Fertilising your crop
Soils of the Swan Coastal Plain are some of the most infertile in the world.
The three main soil types are Spearwood sand, Karrakatta sand and Bassendean sand. Few soils in the world have such low capacity to retain nitrogen.
Fertility and nutrient retention capacity is greatest for Spearwood and least for the Bassendean soil type.
The red/brown Spearwood sands overlay limestone. They were the first choice for vegetable growing but only small areas are now left for vegetable production. They retain phosphorus and potassium to a moderate level and fertility can be improved.
The yellow/brown Karrakatta sands are to the east of the Spearwood sands. They can retain some phosphorus but potassium leaches fairly readily. They have poorer water-holding capacity than Spearwood sands.
Bassendean sands are grey to white on the surface and yellow to pale yellow at depth. They do not retain phosphorus or potassium. Commercial vegetable growing has been forced onto these soils under pressure from urbanisation and they now account for about 40% of production on the Swan Coastal Plain.
Matching fertiliser method to soil
Strategies for growing vegetables on these soils require a steady supply of nitrogen to the crop while minimising waste. Strategies for potassium are similar, with frequent applications during the life of the crop and increased frequency on Bassendean sands.
Phosphorus can be treated differently for the three soils.
On Spearwood sands, phosphorus-containing fertiliser can be broadcast pre-planting at rates determined from soil testing. A similar approach could be followed for Karrakatta sands but in many cases some additional side-dressed phosphorus would be beneficial.
Due to rapid nutrient leaching, soil testing is of very little value on Bassendean sand and regular side dressing at moderate rates is most efficient.
Matching supply and demand
The amount of nutrients needed by a crop changes daily. In a perfect world, we would apply just the right amount of fertiliser to the crop every day. This would meet its needs and it would absorb all fertiliser with no waste. A well-balanced fertiliser program should work towards this objective but it can never be achieved in field cultivation.
In the first few weeks after planting, the total fertiliser requirement of a crop may be less than 1% of its full requirement. At this stage, the crop has only a shallow, confined root system. Placing fertiliser in the root zone, and having it stay at that shallow depth long enough for the crop to get its requirements, presents a challenge.
Later, the crop may be doubling its weight weekly and large amounts of fertiliser will be required to meet these needs. By this time, the crop’s root system is much more extensive and it can recover nutrients for longer time and from greater depth.
Slowing loss of nutrients
Another way to ensure efficient recovery of nutrients — especially nitrogen — is to slow down the rate at which they move through the soil profile. Modifying the soil can be a way of achieving this.
This can be a permanent change — brought about by adding a mineral such as clay — or a semi-permanent change by increasing soil organic matter. Crop residues have this effect over time, but the process can be hastened and better balanced by well-timed use of compost or conditioned manures. Changing the form of nitrogen to less leachable organic forms instead of nitrate can also slow down the rate of loss. A good composting process usually does this.