Chapter 16

Fertilizers for Greenhouse Crops

Inorganic Fertilizer Materials

Many different types of commercial fertilizers are available for use in greenhouses and vary according to their source materials, nutrient quantities, and mechanisms of nutrient release. The commercial fertilizers can be broadly classified into: nitrogen, phosphorus, potassium, sulfur, calcium, and magnesium fertilizers including micronutrients. Some fertilizers contain only one mineral nutrient whereas others contain several. Many of the commonly available inorganic fertilizers are described below, and their analyses are summarized in the following tables.

Nitrogen Fertilizers

Many different chemical and physical forms of nitrogen (N) fertilizers exist. Some of the more common fertilizer nitrogen sources are given in Table 16.3. Plants can use nitrogen in one of two forms: ammonium nitrogen (NH4+) or nitrate nitrogen (NO3?). Ammonium nitrogen is changed to the nitrate nitrogen form by bacteria, a process known as nitrification. The conversion of ammonium to nitrate nitrogen generally occurs very rapidly providing aerobic soil conditions and soil temperatures are above freezing. Loss of ammonia to the atmosphere from ammonia nitrogen (called volatilization), can occur under alkaline conditions, or if lime and ammonium based fertilizers are applied at the same time. Consequently the use of ammonium-based fertilizers should be restricted to acidic soils.

Phosphorous Fertilizers

Many different chemical and physical forms of phosphorous (P) fertilizers exist. Plants absorb most of their phosphorus from the soil solution as orthophosphate (H2PO4¯), regardless of the original source of phosphorus. Although orthophosphate’s negative charge prevents it from being attracted by the soil’s CEC, it does react strongly in the soil, primarily with the large amount of iron and aluminum naturally in the soil, to form products that are very insoluble and thus unavailable to plants.

Potassium Fertilizers

Potassium occurs in the soil in three forms: as exchangeable (available) potassium (K+) adsorbed onto the soil CEC; fixed by certain minerals from which it is released very slowly to available form; and in unavailable mineral forms (most of the potassium in soils). Plants take up potassium as the K?ion. Some of the more common fertilizer potassium sources are given in Table 16.5. Potassium is between nitrogen and phosphorous in mobility. It is not lost as readily as nitrogen, but it will move into the soil to the roots more quickly than phosphorous.

Sulfur, Calcium, and Magnesium Fertilizers

There are numerous sources of calcium, magnesium, and sulfur for fertilizing plants. In addition, materials such as bone meal, wood ash, manures and sludge can contain adequate amounts of these elements. Some of the more common fertilizer sulfur, calcium, and magnesium sources are given in Table 16.6.

Micronutrient Fertilizers

There are many different fertilizers that are marketed as micronutrients. Usually, micronutrients are mixed with fertilizers containing nitrogen phosphorous, and/or potassium. Because micronutrients are needed in such small amounts, the best method to correct a micronutrient deficiency is usually by application is by foliar fertilization. Some of the more common micronutrient sources are given in Table 16.7.

Chelates

Some metal micronutrients are “chelated,” meaning synthetic organic compounds wrap around a metal ion to neutralize it electrically in order to increase its availability. Chelates of zinc, manganese, iron, and copper have become standard products for foliar application in many greenhouses. Chelating agents for various micronutrient metals include EDTA, HEEDTA, NTA, DTPA, and EDDHA.

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