Crop yield across a paddock and between paddocks, on a property, can commonly vary enormously, as many farmers with "yield monitoring" harvester computer setups can readily agree with.
Research suggests such paddock yield variation to be around 30 per cent. How to minimise that variability, especially relating to increasing yield towards the higher yielding parts of a paddock, can be quite a challenge.
Sometimes yield difference within a paddock is because of straight forward factors such as weed and disease variability and soil fertility differences, especially nitrogen. Between paddocks added factors may be sowing time, stored soil water levels at sowing, elevation (for example frost differences) variety choice, soil fertility and disease.
Soil type differences are more than common factors of yield differences and there are a multitude of issues possible, many of them commonly not well assessed. Subsoil constraints include sodic layers (high sodium levels as a percentage of the cation exchange capacity), salinity, acidity and phytotoxic concentrations of chloride.
A light textured soil, sometimes over a moderately shallow clay, that may also have issues like sodicity, at one extreme may only be able to store 90 millimetres of soil water for crop use. Important but not capable of carrying a crop through a long dry period. At the other extreme soil type may be a well textured clay with no top or subsoil constraints like sodicity or high chloride, capable of storing 300mm of plant available water to 1.8 metre depth.
Whatever the soil type, using moisture probes or apps that can reasonable accurately measure stored water, provided sound fallow management is practised, is also an important part of assessing if crops are yielding to expectations and assessing paddock yield variability.
Because of cost (a lot of soil testing and sound interpretation) and complexity of sub as well as top soils, it can be a slow process to gather together a better understanding of physical and chemical variability through the soil layers (around 2.0m) over an extended area. But knowledge of many soil aspects is cumulative and generally because of slow or little long-term change in many soil aspects, progressively improved knowledge occurs.
Much of endeavouring to improve crop yield is based around issues like soil fertility, weeds disease and pests (e.g. nematodes), efficiently storing soil water, sowing time (with an emphasis on earlier with appropriate variety maturity) and the like. But addressing soil issues like sodicity, various toxicities, and sometimes just physical constraints like compacted clay is far more difficult.
Current research indicates some crop types, and varieties within a crop type yield better in specific difficult soil conditions. Good knowledge exists, for example, about acid soil tolerant crops and varieties, regardless of if acidity is in the top or subsoil. Addressing acid top soils is also well addressed with amelioration and lime.
Wheat varieties assessed via a GRDC funded study show some varieties yield better in sodic soils (surface to 60cm) than others, but all tend to be significantly affected in drier seasons (less water extraction). In wetter years yield penalties are commonly low. Top soil amelioration via gypsum can improve germination and establishment, as well as soil water infiltration rate. Research continues to assess adding organic matter and gypsum to the subsoil is another research angle being assessed where various subsoil constraints exist.
Some crops, like barley, tend to have better tolerance of subsoil chlorine toxicity than others.
Wheat has useful tolerance, so does canola, but chickpeas and durum wheat are more susceptible. Zero till stubble retention farming also tends to help reduce salt loads in the top 0-1.2m soil layer compared to cultivation and stubble burnt ways of farming.
Subsoil constraints occur in many different ways with research for many issues still only in preliminary stages. A good profile measure of likely issues is, therefore, an important starting point before big remedial action is contemplated. Electromagnetic induction, for example, is one area of research assessing a feasible way to assess soil constraints.
Next week: Water use efficiency in driest of years.
- Bob Freebairn is an agricultural consultant at Coonabarabran. Email robert.freebairn@bigpond.com or call 0428 752 149.
