using can significantly affect substrate pH over time. Each fertilizer has been carefully formulated, and its ability to cause an acidic or basic effect is known and documented in the fertilizer analysis. In simple terms, “Potential Acidity” or “Potential Basicity” as listed on the fertilizer package refers to the effect that this fertilizer product has on substrate pH. The higher the number for “Potential Acidity,” the more acidity the fertilizer provides and the more acidity there is to react with lime or alkaline water. “Potential Basicity” works the same way. The higher the number for “Potential Basicity,” the greater the potential to provide basic ions (OH-) and raise pH. The most acidifying fertilizers are ammonium sulfate and monoammonium phosphate (MAP), followed by diammonium phosphate (DAP). Less acidifying are urea, ammonium nitrate and anhydrous ammonia. Fertilizers such as sodium and calcium nitrate are not acidifying.

Managing Growing Media pH

Growing media that are too acidic or too alkaline can cause many problems with plant growth as discussed above. Therefore, maintaining pH in the proper range for the desired use of the growing media is critical for successful production of greenhouse crops. Corrective measures to lower pH include: 1) discontinuing the use of basic residue fertilizers, such as calcium nitrate and using acid-residue fertilizers to lower the pH; 2) drenching with aluminum sulfate or iron sulfate to rapidly lower pH; and 3) acidification of irrigation water. The growing media pH would need to be raised if it fell below 5.4. Corrective measures to raise pH include: 1) discontinuing irrigation water acidification, if any is employed; 2) using basic residue fertilizers to raise the substrate pH; and 3) in severe cases, drenching with flowable lime or potassium bicarbonate. Another option in raising the pH is to add limestone to the growing media as a means of adjusting the pH before planting.

Lowering Growing Media pH

Growing media pH can be lowered by various methods. A moderate reduction can be achieved by switching to acidic fertilizer. More substantial corrections can be accomplished through drenches of iron sulfate, acids, or sulfur or acidification of the irrigation water.

Acidic Fertilizers. Plants have the ability to take up several forms of nitrogen (ammonium, nitrate, and urea). Ammonium and urea are acidic forms of nitrogen, meaning they tend to decrease the pH of container medium. Nitrate is a basic form on nitrogen, meaning that it tends to increase the pH of the container medium.

Drenches. Drenches that can be used to restore growing media pH up to the desired level include iron sulfate, aluminum sulfate, and flowable sulfur. Once the pH level has been corrected the drenches are discontinued. At that point remedial or preventative action is taken to ensure that a future pH decline does not occur.

Acidification of Water. Using a fertilizer injector to add acid to the irrigation water directly reduces the alkalinity. A hydrogen ion from the acid will combine with a bicarbonate and carbonate to form carbon dioxide and water.

Raising Growing Media pH

When pH is only moderately low, switching to a more basic fertilizer generally solves the problem. When the pH decline is more severe, that is, when it falls below 5.4, a growing media drench with flowable limestone or potassium bicarbonate may be needed. As previously mentioned, a starter lime charge can be added based on the growing media components used in the mix and the desired starting pH.

Drenches. Consider soil drenches with either flowable lime or potassium bicarbonate. Both materials are incompatible with all types of water-soluble fertilizer and other chemicals, and need to be applied separately as a soil drench. Both materials are fast acting and show most of their effect on the media’s pH within one day. Several factors affect the choice between flowable-lime versus potassium bicarbonate. Flowable lime has the more predictable and stable effect on the media’s pH, without increasing the EC.

Adding Limestone to Growing Media. As previously mentioned, the starter lime charge can be added based on the growing media components used in the mix and the desired starting pH. Limestone sources differ in their composition, particle size, and hardness, which causes them to vary in how reactive they are (i.e., how many lbs/cubic yard are required to raise pH at the start of the crop), and also in how long they continue to react during crop growth.

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