13 12 application is much greater in higher salinity treatments. Sodium (Na+) is also antagonistic to Ca++ and at higher salinities, less Ca++ enters the fruit. Both factors are likely to cause increased BER in susceptible species. Some high-tech greenhouses growing tomatoes try to eliminate NH4+ altogether during the fruiting stage. So, for solanaceous crops when the plant is in the generative stage of flowering and fruiting, and particularly if water is saline, NO3- is the preferred N source, and application of NH4+ should be reduced or avoided in saline situations. Potassium (K+) Potassium is found in the soil in three locations – absorbed to clay and organic material (exchangeable), part of the clay structure (fixed) and dissolved in the soil water (available). K+ moves between these “sinks”. It is very easily taken up by plant roots, but more slowly cycled between the sinks and the rate of mobilisation of K cannot match the rate of K uptake by plants, requiring application of K fertilisers. Unlike N, K as a plant nutrient is found only in the cation (K+) form, but available as fertiliser in different molecules. K has many functions in plants, including transport of water, nutrients and carbohydrates, as well as for enzymes, which affect protein and energy production. The common K fertilisers: Potassium chloride KCl (muriate of potash, MOP) Potassium sulphate K2SO4 (sulphate of potash, SOP) Monopotassium phosphate KH2PO4 (MKP) Potassium nitrate KNO3 (nitrate of potash, NOP) The difference is in the anions. MOP is the cheapest acceptable form of K fertiliser, yet chloride (Cl-) is least required of all the K+ anions and it competes with NO3-. So, in saline conditions, where the impact of Cl- supplied in MOP fertiliser could further reduce crop outcomes, alternative K sources are more appropriate. SOP is roughly 44.9% K+ and 18.4% SO4--. Sulphur concentration in plants is about 6% of N concentration, and so much lower than the K concentration. Using SOP as a soluble K source in fertigation will result in much of the SO4-- being left in the soil or media near the root surface, as the roots will can be lost as ammonia gas (NH3). N is also lost through denitrification – where, under anaerobic conditions (waterlogging), soil microbes use NO3- that has been applied, or has come from the conversion of Urea or NH4+, as their oxygen source, leaving the N behind. This results in losses of N as gasses like N2O (nitrous oxide) and NO (nitric oxide) or molecular nitrogen (N2). Waterlogging is more likely to occur in heavy soils. Fertigation with trickle helps mitigate losses if Urea is applied “little and often”. The high solubility of Urea means it is pushed well under the soil surface in sandy soils, but can rise to the surface where it becomes vulnerable if soil is allowed to dry out. Use of Urea and NH4+ as N sources can also acidify the soil due to plant root chemical reactions needed to absorb NH4+. Sulphur in sulphate of ammonia (SOA) NH4+ fertilisers in time will be converted to sulphuric acid in soil, as well as by microbes, and add to soil acidity. Soilless media With intensive crops grown in media, unlike soil-grown crops, the N uptake is largely decided by which form of N is supplied. If Urea is the N source, the soilless plants requirement for nitrogen is usually higher than the rate at which Urea can be transformed by bacterial activity to NH4-, so N is normally supplied as ammonium and/or nitrate. NH4+ application to the media of soilless plants has a more pronounced effect on media pH than for soil-grown plants because of the limited media volume. So, for light soils with low organic matter and low clay content, and plants grown in soilless media, it is recommended to use the nitrate form of N. Plants Some plants are not sensitive to the N source, like grasses and beans. Some others are sensitive to the N source, like solanaceous crops, (tomato), brassicas and cucurbits. Such crops prefer nitrate and are sensitive to high ammonium concentrations. Sensitivity is moderated by temperature. As temperatures go up, respiration goes up and consumption of sugars (from photosynthesis) by all parts of plant goes up. If sugars are in high demand for root respiration during warm weather, there is less available for amino acid production, and so less absorbed NH4+ can be neutralised, resulting in free NH4+ within the roots, causing cellular damage. This same temperature effect is more pronounced in soilless crops because of the limited media volume. So, under hot conditions and particularly with NH4- sensitive crops, NO3- is the preferred N source. Physiological crop stage While there is an interaction between the cations, calcium (Ca++), magnesium (Mg++) and ammonium (NH4+), it is less important in the vegetative stage. However, it becomes more of an issue in the generative (fruiting) stage when particularly Ca++ needs to get to the developing flowers and early fruit set. While there is direct antagonism between the cations NH4+ and Ca++, there is another effect. Ca++ is transported to the flowers and fruit carried in water pulled up the xylem (transpiration) or pushed up (root pressure). NH4+ reduces root pressure, so plants like tomato and capsicum that rely on root pressure to deliver Ca++ can have the disorder “blossom end rot” (BER) regardless of the Ca++ supply. This is known as “ammonium induced calcium deficiency”. Experiments with tomato at the same total N application have shown the incidence of BER increases with higher ammonium N levels in the applied fertiliser, and that the incidence of BER at the same ammonium N take up more K+ than SO4--. In hard water (high Ca) sulphate, SO4- -, can combine with Ca++ to form gypsum, a common mineral with low solubility. SOP is an acceptable K source in situations where irrigation water supply has low Ca++ content. MKP – normally MKP is considered as a phosphorus source, as the concentration of P in plants is a lot lower than the concentration of K. NOP – N-nitrate has already been discussed above. So, when fertigating valuable crops, particularly in light soils, KNO3 can supply all the K nutrition and some of the N nutrition. For salty or sodic soils, it is well documented that KNO3 and Ca(NO3)2 are the fertilisers of choice to compete with sodium (Na+) and chloride (Cl-), as both Ca++ and K+ replace Na+, and NO3- replaces Cl-. Trickle irrigation wets a small part of the total soil area and roots are confined to the wetted zone. This means there is less available soil volume for roots to extract K, so with trickle or micro sprinklers, it is important to add K. Click for further info Haifa MKP
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