The majority of phosphate (P) is held in the soil (adsorbed) attached to soil mineral particles with only a small portion readily available to plants in soil solution. As soil solution P is used by plants, it is replaced by some of the more weakly adsorbed P. This adsorption mechanism prevents P from leaching, however on volcanic (allophanic) soils P is more strongly adsorbed and there is less P available to soil solution. Hence these soil types, which are found in Waikato, Taranaki and Ohakune, require more soluble P to be applied as fertiliser than soils in Canterbury, Manawatu and Southland.

In New Zealand, the Olsen P test, which measures the weakly adsorbed P, is traditionally used to measure soil phosphate. Organic farmers should use the resin P test, as this measures the P that will slowly release from the Reactive Phosphate Rock (RPR) type products that they use.

Optimum P level

The optimum P level for crops varies, however it is impossible to readily adjust your Olsen P level up and down to suit a particular crop, as the reserve of P that can potentially be released is still present.

Legume crops such as white clover do not appear to need Olsen P levels above 10-12 and maize may get away with 10-15. Cereals and grasses generally require Olsen P levels of 15 to 25 and brassicas a bit more again. Potatoes continue to respond to high levels of phosphate so economics dictate application rates.

The aim should therefore be to set a farm optimum P around your most common crops, hence most farmers should aim for Olsen P of between 20 and 30.Growers on volcanic soils and where cropping is only part of a dominant pastoral regime (dairy farms) should aim for the upper end of this range.

There are several reasons for maintaining Olsen P at levels in the mid-20s. These include:

• Maintaining good P levels gives flexibility as to when P can be applied. If P levels are low (<15), P must be applied at planting and close to the seed. If P levels are good, P application can be delayed and fertiliser can be broadcast;
• The amount of soil P taken up by crops varies according to yield and residue removal. So, depending on fertiliser inputs, the next season's soil test results could differ, affecting the appropriate timing and rate of P required by the next crop. For example, a 12 t wheat crop may remove more than twice the P of a 6 t crop (45-50 kg P vs 20 kg P/ha) and a maize silage crop may remove two or three times as much as a barley seed crop (60-65 kg P vs 20-30 kg P/ha);
• It is important to note that many seeders now do not have fertiliser boxes so there is limited flexibility for application if soil P is marginal and planter P is required;
• Most P is held in the top 25-50 mm of soil, particularly if paddocks are coming out of pasture. Cultivation will dilute this further and shallow sown seed may not have ready access to P for establishment. Cool soil conditions make this problem worse;
• Direct drilling causes minimal disturbance to the soil, and hence immediate availability of P. This increases the need for P to be applied at drilling unless soil P levels are high.

Solubility of fertiliser P

Growers normally use superphosphate, DAP based products (Cropmaster, Cropzeal) or specialty compounds (Nitrophoska, Yara Mila) to supply phosphate. Superphosphate and DAP are highly soluble, while many compound fertilisers are moderate to highly soluble. Lime and Serpentine Reverted Superphosphates have limited water solubility and are designed to reduce germination damage from P fertiliser to small seeds and to improve the flowability of fertiliser through old drills. Reactive phosphate rocks are sparingly soluble and are designed to release P slowly over several years in accordance with soil moisture, temperature and pH conditions.

Responses to P fertiliser

In order to lift soil Olsen P levels, fertiliser P must be supplied over and above that season’s crop removal values. On a medium textured silt loam, 5-8 kg P/ha (60-90 kg Superphosphate) is required to lift the Olsen level 1 point. Sandy and stony soils will be at the lower end of this (3-6 kg P/ha), and clays at the upper end (6-9 kg P/ha); volcanic soils may require more.

If the amount of P removed by crops is not replaced, soil P levels will decline. This may be unwise if the current crop is highly responsive to P or you are going into a high crop removal phase of the rotation. However, this may be a sensible strategy if the paddock is going into a lower legume, grass seed phase.

Some crops, such as potatoes, are inefficient at utilising fertiliser P so require application rates higher that that removed by the crop. In this case, where residues are returned and unused fertiliser P has remained in the soil, P inputs for subsequent crops can be reduced.

Replacement versus development P

The P removed by crops can be applied as a single dressing of up to 50-65 kg P/application (600-750 kg/ha Super, 250 kg/ha DAP). Development P to lift soil P levels usually requires split applications. On highly responsive crops such as potatoes, P is split between pre-plant dressings (Super based) and planter dressings (NPK fertiliser).

Maintaining optimum pH levels improves the chance of better yields from a given amount of P applied.

It is unwise to maintain excessive soil P levels as P can be lost by soil movement (soil erosion through wind and water) particularly on hillsides and undulating ground and under flood irrigation. It only takes 1-2 kg P loss to cause eutrophication in waterways. Soils with Olsen P levels above 60 are at serious risk. Check local regulations around broadcasting phosphatefertiliser close to waterways.

Source: Murray Craighead, Nutrient Solutions Ltd, via FAR