推荐回答(1个)
(1)采样时间:一般认为,在果园果品采摘后至第一次施肥前采集土壤样品,即采样时间以秋季为佳。也有学者认为,在果树开花前的1~2个月均可采样。由于春季采样留给化学分析的时间有限,因此,建议最好在秋季采样。
(2)采样方法:应根据研究目的和果树树龄的不同确定果园土壤的采样方法。
对于定植前或刚栽植的幼树,土壤化验的主要目的是了解土壤肥力的基本性状,为果园土壤长期管理提供依据。由于这时土壤受果树生长和不均匀施肥的影响相对较小,因此,土壤肥力相对一致。这时采集土壤样品时,可以参照农田土壤采样方法进行。在田间按分对角线、棋盘式或蛇形等方法采集多点混合样。每个采样点的取土深度及采样量应均匀一致,土样上层与下层的比例要相同,取样工具应垂直于地面入土,深度相同。若选用小铁铲取土,应先挖成一与铲一样宽、与耕作层或取样要求深度相同深的土坑,将土坑一面铲成垂直面,然后从垂直一面铲取1~2厘米厚的土样。
将各点采集的土样充分混合。混合后的土壤样品往往太多,尚需采用“四分法”去掉多余的土样。具体方法为将混合的土壤摊成圆形,中间划十字分成四份,然后对角线去掉两份,若样品还多,将样品再混合均匀,再反复进行四分法,直至样品最终重量要求0.5~1千克左右为止。每个混合样的样点数量,应根据地形地貌、肥力均衡性和采样地块的大小而定。地形地貌较复杂要多采些,肥力差异较大的地块相应要比肥力均匀的田块要多一些,田块大的要比田块小的多。一般地块面积小于10亩,取5~10个点;10~40亩,取10~15个点;大于40亩取15个点以上。
对于成龄果树,土壤化验的主要目的是评价土壤养分供应状况,以指导果园施肥。因此,采样位置应考虑根系的分布范围和果树施肥的不均匀性。采样应遵循随机、多点、覆盖整个果园的原则。对土壤类型相对一致的果园,可采用X或S形方法,在测定果园选择不少于5~6个果树,在每个果树树冠投影边缘线30厘米左右的范围(因这一区域是吸收根分布相对集中的区域),分东、西、南、北四个方向采4个样;为真实地反映果园土壤的养分供应现状,建议采样点应避过当年和先一年的施肥沟。同一果园的不同样点充分混合,组成混合样,混合方法同上。采样可用土钻或铁锨,深度一般为0~20厘米或0~30厘米。同一果园,若土壤类型和果树树龄等差异较大,建议应分区采样。采样时应将果园土壤表层未分解的有机物、杂草等清除后,再开始采样。
考虑到果园多采用局部施肥方法,且一些果园行间可能套种农作物,施肥量会不同于果树,有学者建议,分区域分别采集混合土壤样品。
生产中常常发现,一些果树生长异常,若怀疑是土壤因素引起,可采集土壤样品。这时采集土壤时,应准确区别健壮树和异常树,分别选5~6株,采集根际及其下层土壤样品;各样点土壤不要混合,送专业实验室分别进行测定。
近年来,一些学者提出以2摩尔/升KCl提取的无机氮(包括铵态氮和硝态氮)作为果园土壤氮素营养诊断指标。这时采样的深度一般在0~60厘米或0~100厘米的范围内,按每20厘米一层采集土壤样品。
采集的样品放入统一的样品袋,用铅笔写好标签,内外各一张。标签内容包括编号、采样地点、采样深度、地块位置、农户、采样时间、采样人等。
在采样的同时,应进行果园生产及土壤施肥等相关内容的调查,以为正确地作出施肥决策提供参考。主要调查内容包括果园土壤类型、果园面积、建园时间、栽植密度、主栽品种、果树长势、果树产量,近年来的施肥状况(包括施肥种类、数量、时期等),是否有缺素等。
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