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Improving Nutrient Efficiencies on Sandy Soils

15 February 2015

 

My weekly in field farm visits takes me pretty much along a far stretch of land that follows the Murray River, composing of predominantly deep red sandy soils. Some of this land is irrigated supporting large scale horticulture and viticulture, while other parts is marginal cropping country.

As we know sandy soils without adequate rainfall or irrigation and fine tune nutrition do not yield well, due to sands inability to store minerals caused by its lack of clay colloids and organic carbon. So in irrigated horticulture frequent and constant nutrition is applied in small doses to minimise losses. While in the marginal cropping area large tracks of land are cropped at lower potential yields.

One of the major inefficiencies can be the loss of valuable applied fertiliser to these soils and that’s dollars down the drain. While free draining is a benefit of these sandy soils, the downside is generally lost nutrients too as there is nothing to stabilise and hold them in the top part of the soil profile.

 So what can we do to improve better nutrient efficiencies in sandy soils?

One of the best strategies in irrigated sandy soils is combining incorporation of Mychorrhizal fungi onto the seed coating prior to sowing or irrigated into the root systems of plants, the addition of a high carbon soil amendment like Biologic Blend and Revive, BioMAX SolubleHumates with your pre-plant fertiliser and the digestion of plant residues(stubble) to assist in returning organic carbon back into the soil with a stubble digesting fungi.

Most plants will have a compatible relationship with Vesicular arbuscular mycorrhizal fungi (VAM). VAMs are fungi which live harmoniously with plant roots. The VAM fungi provide the roots with extra nutrients from the soils that the plants root system may not be able to reach, with an enhanced ability to pull into the plant more phosphorous and zinc. The VAM receives in exchange from the plant, sugars, in which up to 30 percent of the sugar made each day by photosynthesis is purposely directed to the root system to feed soil micro-organisms. VAM also, as with all soil fungi, helps to hold soil particles together.

But for the fungi to really excel in sand you also need soil organic carbon, which comes from the addition of soil conditioners high in carbon and Humates such as humic acid and fulvic acid.

Humates are composed of Humins, Humic Acid and Fulvic and each show different chemical behaviour depending on pH. Fulvic is a great chelator as its soluble in all pH levels and forms bonds with minerals so the plant can easily draw them in. Humic substances are known to enhance biotic and abiotic  degradation of both organic and synthetic toxic components. Humic substances also stimulate the growth of microbes that feed and breakdown toxic components. Humins are the most stable part of this organic carbon and are not soluble at any pH.

Both groups of organic acids, humic acid and fulvic acid carry electrical charges that influence known chemical reactions. Three specific chemical reactions are commonly termed: (1) electrostatic (columbic) attraction (2) complex formation or chelation, and (3) water bridging.

Electrostatic attraction allows more sites for nutrients such as nitrogen, sulphur and trace minerals to be held onto, reducing leaching. Electrically charged sites on humic acid function to dissolve and bind trace minerals to these sites.  Chelation of plant nutrients such as iron (Fe), copper (Cu), zinc (Zn), magnesium (Mg), manganese (Mn), and calcium (Ca) reduces their toxicity as cations, preventing leaching, while increasing uptake by plant roots.

The chelation of heavy toxic metal elements such as mercury (Hg), lead (Pb), and cadmium (Cd) are also influenced by humates. Once chelated these metal complexes become less available for plant uptake.

Humates are also effective to bond and hold onto sodium preventing plant uptake and is a great natural buffer additive to high sodium irrigation water.

Humic and fulvic acids both have water bridging properties. Water bridging is believed to improve nutrient ion mobility, through the soil solution to the root. These mechanisms also help reduce leaching. The humates have the capacity to grab and hold newly applied fertiliser just like it holds minerals already in the soil.

The ability to hold and stabilise minerals applied to sandy soil means a greater cost efficiency for the fertiliser applied as well as the potential to improve gross margin return per hectare. For example, Urea has been known to be short acting. This is due to its high water solubility which means the plant only has a short time at the dinner table before it is lost to the waterways or atmosphere. Humates greatest impact on urea is its ability to hold and release nitrogen slowly, which leads to a greater Nitrogen efficiency. This means a grower can opt to use less or increase production with the current application.

LawrieCo has many examples of restoring lost soil carbon and has achieved excellent results in taking a Sandy soils organic carbon level from 0.21% to 3.31% in the top 15 cm, leading to the restoration of increased soil fertility. If you want to learn more or trial some of these strategies, download our “Dirt to Healthy Soil” Book or contact one of our Field Advisors.