WHAT lays beneath the top 10cm of soil, the depth normally sampled to help with crop and pasture decisions, can have a big impact on aspects like best fertiliser programs and choice of species and variety.
Take for example soil sampling on a variety of soils across the NSW upper central west and Upper Hunter by Grain Orana Alliance (GOA) agronomist Ben OBrien, as part of a GRDC and Qld Dept Agriculture and Fisheries funded project “More Profit from Crop Nutrition”.
A light red soil at Binnaway is typical of what soil testing can identify in layers down the profile. The 10cm soil depth tested typically moderately acidic with pH (calcium chloride) 4.9 with aluminium saturation percentage of 3.8 percent. If one only used this data one would assume low aluminium toxicity and most crops and pastures could be grown, especially if lime was applied every few years.
However the 10 -30cm soil depth layer also tested pH 4.9, but was higher in aluminium saturation, 8.9 percent. Further stratification could see even higher aluminium toxicity in parts of this layer Ben Obrien points out. Future sampling with narrower segment sampling will examine this possibility.
Deeper soil low pH and significant aluminium levels is harder to correct by lime application and uncorrected can spell problems for many crop and pasture species and varieties within a species.
Ongoing research, for example by PhD researcher Hoang Han Nguyen, Charles Sturt University, School of Agricultural and Wine Sciences, indicates that organic amendments can increase pH where it is placed but also the soil layers below it. The research aims to better understand this complex interaction between lime and organic matter amendment and how this impacts soil layers below the place of incorporation.
Knowing that there is an acid soil layer below 0 – 10cm with aluminium toxicity is also important for species and variety choice. For example at these levels lucerne could be severely affected. Crops like faba beans, durum wheat, most barley cultivars, lentils, chickpeas and medics are also highly sensitive to aluminium toxicity.
Deeper soil low pH and significant aluminium levels is harder to correct by lime application.
In species like bread wheat there are several varieties with useful tolerance to aluminium toxicity, especially at soil levels detected in this profile. For example varieties like Beckom, Coolah, Dart, Sunmax, Condo, Crusader, Gregory, Wedgetail and Longswood are rated moderately tolerant or moderately tolerant to tolerant.
Yambla and Tulla (older varieties) are the only currently listed barley varieties with useful tolerance. Most oat varieties have good tolerance to soil aluminium toxicity as do most triticale lines. Most rye-grasses, fescue, sub clovers and chicory have good tolerance to moderate aluminium and species like narrow leaf lupins, cocksfoot, kikuyu, paspalum and yellow serradellas are very tolerant.
Deeper acidity layers are only a small part of what soil tests can detect. For example high sodicity (high sodium as a percentage of the cation exchange capacity) was noted in deeper layers of the soil profile detailed in this analysis, as it also was in one of the sampled heavier clay soils. Sodicity impacts on water infiltration rate and ability of roots to penetrate them. This knowledge may help decide on long term strategies such as permanent perennial tropical grass (can better utilise these layers) rather than cropping.
Some of the soil samples also identified good to excellent fertility of nutrients like nitrogen, phosphorus potassium and sulphur at various depth layers, helping greatly in improving both crop and pasture fertiliser decisions.
Next week. Very early sowing improves dual purpose crop success.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
