perfect potassium source available, is perfectly acceptable to use if watered in. More importantly, with whatever form used, is dilution rate, as work in the US suggests that, once you apply product in the case of liquid applications at over 600L/Ha (6L/100m2
), it cannot be
regarded as being a true foliar application. Richard
Lawrence’s comments relating to magnesium are erroneous as magnesium does not react with potassium and go thick in the tank as stated on page 96. Magnesium will have issues if tank mixed with phosphorous as it forms magnesium phosphate, an insoluble precipitate. Whether this becomes an issue can be related to tank dilution rates as discussed earlier. In relation to his constant comments about sulphur and black layer, this is still relatively anecdotal and there are strong suggestions that other factors may actually play more important roles in this problem than simply moderating sulphur applications. The precise cause of death of creeping bentgrass growing on black-layered sand is unknown. The potential production of hydrogen sulphide by sulphate-reducing bacteria may be one of the toxic components of black-layer development. Research reveals hydrogen sulphide is, indeed, lethal to
creeping
bentgrass turf with as little as seven days’ exposure to 1,000ppm hydrogen
sulphide causing bentgrass turf to die. It is not uncommon, however, to find healthy grass growing on black-layered sand. This suggests that the toxicity of the layers may be variable. It has also
been observed that the roots of A. palustris can grow through and clear the black-layer formed by the interaction of cyanobacteria and sulphate-reducing bacteria. Studies on the physical structure of black-layer have revealed vertical cavities in the layer through which grass roots grow with at least 3mm of clear sand between the root and the blackened sand. For hydrogen sulphide to be released, the environment must be favourable for sulphur-reducing bacteria. Favourable conditions include:
1. Ample amounts of organic matter and water present in the soil. Organic matter is the food and energy source for the bacteria, and water keeps them hydrated.
2. The bacteria also require that the soil have a low aeration status. This is an extremely important requirement because traces of oxygen poison most sulphur- reducing bacteria. Anaerobic conditions must prevail to produce hydrogen sulphide.
3. One other important requirement is an abundance of some form of sulphur, such as sulphate or elemental sulphur. Sulphur is the respiratory molecule for the bacteria, just as oxygen is for humans. If sulphur molecules aren't present in the soil, the sulphur-reducing bacteria can't respire properly and hydrogen sulphide gas isn’t produced.
In relation to comments
relating to oxygen I do agree as, with the widespread use of fertilisers and irrigation, more and more it is aeration which has become a major limiting factor to the attainment of optimal growth. It seems likely that root systems are commonly restricted in extent by the
progressive decrease in aeration that occurs
down a soil profile. Poor aeration
can decrease the uptake of water and induce early wilting. In the case of growing media ‘air space is the percent volume of media or media component that it is filled with
air after the media has achieved container capacity’. Air space is affected by container height i.e. the taller the container, the more drainage and therefore more air space. For a given bulk density, moisture content and container size, air space is equal to the total porosity minus container capacity. However, the principles discussed apply to both turf and nurseries.
When soil becomes compacted, roots can suffer from a lack of water, nutrients and oxygen. As pressure increases on managers, the need for maintaining a well aerated media has increased accordingly. All growing media are a constantly changing environment with nutrients being moved, gaseous exchange constantly occurring and water being in a constant state of flux. Any factor that influences one of these processes will severely affect the growth of plants. If soil compaction develops it can
severely affect this equilibrium. Poor soil aeration or oxygen deficiency is a major factor limiting seedling establishment. Oxygen deficiency in the soil can occur because of improper management ,such as compaction; poor media quality, such as heavy fine-textured soils or layered soils with inadequate drainage; excessive irrigation, rainfall or flooding; usage of excessively small containers for transplant production. Inferior stand establishment can occur due to the inhibitory effects of low aeration on root elongation, proliferation, viability, respiratory capacity, carbohydrate accumulation, hormone synthesis, and water and nutrient uptake. In fact, poor soil aeration affects potassium uptake more than any other major nutrient with levels of uptake being down to only 45% of normal! Within the soil environment there are a number of biological processes occurring. The majority of these are aerobic involving the uptake of oxygen and the evolution of carbon containing compounds. Plant roots respire aerobically and therefore require sufficient oxygen supplies at root surfaces. Anaerobic conditions in the soil induce a series of reduction reactions, both chemical and biochemical. Included in these are denitrification (the processes by which nitrate is reduced to nitrite, then to nitrous oxide and then to elemental nitrogen), manganese reduction, iron reduction and sulphate reduction. Some of the products of these processes are, in fact,
Schematic diagram showing how nitrate and ammonium differ in soil pH effect
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