Neal Stevenson, Agricultural Cropping Consultant
Deep placement of potassium and phosphorous could boost yields for crops like sorghum by as much as 1 tonne a hectare. Traditionally, soil samples are taken from the top 10 centimetres, which means soil tests often fail to detect deficiencies 20cm beneath the surface.
It’s becoming a big problem. This is more prevalent in continues cropping land with minimum tillage, phosphorus and potassium are often drawn up by the crops at depth well below the top layer of the soil especially later in the season. After continued cropping rotation and minimal soil disturbance, the soil can start to be mined of phosphorus (P) and potassium (K) leaving the roots at depth hungry and less able to access P and K.
Although more research needs to be done, trials show that even when more conventional tillage practices are incorporated in farming system and soil hard pans are broken mechanically, crops often still do better when phosphorus and potassium deficiencies at depth are addressed.
The move to zero till means nutrients are deposited on the soil surface and aren’t incorporated into the soil profile, causing nutrient stratification.The solution starts with sampling both at 0-10cm and 10-30cm depths, and preferably, 30-90cm. The results need to be interpreted carefully, too, with an expert eye for ratios with other elements and soil conditions that might limit uptake as well as simple threshold values.
Only about a quarter of the phosphorous and potassium applied by fertiliser will be available to the crop in the first year and, to minimise soil structure damage, aim to fertilise at depth only every four to five years. If the 10-30cm samples indicate phosphorus and potassium deficiencies, using a knife-point tined implement to place the fertiliser about 20cm deep will help.
The ideal application time is during the fallow period after harvest but, if costs are prohibitive or the ground is too hard, it can be done at lower rates in a single pass during sowing.
Chickpeas, which can tolerate being sown at greater depths, are the ideal crop for this application, while winter cereals struggle at depth with so much soil over the top.
And while trial results have shown yield improvements of 500-1000 kilograms a hectare per year after deep placement of potassium and phosphorus, it’s not a silver bullet. There’s no point trying to get the phosphorus and potassium right while ignoring nitrogen, sulphur and trace element levels.
Consult an agronomist to create a practical program that takes your machinery resources into account as well as how the soil performs across the crop rotation.
Case study: making it practical
A grower, who operates a livestock and cropping enterprise west of Roma, purchased a cropping block with a long history of low inputs and poor yields.
We took samples at 0-10cm (comprehensive), 10-30cm (comprehensive) and 30-90cm (nitrogen). Queensland Department of Agriculture and Fisheries trial data indicated a yield response when the Colwell P value was below 6 milligrams a kilogram in the 10-30cm zone.Test results from the 0-10cm and 10-30cm zones returned Colwell P values of 3mg/kg and 2mg/kg respectively.
While there was good soil moisture at the time of sampling, it would be at least four months before the planting window opened for the winter crop. Because sowing conditions are rarely ideal, seed is usually sown deep into the profile.
Out of necessity, the grower modified his tined planter so he could sow barley and wheat into moisture, while moving enough dirt away from the seed so there was only 5cm of soil above it. We calculated the highest amount of Mono Ammonium Phosphate (MAP) we could apply with the seed without causing damaging nitrogen burn. The result was seed and fertiliser placed up to 20cm into the soil profile in a single pass.