Nitrogen (N) – The unstable essential
Nitrogen is the most abundant mineral found in the plant, so there will be an inevitable price to pay if this mineral is not managed effectively.
This is the main mineral found in the green pigment, chlorophyll, so it is absolutely essential for photosynthesis, the most important of plant processes. Nitrogen is also the basis for plant amino acid production, so it has a major impact on plant growth and vitality. These nitrogen-based, protein building blocks are both structural and functional. Enzymes, for example, are proteins and therefore nitrogen dependant. They drive every biochemical reaction upon which all life is based. Nitrogen is also a major component of DNA and RNA, the genetic material that allows cells (and eventually, plants) to grow and reproduce.
There are no surprises that we have become enamoured with this particular input, because it is hugely important. The problem is that the sources of, requirements for, and application timing for nitrogen are often misunderstood. As a result of this confusion, nitrogen has become the most misused and abused of all minerals.
Nitrogen can cycle between the soil, the plant and the atmosphere (just like carbon). The air we breathe contains almost 79% nitrogen and we were supposed to source significant amounts of this “free gift” from the atmosphere, via nitrogen-fixing organisms. Nitrogen is also stored and released by humus in the soil and, as crop residues decompose, some of their protein reverts to plant-available nitrogen. The fertiliser from the bag actually accounts for much less than half of the nitrogen used to produce your crop, so it is so important to manage the nitrogen cycle efficiently.
When we talk composting strategies, we strive to build a layer cake based upon carbon and nitrogen. The green layer (nitrogen) is based on the nitrogen-dominant chlorophyll. The alternating brown layer (carbon) involves organic matter, where nitrogen has departed and returned to the atmosphere as nitrogen gas.
There are two forms of nitrogen in your soil – ammonium nitrogen and nitrate nitrogen. Ideally, we like to see equal amounts of each (a 1:1 ratio). The maximum soil requirement for each form of nitrogen is 20 ppm. However, plants can thrive with less than this.
The ideal ratio between these two forms of nitrogen is different in the plant, when compared to the soil. In the leaf, we are seeking three parts of ammonium nitrogen to one part nitrate nitrogen (a 3:1 ratio). This different ratio in the leaf is partially related to an inflow of ammonium nitrogen from the atmosphere, directly into the leaf, via nitrogen-fixing organisms living on the leaf surface. Nitrogen-fixing organisms in the soil also constantly boost the ammonium component within the plant. This key 3:1 ratio between ammonium and nitrate nitrogen in the plant is a really important, but often unrecognised, player in plant health and resilience. Mismanagement of this ratio is tragically common. In fact, this ratio is often inverted and, when this happens, the season is set to be dominated by stress and strife. This nitrate excess is a calling card for insects and disease and the battle begins, as every pest arrives to party.
Nitrogen fixation is dependent on a trace mineral called molybdenum, which is lacking in many soils. Cobalt is also important to ensure access to the atmospheric bounty. The third component of successful conversion of nitrogen gas from the atmosphere to ammonium nitrogen in the soil is an ongoing supply of soluble phosphorus to enable the creation of ATP (adenosine tri-phosphate), which fires the nitrogenase enzyme. This is the enzyme that allows the harvest of atmospheric nitrogen.
The best way to ensure a constant trickle feed of soluble phosphate for this purpose is to ensure that your soil contains beneficial fungi. These creatures are missing in many soils, but they exude acids that break the bond between locked-up calcium and phosphorus and deliver both minerals into soil solution. All cellulose-digesting fungi offer this benefit (along with humus creation), but the most important of these organisms is mycorrhizal fungi. These key organisms are missing in 90% of our soils, but the good news is that these creatures can be successfully re-introduced for as little as $20 per hectare (Nutri-Life Platform® from NTS).
While it is always important to ensure sufficient nitrogen supply for maximum yield, it is also critically important that we do not assume that, if a little works well, then more will work better. Nitrogen is the mineral most often abused in terms of this ‘more-on’ approach. Excess nitrogen can reduce resilience and lead to reduced uptake of minerals like potassium, calcium and boron.
One important strategy to maximise response (in the right form) while minimising N inputs, involves the foliar application of urea. The vast majority of soil-applied urea ends up as nitrate nitrogen in the plant. Now, there is a hugely energy-intensive process involved to convert nitrate nitrogen in the leaf into protein. This can suck up 17% of the plants photosynthates, which could have been used much more effectively, including the boosting of your bank account via increased yield at season’s end.
The conversion of nitrate N to protein involves three steps – nitrates to amines, amines to amino acids, and then amino acids to protein. The energy-sucking stage of this three-step process is the conversion of nitrates into amines. The irony here is that urea is an amine. The foliar route into the plant is at least 12 times more efficient, so we can use much less urea when it is applied as a foliar. However, the key consideration here is that we are supplying an amine, which is easily converted to amino acids and then proteins. We have effectively avoided the nitrate-based energy drawdown and used much less N in the process. Urea can be very successfully foliar applied at rates of 8 – 20 kg per hectare, but it should always be combined with humic acid to buffer the N and magnify the nitrogen uptake.
Phosphorus (P) – The energiser
Phosphorus is essential for efficient photosynthesis. It is the “energy mineral” required throughout the process of glucose production. This begins with adenosine-tri-phosphate (ATP), often called “the battery of life”. However, the critical P link continues with a suite of phosphate-based enzymes that drive the sugar factories in the leaf (chloroplasts).
Phosphorus is also essential to plant immunity, as many of the processes surrounding this natural protection system are phosphate-based.
This mineral drives all stages of the crop cycle, from early root growth to the vegetative phase, and it is in even more demand for fruit and seed filling.
Phosphorus-based fertilisers are among the most expensive and unstable of all mineral inputs. This relates to the very low solubility of phosphate complexes. When phosphate anions react with cations in the soil, such as calcium, iron, aluminium and manganese, an insoluble compound is formed and you have effectively lost your fertiliser investment. It is estimated that 73% of applied phosphate is destined to lock up in this manner. It has now become part of the huge bank of insoluble phosphorus found in most farmed soils. The good news is that you can learn the tricks to reclaim this frozen reserve and you can also stabilise your P, to avoid these lockups.
Your soil should ideally contain between 50 – 70 ppm of this key mineral. It is common to see P deficiencies in broadacre soils while, conversely, gross excesses are almost the norm in intensive horticulture and home gardens. When P has been oversupplied, you can expect to see plant shortages of zinc, iron, potassium and, even calcium uptake is impacted.
Humic acid is the best tool to stabilise your soluble phosphate inputs to prevent lockups. The humic acid and water soluble phosphate combine to create a phosphate humate that remains stable and plant available throughout the season. Soluble humate granules should be combined at rates of 5% with DAP/MAP (i.e., 5 kg of soluble humate granules with every 100 kg of DAP/MAP per hectare – up to 10 kg/ha banded or 20 kg/ha broadcast).
Some of the strategies to reclaim your locked-up phosphate include utilisation of compost, legume-based cover crops, fulvic acid, the introduction of mycorrhizal fungi and the use of stubble digestion programs. Cellulose-digesting fungi that can digest crop residues release acid exudates that increase the availability of locked-up P, so you can effectively kill two birds with one stone. You can turn residues into invaluable humus, while recycling your frozen P reserves.
Magnesium can stimulate the uptake of phosphorus, while excess potassium can inhibit uptake.
Slow-release phosphate fertilisers, like NTS Soft Rock™ are much more effective in crops like pastures and orchard crops that do not require fast-food phosphate. They will release their phosphate, calcium, silica and trace minerals over several years and provide a much better investment. Always remember that the water soluble alternative will only deliver 27% of their P lode, before locking up. However, there is a role for DAP/MAP in short term row crops, where instant phosphate is required. Just ensure that you stabilise this rapid-release form with humates.
If you are interested in gaining more in-depth information on nitrogen management, please take a look at this video from my recent seminar in South Africa:
In the next instalment of these Soil Therapy™ guidelines, we will consider the dynamics of the key cations – calcium, magnesium and potassium – by taking a look at their requirements and interrelationships.
PLEASE NOTE: All prices referenced above are in AUD. Prices valid at time of publication and are subject to change.