Phosphorus and potassium nutrition

Positive grain yield increases with deep phosphorus (P) have been inconsistent across the northern region, with crop type (cereal or legume) and seasonal growing conditions affecting the outcome. Cereal crops (wheat, barley and sorghum) are responding (mainly) due to growing both a larger plant biomass and producing more yield. Chickpeas have not been as consistent, with some very good biomass results not always converting to higher yields, or in some cases no effects on either biomass or yield. Negative yield responses are rare in any species, provided soil is allowed to settle after deep tillage. Potassium (K) responses have generally been additive to phosphorus, meaning we have to overcome the P limit before K can have an effect, but we have also seen evidence of P by K interactions where the application of P helps overcome a low K situation. Potassium responses are most widespread in Central Queensland, while in Southern Queensland and Northern New South Wales they have tended to be restricted to the upland slopes (predominantly) and some grey box soils where lower soil K supply levels have been measured. Soil testing is a good indicator of response (suggested 10-30 cm profile critical values of <0.2 cmol/kg), while chemical analyses of plant biomass have suggested chickpeas are a good indicator of low K soil supply.