19 January 1996

Spread phosphate in spring for boost

PHOSPHATE spread in early spring can boost early grass by up to 20%, according to Scottish Agricultural College grassland specialist Graham Swift.

That response is best when phosphate is applied as an NP compound rather than as straights.

The NP compound produced 30% more spring grass than straight N and no P on very low P soils (under index 1), 10% more on low P soils, and 5% more on moderate status soil in an SAC trial.

The equivalent N and P applied as straights (90kg/ha N and 45kg/ha P) increased grass growth by 20% on very low P soils and 5% on low P soils – compared with straight N and no P.

As well as boosting grass growth, a spring application of P gives higher P levels in the grass, says Mr Swift. This can improve the Ca:P balance in the animals diet and have health benefits, particularly on fertility. It could especially benefit a spring-calving herd.

In the SAC trials, the increase in P levels was about 4% greater with the NP compound than with N and P applied as straights (see table). SAC recommends an annual dressing of 40kg/ha (32 units/acre) P on moderate P soils (index 2) and 80kg/ha (6kg units/acre) P on low P soils (index 1).

For those producers considering using a basic slag known as Steelworks Slag to supply phosphate to gbrass, he urges caution.

It has 2% phosphate compared with 12% in the old basic slag. Its other ingredients, including 43% calcium and trace elements are similar apart from a higher iron content.

Newcastle work

However, trials at Newcastle University have shown that it encourages higher grass yields than equivalent amounts of lime. But at £28-£3/t delivered it is expensive on the basis of its low P content, says Mr Swift. Recommended rate is 1-1.5t/acre, applied every three to five years. At 1.5t/acre, the slag provides 75kg/ha (60 units/acre) available P.

The SAC is currently comparing five phosphate fertilisers with triple super phosphate. One is Scotphos slag (0.16.0) containing 50% Steelworks Slag: 50% Gafsa rock phosphate and another is Fibrophos (0.24.12) a by-product of poultry manure. The three-year trial is funded by Scotphos.


Grass phosphate content (g/kg)

90kg/ha N90kg/ha N

Soil P/fertiliser90kg N+ 45kg P2O5/ha + 45kg P2O5/ha

status+ nil P2O5/haas straightsas NP compound

very low2.43.03.1

low3.03.33.4

moderate3.33.63.7