PhosPhorus' essential rolesPhosphorus plays four essential roles in all plant (and animal) tissues:

1. As a component of DNA, which provides the molecular building blocks of genes and chromosomes that pass genetic information from one generation to the next;

2. As a component of RNA, the mol-ecules that translate the genetic code embedded in DNA to produce proteins and enzymes, which deter-mine the structure and function of plant tissues;

3. As P-containing molecules called ATP and ADP that are the “energy storage batteries” in plant tissues. These molecules capture energy produced when sugars are broken down (recall that sugars are pro-duced in leaves by photosynthesis from atmospheric carbon dioxide and water). They release this energy elsewhere in the plant when needed to power vital metabolic processes that require energy input;

4. As a component of phospholipids that form membranes which regu-late passage of water, nutrients, and other soluble compounds between neighbouring cells and organelles within each cell.

Ken G. CassmanIngleby

Environmental Committee adviser

INGLEBY Focus on SoilArticles on soil by Ken G. Cassman December 2016 # 5

Phosphorus is one of three primary macro-nutrients essential for crop growth, and represents the “P” in NPK. But after the complexities of managing nitrogen and potassium, it is refreshing to consider phos-phorus because it behaves in a more predictable manner.

Phosphorus (P) s u f f i c i e n c y levels in plant

tissues range from 0.1- 0.5%. Like nitrogen, P concentrations are greatest in seeds such that 60-80% of total plant P is removed with harvested grain, oilseeds, and fruit (see Table below).

Although oil crops like canola, soya and sunflower have seed P concen-trations 1.5 to 2.6 times higher than in cereal grains like wheat, barley, corn and sorghum, the total amount of P removed with harvest tends to be greater in the higher-yielding cereals.

Phosphorus removal in legume hay crops is smaller than P removed by grain or oilseeds because P concentra-tions in vegetative tissues are typically less than one-third the concentrations in reproductive structures (i.e. seeds).

table 1: PhosPhorus reMoVal

Crop typical yieldP in grain1,

oilseed1, or hay removed at harvest

(kg/ha)1

t/ha % (elemental) elemental P as P2O52

Wheat 6.0 0.38 22 52

Barley 6.0 0.36 22 49

Maize 8.0 0.27 22 49

Canola 3.0 0.70 21 48

Sorghum 6.0 0.34 20 47

Soybean 3.5 0.52 18 42

Sunflower 3.0 0.60 18 29

Lucerne (for hay) 6.0 0.26 16 36

1. Assumes only grain and seed are harvested from oilseed and cereal crops2. Fertiliser P content reported as P2O5, which is 44% P on elemental basis

PH O S P H O R U S

STRAIGHTFORWARD

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Soil P occurs in three forms: a) as contained in soil organic matter,b) as contained in inorganic minerals,

andc) as soluble inorganic P ions in the

water film surrounding soil parti-cles (also called the soil solution).

Roots take up P from the soil solution primarily as the negatively charged (H2PO4)-1 ion. The concentration of this ion in the soil solution is main-tained at very low levels as controlled by solubility of P-containing inorganic minerals such as calcium phosphates (called “apatites”), and aluminium- and iron-phosphates. In most soils, the P concentration is so low that the total amount of soluble P at any one time is considerably less than the amount required to support crop growth for one day. Hence, soil solution P must be replaced rapidly from reserves in organic matter and inorganic minerals as it is removed by root uptake.

In temperate soils, P in soil organic matter (SOM) accounts for 20-50% of total soil P. The rate of supply from organic sources depends on the amount of SOM and its decom-position rate, which in turn largely depends on adequate soil moisture

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INGLEBY Focus on SoilArticles on soil by Ken G. Cassman December 2016 # 5

and temperature. Favourable soil pH between 6.0-7.0 supports microbial activity and favours the presence of P as (H2PO4)-1, which is the preferred ion for root uptake.

Phosphorus uptake by roots is largely determined by root length density because P does not readily move to the root surface due to its low solubility. Hence, plants make heroic efforts to find P through enormous investment in root growth and extension.

For example, typical cereal root systems have more than 100 kilome-tres of root length per cubic metre of topsoil! And even that amount of root length is further augmented by mycor-rhizal fungi, which live inside root tissue and project lots of thin hyphal threads into soil. These threads behave like microscopic roots in acquiring P from the soil solution, and then transport it back to the root where it is shared with the plant. In return, the plant provides sugars, obtained from photosynthesis in the leaf canopy, to support growth of the fungus.

One centimetre of root length can have up to 5 metres of attached mycor-rhizal hyphal threads—improving the P-scavenging capacity by 500 times. An additional benefit of mycorrhizal fungi is their production of glomalin, a glycoprotein that contributes to formation of stable soil aggregates and improved soil structure. Fortunately mycorrhizal fungi are plentiful in arable soils and research to date has shown little benefit from using commercially produced inoculum on growth and yield of major food crops.

Cost-effective management of P on Ingleby farms is straightforward because typical soil tests provide a useful “index” to determine whether a soil requires P fertiliser to avoid yield loss from P deficiency. Although the type of soil P test may differ depending on soil pH or laboratory preference, each has been well calibrated with crop response to applied P.

None of them, however, give reliable estimates of P reserves in soil so that soil P status must be monitored by tracking the P balance through records of inputs from fertiliser, manures and compost versus removal with harvest, as we do in the Ingleby Sustainability reports.

At high latitudes where soils are often cool during early crop growth when root systems are not yet well established, a “starter” P fertiliser application banded near the seed row is often needed to avoid early-season P deficiency.

In fields with heterogeneous soils and associated large differences in soil P supply, zone management using different P fertiliser rates to account for this variability increases P ferti-liser efficiency and profits. Or vari-able P fertiliser rates may be used in different zones within a field if those zones consistently produce different yields, with more P applied to replace P removal in high-yield zones, and less in zones with low yields.

A number of P fertiliser formulations are widely available, and they vary in terms of P concentration, immediate P solubility, and content of other plant nutrients such as nitrogen, calcium and sulphur (see table 2). Fertiliser price closely follows the content of water soluble P and the value of other nutri-ents it contains.

Current annual global use of P as fertiliser applied in crop production is about 22 million metric tonnes and there are concerns about the supply of good quality phosphate rock to support future demand. More than 80% of the high quality reserves are found in Morocco, Tunisia, and Jordan, and some projections suggest that peak P production will occur before end of this century. After that, P ferti-liser costs are likely to rise dramatically.

The spectre of peak P is a clarion call for efficient use of P fertilisers based on soil testing, zone management where appropriate, and maintenance of soil quality more generally. For example, while there is little risk of P losses from leaching due to low P solu-bility in the soil solution, most P is lost via soil erosion, which results in pollu-tion of surface waters, streams and rivers. Hence erosion prevention repre-sents the “front line” in defence of soil P status. Likewise, because P uptake depends heavily on a healthy and extensive root system and an enabling microbial community, maintaining soil quality in terms of good soil tilth, aera-tion, and drainage, and avoiding deple-tion of SOM, is an essential component of a long-term strategy for achieving efficient P fertiliser use.

table 2: P fertiliser forMulations

fertiliser nameelemental

compositionP2o5

content% water

soluble Pother nutrients

(elemental basis)

Triple superphosphate Ca(H2PO4)2 46% 85% 17% Ca

Mono-ammonium phosphate NH4H2PO4 48% 92% 11% N

Di-ammonium phosphate (NH4)2HPO4 46% 95% 18% N

Ammonium poly-phosphate (liquid)

NH4H2PO4 + (NH4)2HP2O7 34% 100% 10% N

Simple superphosphate Ca(H2PO4)2 + CaSO4 20% 85% 9% S, 23% Ca

Rock phosphate 3Ca(PO4)2•CaF2 32% <1% 36% Ca

Corn leaves showing symptoms of P deficiency during early vegetative growth (purpling at leaf margins)

Non-mycorrhizal roots

Mycorrhizal roots with hyphal network

Drawing by Andrea Sanz

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