Accelerating Change toured the Riverina in January to have a close look at the soil and agronomic factors influencing summer cropping performance this year, above and below the surface. 46 farmers and agronomist came together to discuss feedbase planning, ground preparation, crop nutrition, weed control, irrigation management and the impact of sub soil constraints on yield. The tour kicked off at Mark & Phillipa Flemming’s at Tocumwal to look at grazed millet and conserved sorghum, and how these crops fit into their feeding system. The next step was Malcolm Holm & Jenny Wheeler’s farm at Blighty, where the group visited two maize crops that were subject to the same agronomic management but were performing vastly differently. The final stop was at Rob & Gaye Singleton’s at Finley where the group looked at an additional three maize sites, where a comparison was made on the impact of crop rotations and soil preparation on the development and yield potential of this year’s maize crop.
The following take home messages were discussed on the tour:
Choose summer crops based on fit into your overall system.
There was lots of discussion on the tour around tactical decisions of summer cropping and how you fit them into your overall feeding systems. The Flemming’s use summer cropping opportunistically, and grow sorghum and millet when water price is low. They graze the millet directly because of it’s ease of management, and conserve sorghum as well as maize. They usually have a full TMR system over summer and have appropriate feeding infrastructure to allow them to do this. This means they can balance the NDF of millet and sorghum with other sources of feed easily. Their feeding infrastructure also means they are not forced into irrigating pastures or crops for direct grazing when water price is high. Other farmers on the tour look to grow sorghum and maize when water prices are high, in order to drive higher returns on investment on their water applied, compared to pasture they may normally water through. This highlights that it is important to decide on summer cropping options with your own resources, goals, feeding system and farming philosophy at front of mind.
Agronomic timing is critical but not the only factor that determines yield
Accelerating Change has spoken in previous workshops about following the ‘recipe’ for summer crops, to ensure the result is more sponge-like rather than pancake. Maize and sorghum have consistent agronomic needs in terms of timing of sowing, nutrient application and irrigation. It is critical to get these things right in order to reach full yield potential. However, it is also important to note that even if the time of planting is optimal and agronomy follows ‘best practice’, yield potential may be limited by un-diagnosed soil constraints. Having a look beneath the soil may reveal factors that limit productivity. The impact of sub soil constraints was evident on both the Holm farm and the Singleton farm maize crops. Both farms had variability in yield potential and stage of development of their maize crops across bays, even though agronomic management were similar. It is important to look below the surface before you start preparing for your summer crop. Once it’s in and growing there will be little you can do to address constraints.
Know your soil inside and out!
Variability in yield across the maize sites we looked at can be attributed to variability in the availability of top soil and the effective root zone (sub soil). Topsoil depth is a relatively fixed property, which is often modified from landforming. The take home message for farmers and agronomists was ‘topsoil is your horsepower and your subsoil is your water tank’. The less top soil you have, the less productivity capacity the soil has overall. In a lot of our region, top soil depths are limited due to soil type and older practices of cutting and filling. Knowing the depth of the top soil and the variability across a field or farm will assist you to understand the yield potential of the site you select to grow summer crops (or any crops) but also assist in soil management decisions. There is a delicate balance between cultivation practices that will ensure appropriate seed soil contact and a uniform seed bed and practices to preserve top soil. Variability in yield potential is also related to Effective Root Zone depth (ERZ).
Effective root zone is how much of your water tank you are actually using
There was vast differences in the maize sites the tour visited in terms of root development down the profile. Some sites demonstrated hard panning (consolidated or compacted layers beneath the topsoil) which drastically stunted root development and impacted crop performance. The shallower the root system, the smaller the ‘water bucket’ and less efficient the delivery of nutrients for crop uptake. Management will determine how the root zone develops. If there is a hard pan or a hostile sub soil that is not addressed, plant roots will not be able to elongate past these barriers. Additionally, if soil is kept too wet at the start of crop development, roots won’t push down to find water. But if it is kept too dry, yield potential is restricted from early in the crops life. Maintaining a moist soil is critical, avoiding prolonged saturation of layers in the profile. This can be difficult to achieve under border check irrigation because an irrigation applies a large volume of water and control of soil moisture conditions is difficult.
At one site, the crop had an effective root zone depth of less than 30cm resulting from consolidated soil below 10cm, caused by soil slaking and unstable conditions from a lack of organic matter. If this was treated, the plants root system could elongate to greater depths and achieve improved crop performance and nutrient uptake, as observed in the neighbouring bay just a few metres away. In this bay the crop’s biomass, height and growth stage were more advanced. This growth response can directly related back to improved soil structure in that bay relating to different ground preparation. The greater the root system (and drainage) the crop develops, the bigger the bucket to fill and drain, leading to greater water efficiency.
Above: maize crop at different stages of development due to differences in effective root zone depth across the check bank.
Get deep ripping right for productive purposes
When evaluating the Effective Root Zone depth, many sites had a hard pan at depth that was a significant limitation to root development, water movement and aeration. Soil structure was deemed ‘massive’ or structureless. Deep ripping can be used to shatter soils and improve structure, but for a deep rip to be effective the soil needs to be stabilised as well as shattered.
Deep ripping depth should aim for a depth that will just shatter the hard pan you want to remediate. Stability is achieved from organic matter and top soils from the surface moving down the profile, application of gypsum, and the movement of nutrients. Best results are achieved where organic matter moves down the rip line behind the ripping tine. This keeps the rip line open and stabilised, providing a space for root growth. It also assists with cycling organic matter at depth. If you don’t stablise the soil when you rip, the soil may slump back down and the full benefit of deep ripping may be compromised.
Above: maize site that was pugged over winter and multidisced, with hard pan at depth and stunted root development.
Below: maize site that was deep ripped after having a deeper rooted cereal crop in over winter that was mulched in. Effective root elongation through the profile.
Know the soil constraint you’ve got before you actually remediate.
It’s particularly important to know the difference between a physical soil constraint, including structure, and other problems associated with soil chemistry. One of the maize sites showed the importance of knowing the difference between slaking and dispersive soils. According to Dairy Australia’s Fert$mart Program, soil slaking is related to structural stability, which is the soil’s ability to retain aggregates and pore spaces under various environmental conditions. This means the soil slumps and consolidates resulting from a lack of organic matter. If you have soil slaking problems, gypsum responses are often limited and treatment is achieved by building organic matter through correct tillage and agronomic practices. Dispersion is usually a problem of soil chemistry (namely, high levels of exchangeable sodium and potassium). Dispersion can be treated with calcium ameliorants including gypsum.
Building organic matter in a dairy system-it’s tricky but worthwhile!
Most of the maize sites we visited on the tour would benefit from building organic matter. This is difficult as the whole point of a dairy feed system is to remove as much biomass as possible to feed cows efficiently. Lack of organic matter causes a whole range of issues like slaking and a crusting top, hard panning at depth which stunts root growth and water infiltration, lack of nutrient availability through reduced activity of soil biota and less buffering against pH changes, and increases the impact of water logging. Increasing organic matter will also assist to improve the results of remediation such as deep ripping, and build top soil. It was interesting to note that while a lot of farmers look to application of manure and effluent to build organic matter, the actual level of carbon in manure and effluent can be very low. The main benefit of applying manure and effluent is the nutrient content, and large volumes need to be added over a long period of time to have an influence on organic matter levels.
Below: one of the maize sites that had low levels of organic matter struggled in performance this year.
Improving and preserving soil structure-it’s a long term plan
Rotations as well as seasonal management will contribute to preserving and improving structure. In irrigated systems with high clay percentages, such as deep cracking clay soils, opportunities to dry the profile, achieve cracking and self-mulching of topsoil and organic matter are often limited. This means that the clay component of the soils cannot enact normal swelling and shrinking processes to maintain structure. The lack of shrink-swell can aid the formation of hard pans in soils where slaking and dispersion under constant wet or saturated conditions occurs. Growing a rotational crop such as a cereal crop and taking it to the grain stage can give the soil an opportunity to dry at depth, use up subsoil moisture and improve the structure of subsoil clay. The straw retained after harvest will also assist with building organic matter and an improved root system will improve the chances of achieving optimal yield potential. Farmers discussed whether this would be feasible or not in a dairy system, how cost effective accessing the required machinery would be and what other options might be able to achieve the same results.
Below: a maize site with hard panning at depth and stunted root development
Soil moisture monitoring-not just to inform irrigation!
Soil moisture monitoring can also assist you to identify at what depth in the profile the crop is drawing water from. This will help you identify the effective root zone which will influence your irrigation management, but also inform what management decisions you take going into the next season.
Weed control-get onto it early
Summer crops that are a grass species really need to have early and active weed control measures in place because once they are up and going the options you have to attack grass weeds are limited. This can be a real issues in paddocks that are rotated in and out of pastures. This will effect yield and quality of the crop.