While occasionally most cropping areas experience a season where fallow soil water capture and storage is not that important (for some areas 2016), for most years and most areas it is a vital aspect of maximising crop yield.
Rotations, soil fertility, disease and pest management and other aspects of crop agronomy (such as variety choice and sowing time) are also important. But insufficient water, especially during late tillering onwards, overrides every other crop growing consideration. It might be old knowledge, but the highest efficiency in fallow water capture and storage remains critical.
SoilWater App (designed by a University Southern Queensland team lead by Soil Scientist Dr David Freebairn) indicates how different soils for any given area are capable of storing specific amounts of plant available soil water (PAW).
For example, a deep cracking clay can store 300mm of PAW to 1.8 metres deep, the depth crops can grow on these soils. At the other extreme a sandy surface soil over a light clay may only be capable of storing 90mm of PAW to an impervious depth layer of 0.8m.
However, even on shallower and lighter soils these lower amounts of stored PAW can commonly be the difference between a poor or failed crop and a moderately yielding one, especially if rainfall is deficient at vital periods like flowering and grain filling (the situation for many in 2017). Every mm of stored plant available water can be worth up to 30kg/hectare cereal grain, or for 90mm of stored PAW that can equate to 2.7 t/ha extra yield.
Apps like “SWApp” are based on extensive research findings from many different soil types, environments and seasons. Other soil types and their broad PAW holding capacity include shallow medium clay (76mm), re-brown earth (120mm), shallow heavy clay (122mm), clay loam (140mm), loamy clay (170mm), deep red loam (182mm). grey cracking clay (200mm), deep clay loam (200mm) and a moderate cracking clay (220mm).
SWApp and other apps estimating how much PAW is being stored depends on many aspects such as stubble cover, a feature that adds to the apps accuracy. One can enter the closest weather station or one’s own rainfall data for the app to predict PAW for any period.
SWApp’s ability to predict accurately is dependent on timely control of fallow weeds. Sounds straight forward but is commonly the main reason Apps overpredict stored soil water during the fallow.
Timely fallow weed control is commonly interrupted for a multitude of factors including breakdown, other commitments such as harvest or the Christmas break, and sometimes weather events. Livestock are often used to control fallow weeds but all too often fail to prevent significant weed water use, as well as allowing weed seed set.
It is best to budget on five or so fallow weed control treatments (herbicides are generally more effective at conserving moisture than ploughing) and aim for early control before weeds stress or set seed. In summer it is common to spray weeds 10 to 14 days after rain. It only takes three or four days for weeds to germinate and another few days for sufficient leaf area for herbicide capture. While timely weed control often means an extra one or two treatments over the summer the improved reliability often more than makes up for added costs.
Soil moisture probes are another method to estimate PAW and if well used, are an accurate way to measure fallow PAW accumulation as well as to monitor in-crop soil water balance. Knowing how much PAW one has accumulated or has left in a crop at a given time has many advantages other than helping predict how a crop is growing.
This past dry winter - early spring for example, knowing PAW helped with decisions such as to cut for hay/silage or to leave for grain. PAW knowledge also helps for decisions like adding more in-crop nitrogen.
