Counter Point...
Jerry Spencer of Endeavour Turf Products took exception to an article in the August/September issue. So, from the other side of the world, he wants to put the record straight
I
have thought about writing this for several weeks, since reading an article entitled ‘The little things are important’ by Richard Lawrence, the managing director of
Environmental Turf Technology. After all, is it really worth the effort making comment on an article written on the other side of the world by someone I don’t know? Generally, I agree with some of the comments Mr Lawrence makes but, on some, I do take considerable umbrage as many of his statements are categorically incorrect and totally mislead the reader. For example he states (on page 95 - issue 25) that nitrate is acidic. Wrong, and not just a little bit incorrect, this statement is utterly incorrect. Repeated applications of nitrate containing fertiliser materials generally increases rhizosphere soil pH. The plant causes the increase in pH itself as nitrate is taken into the root and the hydroxyl ion (OH) is released into the soil. However, when ammonium is the primary form of nitrogen accumulated in the plant, the plant releases acid and the pH adjacent to the root decreases. This rhizosphere pH can be as much as
Material Urea
Ammonium nitrate Ammonium sulphate DAP
Potassium nitrate MAP DAP
Potassium chloride Potassium sulphate Potassium nitrate
Nitrogen % 45
33.5 20.5
18-21 14 27
22.5
Potassium % 50
41.5 38
0 0
0.5
two pH units different than the bulk soil pH, and can substantially alter the population of micro-organisms inhabiting the rhizosphere. Fertiliser
supplied as the ammonium form in cool weather may supply substantial
amounts of ammonium to the plant but, at high temperatures, most of the nitrogen to the plant will be nitrate.
When evaluated in solution culture, most plants can take up ammonium and nitrate by the roots
equally as well. In soils, however,
the movement of nitrate and
Bearing this in mind, as a general guide, when the temperature is below 25°C and/or is in shade ammonium is the preferable N source. Above this temperature and nitrate is preferable.
CaCo3 equivalent per kg of Nitrogen
1.8 -1.8 -1.8 -3.5 1.8 -1
Kg lime required to
neutralise each 100 kg of fertiliser material
81 60
110
65 90
0 0
ammonium to the root surface can alter the uptake of the nitrogen forms. Nitrate primarily arrives to the root surface in the flow of water that is transpired by the plant. Since none of the nitrate is adsorbed to soil particles it is abundant in the soil water and the movement of the nitrate to the root rarely limits its uptake. Ammonium, however, is attracted to the soil particles, so only a portion of the ammonium is in the soil water at any one time. Ammonium concentrations at the root surface can limit plant uptake in certain situations. In soils of high cation exchange capacity or high fixation capacity (prevalent clay minerals are vermiculite, mica, and hydrous mica) the amount of ammonium in solution is insufficient to support optimum nitrogen uptake by the plant. Under dry conditions soil moisture may also limit the movement of ammonium to the root, and plant uptake may be hindered. This means that, in soils with high quantities of ammonium fixing clays or when dry soil conditions are prevalent, ammonium will be inferior to nitrate in providing nitrogen to the plant.
In the case of potassium, which form to use depends entirely on whether you are utilising a granular or soluble/liquid programme and what the status is regarding water. Potassium chloride, although far from the most
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